Related
I have a data class:
public class MyData {
final Integer alpha;
final Double beta;
final Integer foo;
final Double bar;
}
I need .equals and .hashCode to have the conventional definition involving all four fields. But I have another important requirement:
Given a large number of MyData objects, I need to rapidly check whether a new MyData object matches any of the existing ones on the .alpha and .beta fields only.
Three approaches I don't want to take:
Composite object:
public class MyData {
final MyDataKey alphaAndBeta;
final Integer foo;
final Double bar;
}
public class MyDataKey {
final Integer alpha;
final Double beta;
}
While I could then do lookups against a HashSet<MyDataKey>, it's inelegant because all other uses of the object will need to refer to dataObj.alphaAndBeta.alpha instead of dataObj.alpha.
Comparator:
public class OnlyAlphaAndBeta implements Comparator<MyData> {
int compare(MyData a, MyData b) {...}
}
This would then let a new TreeSet<MyData>(new OnlyAlphaAndBeta()) do the lookups I want; but with O(log(N)) instead of O(1).
Multi-level lookup:
public class MyDataLookup {
Map<Integer, Set<Double>> existingAlphaBeta;
boolean contains(MyData query) {
Set<Double> betas = this.existingAlphaBeta.get(query.alpha);
if (betas == null) {
return false;
}
return betas.contains(query.beta);
}
boolean add(MyData toInsert) {...};
}
This does the job, in O(1), but what if the key was more than just 2 fields? I could keep nesting Map<A, Map<B, Map<C, ...>>> for each field in the key, but this doesn't seem right. Surely I'd rather compute just one hash and look it up in one table.
I think what I'm looking for is something like HashSet, but which can be specialized to use something other than the .equals and .hashCode methods, analogous to how Comparator redefines ordering for SortedSet. Such a collection wouldn't fulfill the Set contract anymore, but it would be "set-like".
Does something like this exist in any of the big, well-maintained Java utility libraries? Alternately, am I overlooking some obvious way of accomplishing my goals?
Using a Map is the right approach, but you can encapsulate it in a Set implementation having precisely the intended behavior of “a Set with custom equals”.
public class CustomSet<E> extends AbstractSet<E> {
private final Function<E, Object> theKeyFunction;
private final HashMap<Object, E> backend = new HashMap<>();
public CustomSet(Function<E,Object> keyFunction) {
theKeyFunction = Objects.requireNonNull(keyFunction);
}
#Override
public int size() {
return backend.size();
}
#Override
public boolean add(E e) {
Objects.requireNonNull(e);
return backend.putIfAbsent(theKeyFunction.apply(e), e) == null;
}
#Override
public boolean contains(Object o) {
if(o == null) return false;
#SuppressWarnings("unchecked") E e = (E)o;
Object key;
try { key = theKeyFunction.apply(e); }
catch(ClassCastException ex) { return false; }
return backend.containsKey(key);
}
#Override
public boolean remove(Object o) {
if(o == null) return false;
#SuppressWarnings("unchecked") E e = (E)o;
Object key;
try { key = theKeyFunction.apply(e); }
catch(ClassCastException ex) { return false; }
return backend.remove(key) != null;
}
#Override
public void clear() {
backend.clear();
}
#Override
public boolean retainAll(Collection<?> c) {
return backend.values().retainAll(c);
}
#Override
public boolean removeIf(Predicate<? super E> filter) {
return backend.values().removeIf(filter);
}
#Override
public void forEach(Consumer<? super E> action) {
backend.values().forEach(action);
}
#Override
public Iterator<E> iterator() {
return backend.values().iterator();
}
#Override
public Spliterator<E> spliterator() {
return backend.values().spliterator();
}
#Override
public Object[] toArray() {
return backend.values().toArray();
}
#Override
public <T> T[] toArray(T[] a) {
return backend.values().toArray(a);
}
}
To keep it simple, this Set does not support null.
This class overrides some methods it doesn’t have to, to provide better performance when iterating or streaming over it. Besides that, it’s rather simple. If you think, “but a Set that internally uses a Map is quite inefficient”, look at the source code of HashSet or TreeSet…
This set implementation can be tested like
record Person(String name, int age) {}
Set<Person> nameSet = new CustomSet<>(Person::name);
Set<Person> ageSet = new CustomSet<>(Person::age);
for(String name: List.of("John", "Paul", "George", "Ringo")) {
for(int age: new int[] { 20, 24, 27, 31 }) {
Person p = new Person(name, age);
if(nameSet.add(p)) System.out.println("added " + p + " to nameSet");
if(ageSet.add(p)) System.out.println("added " + p + " to ageSet");
}
}
System.out.println();
System.out.println("nameSet: " + nameSet);
System.out.println("ageSet: " + ageSet);
System.out.println();
Person p = new Person("Paul", 100);
System.out.println("nameSet contains " + p + "? " + nameSet.contains(p));
System.out.println("ageSet contains " + p + "? " + ageSet.contains(p));
p = new Person("Bob", 27);
System.out.println("nameSet contains " + p + "? " + nameSet.contains(p));
System.out.println("ageSet contains " + p + "? " + ageSet.contains(p));
added Person[name=John, age=20] to nameSet
added Person[name=John, age=20] to ageSet
added Person[name=John, age=24] to ageSet
added Person[name=John, age=27] to ageSet
added Person[name=John, age=31] to ageSet
added Person[name=Paul, age=20] to nameSet
added Person[name=George, age=20] to nameSet
added Person[name=Ringo, age=20] to nameSet
nameSet: [Person[name=George, age=20], Person[name=John, age=20], Person[name=Ringo, age=20], Person[name=Paul, age=20]]
ageSet: [Person[name=John, age=20], Person[name=John, age=24], Person[name=John, age=27], Person[name=John, age=31]]
nameSet contains Person[name=Paul, age=100]?true
ageSet contains Person[name=Paul, age=100]?false
nameSet contains Person[name=Bob, age=27]?false
ageSet contains Person[name=Bob, age=27]?true
demonstrating the different understanding of equality of the two sets, which leads to the same warning as applies to TreeSet with comparators not consistent with equals. Mixing sets with different key functions can lead to the same weird behavior as mixing sorted sets with different comparators or mixing such sets with an ordinary hash set.
If the key consists of multiple properties, a dedicated key object is the way to go, but that doesn’t mean that the application domain object has to be a composed object:
record MyData(int alpha, double beta, int foo, double bar) {}
Set<MyData> set = new CustomSet<>(d -> {
record Key(int alpha, double beta) {}
return new Key(d.alpha(), d.beta());
});
set.add(new MyData(1, 1.0, 100, 1.23));
System.out.println(set.contains(new MyData(1, 1.0, -1, Double.NaN))); // true
Solutions for older Java versions without record are a bit more verbose, but the principle stays the same. If you don’t need maximum performance, you can also use List keys as they have a working equals and hashCode implementation:
// Java 8 compatible
Set<MyData> set = new CustomSet<>(d -> Arrays.asList(d.alpha(), d.beta()));
I promise that I did try searching for an answer before writing this question. But 24 hours later I had a much more productive search, and found several viable answers:
Guava Equivalence
Eclipse Collections HashingStrategy
commons-collections AbstractHashedMap
Trove4j HashingStrategy
Keep a Map of the MyData values where the map key is defined uniquely by alpha and beta. In this example I'm concatenating them as Strings:
class MyDataLookup {
Map<String, MyData> map = new HashMap<>();
public MyData put(MyData value) {
return map.put(getKey(value), value);
}
public boolean contains(MyData value) {
return map.containsKey(getKey(value));
}
private static String getKey(MyData value) {
return value.alpha +"_"+ value.beta;
}
}
From this you can easily change the definition of containment by modifying MyDataLookup#getKey (say to include foo as well).
I want to return two objects from a Java method and was wondering what could be a good way of doing so?
The possible ways I can think of are: return a HashMap (since the two Objects are related) or return an ArrayList of Object objects.
To be more precise, the two objects I want to return are (a) List of objects and (b) comma separated names of the same.
I want to return these two Objects from one method because I dont want to iterate through the list of objects to get the comma separated names (which I can do in the same loop in this method).
Somehow, returning a HashMap does not look a very elegant way of doing so.
If you want to return two objects you usually want to return a single object that encapsulates the two objects instead.
You could return a List of NamedObject objects like this:
public class NamedObject<T> {
public final String name;
public final T object;
public NamedObject(String name, T object) {
this.name = name;
this.object = object;
}
}
Then you can easily return a List<NamedObject<WhateverTypeYouWant>>.
Also: Why would you want to return a comma-separated list of names instead of a List<String>? Or better yet, return a Map<String,TheObjectType> with the keys being the names and the values the objects (unless your objects have specified order, in which case a NavigableMap might be what you want.
If you know you are going to return two objects, you can also use a generic pair:
public class Pair<A,B> {
public final A a;
public final B b;
public Pair(A a, B b) {
this.a = a;
this.b = b;
}
};
Edit A more fully formed implementation of the above:
package util;
public class Pair<A,B> {
public static <P, Q> Pair<P, Q> makePair(P p, Q q) {
return new Pair<P, Q>(p, q);
}
public final A a;
public final B b;
public Pair(A a, B b) {
this.a = a;
this.b = b;
}
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((a == null) ? 0 : a.hashCode());
result = prime * result + ((b == null) ? 0 : b.hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj == null) {
return false;
}
if (getClass() != obj.getClass()) {
return false;
}
#SuppressWarnings("rawtypes")
Pair other = (Pair) obj;
if (a == null) {
if (other.a != null) {
return false;
}
} else if (!a.equals(other.a)) {
return false;
}
if (b == null) {
if (other.b != null) {
return false;
}
} else if (!b.equals(other.b)) {
return false;
}
return true;
}
public boolean isInstance(Class<?> classA, Class<?> classB) {
return classA.isInstance(a) && classB.isInstance(b);
}
#SuppressWarnings("unchecked")
public static <P, Q> Pair<P, Q> cast(Pair<?, ?> pair, Class<P> pClass, Class<Q> qClass) {
if (pair.isInstance(pClass, qClass)) {
return (Pair<P, Q>) pair;
}
throw new ClassCastException();
}
}
Notes, mainly around rustiness with Java & generics:
both a and b are immutable.
makePair static method helps you with boiler plate typing, which the diamond operator in Java 7 will make less annoying. There's some work to make this really nice re: generics, but it should be ok-ish now. (c.f. PECS)
hashcode and equals are generated by eclipse.
the compile time casting in the cast method is ok, but doesn't seem quite right.
I'm not sure if the wildcards in isInstance are necessary.
I've just written this in response to comments, for illustration purposes only.
In the event the method you're calling is private, or called from one location, try
return new Object[]{value1, value2};
The caller looks like:
Object[] temp=myMethod(parameters);
Type1 value1=(Type1)temp[0]; //For code clarity: temp[0] is not descriptive
Type2 value2=(Type2)temp[1];
The Pair example by David Hanak has no syntactic benefit, and is limited to two values.
return new Pair<Type1,Type2>(value1, value2);
And the caller looks like:
Pair<Type1, Type2> temp=myMethod(parameters);
Type1 value1=temp.a; //For code clarity: temp.a is not descriptive
Type2 value2=temp.b;
You may use any of following ways:
private static final int RETURN_COUNT = 2;
private static final int VALUE_A = 0;
private static final int VALUE_B = 1;
private static final String A = "a";
private static final String B = "b";
1) Using Array
private static String[] methodWithArrayResult() {
//...
return new String[]{"valueA", "valueB"};
}
private static void usingArrayResultTest() {
String[] result = methodWithArrayResult();
System.out.println();
System.out.println("A = " + result[VALUE_A]);
System.out.println("B = " + result[VALUE_B]);
}
2) Using ArrayList
private static List<String> methodWithListResult() {
//...
return Arrays.asList("valueA", "valueB");
}
private static void usingListResultTest() {
List<String> result = methodWithListResult();
System.out.println();
System.out.println("A = " + result.get(VALUE_A));
System.out.println("B = " + result.get(VALUE_B));
}
3) Using HashMap
private static Map<String, String> methodWithMapResult() {
Map<String, String> result = new HashMap<>(RETURN_COUNT);
result.put(A, "valueA");
result.put(B, "valueB");
//...
return result;
}
private static void usingMapResultTest() {
Map<String, String> result = methodWithMapResult();
System.out.println();
System.out.println("A = " + result.get(A));
System.out.println("B = " + result.get(B));
}
4) Using your custom container class
private static class MyContainer<M,N> {
private final M first;
private final N second;
public MyContainer(M first, N second) {
this.first = first;
this.second = second;
}
public M getFirst() {
return first;
}
public N getSecond() {
return second;
}
// + hashcode, equals, toString if need
}
private static MyContainer<String, String> methodWithContainerResult() {
//...
return new MyContainer("valueA", "valueB");
}
private static void usingContainerResultTest() {
MyContainer<String, String> result = methodWithContainerResult();
System.out.println();
System.out.println("A = " + result.getFirst());
System.out.println("B = " + result.getSecond());
}
5) Using AbstractMap.simpleEntry
private static AbstractMap.SimpleEntry<String, String> methodWithAbstractMapSimpleEntryResult() {
//...
return new AbstractMap.SimpleEntry<>("valueA", "valueB");
}
private static void usingAbstractMapSimpleResultTest() {
AbstractMap.SimpleEntry<String, String> result = methodWithAbstractMapSimpleEntryResult();
System.out.println();
System.out.println("A = " + result.getKey());
System.out.println("B = " + result.getValue());
}
6) Using Pair of Apache Commons
private static Pair<String, String> methodWithPairResult() {
//...
return new ImmutablePair<>("valueA", "valueB");
}
private static void usingPairResultTest() {
Pair<String, String> result = methodWithPairResult();
System.out.println();
System.out.println("A = " + result.getKey());
System.out.println("B = " + result.getValue());
}
I almost always end up defining n-Tuple classes when I code in Java. For instance:
public class Tuple2<T1,T2> {
private T1 f1;
private T2 f2;
public Tuple2(T1 f1, T2 f2) {
this.f1 = f1; this.f2 = f2;
}
public T1 getF1() {return f1;}
public T2 getF2() {return f2;}
}
I know it's a bit ugly, but it works, and you just have to define your tuple types once. Tuples are something Java really lacks.
EDIT: David Hanak's example is more elegant, as it avoids defining getters and still keeps the object immutable.
Before Java 5, I would kind of agree that the Map solution isn't ideal. It wouldn't give you compile time type checking so can cause issues at runtime. However, with Java 5, we have Generic Types.
So your method could look like this:
public Map<String, MyType> doStuff();
MyType of course being the type of object you are returning.
Basically I think that returning a Map is the right solution in this case because that's exactly what you want to return - a mapping of a string to an object.
Apache Commons has tuple and triple for this:
ImmutablePair<L,R> An immutable pair consisting of two Object
elements.
ImmutableTriple<L,M,R> An immutable triple consisting of
three Object elements.
MutablePair<L,R> A mutable pair consisting of
two Object elements.
MutableTriple<L,M,R> A mutable triple
consisting of three Object elements.
Pair<L,R> A pair consisting of
two elements.
Triple<L,M,R> A triple consisting of three elements.
Source: https://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/tuple/package-summary.html
Alternatively, in situations where I want to return a number of things from a method I will sometimes use a callback mechanism instead of a container. This works very well in situations where I cannot specify ahead of time just how many objects will be generated.
With your particular problem, it would look something like this:
public class ResultsConsumer implements ResultsGenerator.ResultsCallback
{
public void handleResult( String name, Object value )
{
...
}
}
public class ResultsGenerator
{
public interface ResultsCallback
{
void handleResult( String aName, Object aValue );
}
public void generateResults( ResultsGenerator.ResultsCallback aCallback )
{
Object value = null;
String name = null;
...
aCallback.handleResult( name, value );
}
}
While in your case, the comment may be a good way to go, in Android, you can use Pair . Simply
return new Pair<>(yourList, yourCommaSeparatedValues);
Use of following Entry object
Example :
public Entry<A,B> methodname(arg)
{
.......
return new AbstractMap.simpleEntry<A,B>(instanceOfA,instanceOfB);
}
Regarding the issue about multiple return values in general I usually use a small helper class that wraps a single return value and is passed as parameter to the method:
public class ReturnParameter<T> {
private T value;
public ReturnParameter() { this.value = null; }
public ReturnParameter(T initialValue) { this.value = initialValue; }
public void set(T value) { this.value = value; }
public T get() { return this.value; }
}
(for primitive datatypes I use minor variations to directly store the value)
A method that wants to return multiple values would then be declared as follows:
public void methodThatReturnsTwoValues(ReturnParameter<ClassA> nameForFirstValueToReturn, ReturnParameter<ClassB> nameForSecondValueToReturn) {
//...
nameForFirstValueToReturn.set("...");
nameForSecondValueToReturn.set("...");
//...
}
Maybe the major drawback is that the caller has to prepare the return objects in advance in case he wants to use them (and the method should check for null pointers)
ReturnParameter<ClassA> nameForFirstValue = new ReturnParameter<ClassA>();
ReturnParameter<ClassB> nameForSecondValue = new ReturnParameter<ClassB>();
methodThatReturnsTwoValues(nameForFirstValue, nameForSecondValue);
Advantages (in comparison to other solutions proposed):
You do not have to create a special class declaration for individual methods and its return types
The parameters get a name and therefore are easier to differentiate when looking at the method signature
Type safety for each parameter
All possible solutions will be a kludge (like container objects, your HashMap idea, “multiple return values” as realized via arrays). I recommend regenerating the comma-separated list from the returned List. The code will end up being a lot cleaner.
Keep it simple and create a class for multiple result situation. This example accepts an ArrayList and a message text from a databasehelper getInfo.
Where you call the routine that returns multiple values you code:
multResult res = mydb.getInfo();
In the routine getInfo you code:
ArrayList<String> list= new ArrayList<String>();
add values to the list...
return new multResult("the message", list);
and define a class multResult with:
public class multResult {
public String message; // or create a getter if you don't like public
public ArrayList<String> list;
multResult(String m, ArrayList<String> l){
message = m;
list= l;
}
}
As I see it there are really three choices here and the solution depends on the context. You can choose to implement the construction of the name in the method that produces the list. This is the choice you've chosen, but I don't think it is the best one. You are creating a coupling in the producer method to the consuming method that doesn't need to exist. Other callers may not need the extra information and you would be calculating extra information for these callers.
Alternatively, you could have the calling method calculate the name. If there is only one caller that needs this information, you can stop there. You have no extra dependencies and while there is a little extra calculation involved, you've avoided making your construction method too specific. This is a good trade-off.
Lastly, you could have the list itself be responsible for creating the name. This is the route I would go if the calculation needs to be done by more than one caller. I think this puts the responsibility for the creation of the names with the class that is most closely related to the objects themselves.
In the latter case, my solution would be to create a specialized List class that returns a comma-separated string of the names of objects that it contains. Make the class smart enough that it constructs the name string on the fly as objects are added and removed from it. Then return an instance of this list and call the name generation method as needed. Although it may be almost as efficient (and simpler) to simply delay calculation of the names until the first time the method is called and store it then (lazy loading). If you add/remove an object, you need only remove the calculated value and have it get recalculated on the next call.
Can do some thing like a tuple in dynamic language (Python)
public class Tuple {
private Object[] multiReturns;
private Tuple(Object... multiReturns) {
this.multiReturns = multiReturns;
}
public static Tuple _t(Object... multiReturns){
return new Tuple(multiReturns);
}
public <T> T at(int index, Class<T> someClass) {
return someClass.cast(multiReturns[index]);
}
}
and use like this
public Tuple returnMultiValues(){
return Tuple._t(new ArrayList(),new HashMap())
}
Tuple t = returnMultiValues();
ArrayList list = t.at(0,ArrayList.class);
I followed a similar approach than the described in the other answers with a few tweaks based on the requirement I had, basically I created the following classes(Just in case, everything is Java):
public class Pair<L, R> {
final L left;
final R right;
public Pair(L left, R right) {
this.left = left;
this.right = right;
}
public <T> T get(Class<T> param) {
return (T) (param == this.left.getClass() ? this.left : this.right);
}
public static <L, R> Pair<L, R> of(L left, R right) {
return new Pair<L, R>(left, right);
}
}
Then, my requirement was simple, in the repository Class that reaches the DB, for the Get Methods than retrieve data from the DB, I need to check if it failed or succeed, then, if succeed, I needed to play with the returning list, if failed, stop the execution and notify the error.
So, for example, my methods are like this:
public Pair<ResultMessage, List<Customer>> getCustomers() {
List<Customer> list = new ArrayList<Customer>();
try {
/*
* Do some work to get the list of Customers from the DB
* */
} catch (SQLException e) {
return Pair.of(
new ResultMessage(e.getErrorCode(), e.getMessage()), // Left
null); // Right
}
return Pair.of(
new ResultMessage(0, "SUCCESS"), // Left
list); // Right
}
Where ResultMessage is just a class with two fields (code/message) and Customer is any class with a bunch of fields that comes from the DB.
Then, to check the result I just do this:
void doSomething(){
Pair<ResultMessage, List<Customer>> customerResult = _repository.getCustomers();
if (customerResult.get(ResultMessage.class).getCode() == 0) {
List<Customer> listOfCustomers = customerResult.get(List.class);
System.out.println("do SOMETHING with the list ;) ");
}else {
System.out.println("Raised Error... do nothing!");
}
}
In C++ (STL) there is a pair class for bundling two objects. In Java Generics a pair class isn't available, although there is some demand for it. You could easily implement it yourself though.
I agree however with some other answers that if you need to return two or more objects from a method, it would be better to encapsulate them in a class.
Why not create a WhateverFunctionResult object that contains your results, and the logic required to parse these results, iterate over then etc. It seems to me that either:
These results objects are intimately tied together/related and belong together, or:
they are unrelated, in which case your function isn't well defined in terms of what it's trying to do (i.e. doing two different things)
I see this sort of issue crop up again and again. Don't be afraid to create your own container/result classes that contain the data and the associated functionality to handle this. If you simply pass the stuff around in a HashMap or similar, then your clients have to pull this map apart and grok the contents each time they want to use the results.
public class MultipleReturnValues {
public MultipleReturnValues() {
}
public static void functionWithSeveralReturnValues(final String[] returnValues) {
returnValues[0] = "return value 1";
returnValues[1] = "return value 2";
}
public static void main(String[] args) {
String[] returnValues = new String[2];
functionWithSeveralReturnValues(returnValues);
System.out.println("returnValues[0] = " + returnValues[0]);
System.out.println("returnValues[1] = " + returnValues[1]);
}
}
This is not exactly answering the question, but since every of the solution given here has some drawbacks, I suggest to try to refactor your code a little bit so you need to return only one value.
Case one.
You need something inside as well as outside of your method. Why not calculate it outside and pass it to the method?
Instead of:
[thingA, thingB] = createThings(...); // just a conceptual syntax of method returning two values, not valid in Java
Try:
thingA = createThingA(...);
thingB = createThingB(thingA, ...);
This should cover most of your needs, since in most situations one value is created before the other and you can split creating them in two methods. The drawback is that method createThingsB has an extra parameter comparing to createThings, and possibly you are passing exactly the same list of parameters twice to different methods.
Case two.
Most obvious solution ever and a simplified version of case one. It's not always possible, but maybe both of the values can be created independently of each other?
Instead of:
[thingA, thingB] = createThings(...); // see above
Try:
thingA = createThingA(...);
thingB = createThingB(...);
To make it more useful, these two methods can share some common logic:
public ThingA createThingA(...) {
doCommonThings(); // common logic
// create thing A
}
public ThingB createThingB(...) {
doCommonThings(); // common logic
// create thing B
}
Pass a list to your method and populate it, then return the String with the names, like this:
public String buildList(List<?> list) {
list.add(1);
list.add(2);
list.add(3);
return "something,something,something,dark side";
}
Then call it like this:
List<?> values = new ArrayList<?>();
String names = buildList(values);
You can utilize a HashMap<String, Object> as follows
public HashMap<String, Object> yourMethod()
{
.... different logic here
HashMap<String, Object> returnHashMap = new HashMap<String, Object>();
returnHashMap.put("objectA", objectAValue);
returnHashMap.put("myString", myStringValue);
returnHashMap.put("myBoolean", myBooleanValue);
return returnHashMap;
}
Then when calling the method in a different scope, you can cast each object back to its initial type:
// call the method
HashMap<String, Object> resultMap = yourMethod();
// fetch the results and cast them
ObjectA objectA = (ObjectA) resultMap.get("objectA");
String myString = (String) resultMap.get("myString");
Boolean myBoolean = (Boolean) resultMap.get("myBoolean");
I noticed there is no no-custom class, n-length, no-cast, type-safe answers yet to returning multiple values.
Here is my go:
import java.util.Objects;
public final class NTuple<V, T extends NTuple<?, ?>> {
private final V value;
private final T next;
private NTuple(V value, T next) {
this.value = value;
this.next = next;
}
public static <V> NTuple<V, ?> of(V value) {
return new NTuple<>(value, null);
}
public static <V, T extends NTuple<?, ?>> NTuple<V, T> of(V value, T next) {
return new NTuple<>(value, next);
}
public V value() {
return value;
}
public T next() {
return next;
}
public static <V> V unpack0(NTuple<V, ?> tuple) {
return Objects.requireNonNull(tuple, "0").value();
}
public static <V, T extends NTuple<V, ?>> V unpack1(NTuple<?, T> tuple) {
NTuple<?, T> tuple0 = Objects.requireNonNull(tuple, "0");
NTuple<V, ?> tuple1 = Objects.requireNonNull(tuple0.next(), "1");
return tuple1.value();
}
public static <V, T extends NTuple<?, NTuple<V, ?>>> V unpack2(NTuple<?, T> tuple) {
NTuple<?, T> tuple0 = Objects.requireNonNull(tuple, "0");
NTuple<?, NTuple<V, ?>> tuple1 = Objects.requireNonNull(tuple0.next(), "1");
NTuple<V, ?> tuple2 = Objects.requireNonNull(tuple1.next(), "2");
return tuple2.value();
}
}
Sample use:
public static void main(String[] args) {
// pre-java 10 without lombok - use lombok's var or java 10's var if you can
NTuple<String, NTuple<Integer, NTuple<Integer, ?>>> multiple = wordCount("hello world");
String original = NTuple.unpack0(multiple);
Integer wordCount = NTuple.unpack1(multiple);
Integer characterCount = NTuple.unpack2(multiple);
System.out.println(original + ": " + wordCount + " words " + characterCount + " chars");
}
private static NTuple<String, NTuple<Integer, NTuple<Integer, ?>>> wordCount(String s) {
int nWords = s.split(" ").length;
int nChars = s.length();
return NTuple.of(s, NTuple.of(nWords, NTuple.of(nChars)));
}
Pros:
no-custom container class - no need to write a class just for a return type
n-length - can handle any number of return values
no-cast - no need to cast from Object
type-safe - the types are checked via Java's generics
Cons:
inefficient for large numbers of return values
according to my experience with python's multiple return values, this should not happen in practice
heavy type declarations
can be alleviated by lombok/Java 10 var
In C, you would do it by passing pointers to placeholders for the results as arguments:
void getShoeAndWaistSizes(int *shoeSize, int *waistSize) {
*shoeSize = 36;
*waistSize = 45;
}
...
int shoeSize, waistSize;
getShoeAndWaistSize(&shoeSize, &waistSize);
int i = shoeSize + waistSize;
Let's try something similar, in Java.
void getShoeAndWaistSizes(List<Integer> shoeSize, List<Integer> waistSize) {
shoeSize.add(36);
waistSize.add(45);
}
...
List<Integer> shoeSize = new List<>();
List<Integer> waistSize = new List<>();
getShoeAndWaistSizes(shoeSize, waistSize);
int i = shoeSize.get(0) + waistSize.get(0);
PASS A HASH INTO THE METHOD AND POPULATE IT......
public void buildResponse(String data, Map response);
I want to return two objects from a Java method and was wondering what could be a good way of doing so?
The possible ways I can think of are: return a HashMap (since the two Objects are related) or return an ArrayList of Object objects.
To be more precise, the two objects I want to return are (a) List of objects and (b) comma separated names of the same.
I want to return these two Objects from one method because I dont want to iterate through the list of objects to get the comma separated names (which I can do in the same loop in this method).
Somehow, returning a HashMap does not look a very elegant way of doing so.
If you want to return two objects you usually want to return a single object that encapsulates the two objects instead.
You could return a List of NamedObject objects like this:
public class NamedObject<T> {
public final String name;
public final T object;
public NamedObject(String name, T object) {
this.name = name;
this.object = object;
}
}
Then you can easily return a List<NamedObject<WhateverTypeYouWant>>.
Also: Why would you want to return a comma-separated list of names instead of a List<String>? Or better yet, return a Map<String,TheObjectType> with the keys being the names and the values the objects (unless your objects have specified order, in which case a NavigableMap might be what you want.
If you know you are going to return two objects, you can also use a generic pair:
public class Pair<A,B> {
public final A a;
public final B b;
public Pair(A a, B b) {
this.a = a;
this.b = b;
}
};
Edit A more fully formed implementation of the above:
package util;
public class Pair<A,B> {
public static <P, Q> Pair<P, Q> makePair(P p, Q q) {
return new Pair<P, Q>(p, q);
}
public final A a;
public final B b;
public Pair(A a, B b) {
this.a = a;
this.b = b;
}
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((a == null) ? 0 : a.hashCode());
result = prime * result + ((b == null) ? 0 : b.hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj == null) {
return false;
}
if (getClass() != obj.getClass()) {
return false;
}
#SuppressWarnings("rawtypes")
Pair other = (Pair) obj;
if (a == null) {
if (other.a != null) {
return false;
}
} else if (!a.equals(other.a)) {
return false;
}
if (b == null) {
if (other.b != null) {
return false;
}
} else if (!b.equals(other.b)) {
return false;
}
return true;
}
public boolean isInstance(Class<?> classA, Class<?> classB) {
return classA.isInstance(a) && classB.isInstance(b);
}
#SuppressWarnings("unchecked")
public static <P, Q> Pair<P, Q> cast(Pair<?, ?> pair, Class<P> pClass, Class<Q> qClass) {
if (pair.isInstance(pClass, qClass)) {
return (Pair<P, Q>) pair;
}
throw new ClassCastException();
}
}
Notes, mainly around rustiness with Java & generics:
both a and b are immutable.
makePair static method helps you with boiler plate typing, which the diamond operator in Java 7 will make less annoying. There's some work to make this really nice re: generics, but it should be ok-ish now. (c.f. PECS)
hashcode and equals are generated by eclipse.
the compile time casting in the cast method is ok, but doesn't seem quite right.
I'm not sure if the wildcards in isInstance are necessary.
I've just written this in response to comments, for illustration purposes only.
In the event the method you're calling is private, or called from one location, try
return new Object[]{value1, value2};
The caller looks like:
Object[] temp=myMethod(parameters);
Type1 value1=(Type1)temp[0]; //For code clarity: temp[0] is not descriptive
Type2 value2=(Type2)temp[1];
The Pair example by David Hanak has no syntactic benefit, and is limited to two values.
return new Pair<Type1,Type2>(value1, value2);
And the caller looks like:
Pair<Type1, Type2> temp=myMethod(parameters);
Type1 value1=temp.a; //For code clarity: temp.a is not descriptive
Type2 value2=temp.b;
You may use any of following ways:
private static final int RETURN_COUNT = 2;
private static final int VALUE_A = 0;
private static final int VALUE_B = 1;
private static final String A = "a";
private static final String B = "b";
1) Using Array
private static String[] methodWithArrayResult() {
//...
return new String[]{"valueA", "valueB"};
}
private static void usingArrayResultTest() {
String[] result = methodWithArrayResult();
System.out.println();
System.out.println("A = " + result[VALUE_A]);
System.out.println("B = " + result[VALUE_B]);
}
2) Using ArrayList
private static List<String> methodWithListResult() {
//...
return Arrays.asList("valueA", "valueB");
}
private static void usingListResultTest() {
List<String> result = methodWithListResult();
System.out.println();
System.out.println("A = " + result.get(VALUE_A));
System.out.println("B = " + result.get(VALUE_B));
}
3) Using HashMap
private static Map<String, String> methodWithMapResult() {
Map<String, String> result = new HashMap<>(RETURN_COUNT);
result.put(A, "valueA");
result.put(B, "valueB");
//...
return result;
}
private static void usingMapResultTest() {
Map<String, String> result = methodWithMapResult();
System.out.println();
System.out.println("A = " + result.get(A));
System.out.println("B = " + result.get(B));
}
4) Using your custom container class
private static class MyContainer<M,N> {
private final M first;
private final N second;
public MyContainer(M first, N second) {
this.first = first;
this.second = second;
}
public M getFirst() {
return first;
}
public N getSecond() {
return second;
}
// + hashcode, equals, toString if need
}
private static MyContainer<String, String> methodWithContainerResult() {
//...
return new MyContainer("valueA", "valueB");
}
private static void usingContainerResultTest() {
MyContainer<String, String> result = methodWithContainerResult();
System.out.println();
System.out.println("A = " + result.getFirst());
System.out.println("B = " + result.getSecond());
}
5) Using AbstractMap.simpleEntry
private static AbstractMap.SimpleEntry<String, String> methodWithAbstractMapSimpleEntryResult() {
//...
return new AbstractMap.SimpleEntry<>("valueA", "valueB");
}
private static void usingAbstractMapSimpleResultTest() {
AbstractMap.SimpleEntry<String, String> result = methodWithAbstractMapSimpleEntryResult();
System.out.println();
System.out.println("A = " + result.getKey());
System.out.println("B = " + result.getValue());
}
6) Using Pair of Apache Commons
private static Pair<String, String> methodWithPairResult() {
//...
return new ImmutablePair<>("valueA", "valueB");
}
private static void usingPairResultTest() {
Pair<String, String> result = methodWithPairResult();
System.out.println();
System.out.println("A = " + result.getKey());
System.out.println("B = " + result.getValue());
}
I almost always end up defining n-Tuple classes when I code in Java. For instance:
public class Tuple2<T1,T2> {
private T1 f1;
private T2 f2;
public Tuple2(T1 f1, T2 f2) {
this.f1 = f1; this.f2 = f2;
}
public T1 getF1() {return f1;}
public T2 getF2() {return f2;}
}
I know it's a bit ugly, but it works, and you just have to define your tuple types once. Tuples are something Java really lacks.
EDIT: David Hanak's example is more elegant, as it avoids defining getters and still keeps the object immutable.
Before Java 5, I would kind of agree that the Map solution isn't ideal. It wouldn't give you compile time type checking so can cause issues at runtime. However, with Java 5, we have Generic Types.
So your method could look like this:
public Map<String, MyType> doStuff();
MyType of course being the type of object you are returning.
Basically I think that returning a Map is the right solution in this case because that's exactly what you want to return - a mapping of a string to an object.
Apache Commons has tuple and triple for this:
ImmutablePair<L,R> An immutable pair consisting of two Object
elements.
ImmutableTriple<L,M,R> An immutable triple consisting of
three Object elements.
MutablePair<L,R> A mutable pair consisting of
two Object elements.
MutableTriple<L,M,R> A mutable triple
consisting of three Object elements.
Pair<L,R> A pair consisting of
two elements.
Triple<L,M,R> A triple consisting of three elements.
Source: https://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/tuple/package-summary.html
Alternatively, in situations where I want to return a number of things from a method I will sometimes use a callback mechanism instead of a container. This works very well in situations where I cannot specify ahead of time just how many objects will be generated.
With your particular problem, it would look something like this:
public class ResultsConsumer implements ResultsGenerator.ResultsCallback
{
public void handleResult( String name, Object value )
{
...
}
}
public class ResultsGenerator
{
public interface ResultsCallback
{
void handleResult( String aName, Object aValue );
}
public void generateResults( ResultsGenerator.ResultsCallback aCallback )
{
Object value = null;
String name = null;
...
aCallback.handleResult( name, value );
}
}
While in your case, the comment may be a good way to go, in Android, you can use Pair . Simply
return new Pair<>(yourList, yourCommaSeparatedValues);
Use of following Entry object
Example :
public Entry<A,B> methodname(arg)
{
.......
return new AbstractMap.simpleEntry<A,B>(instanceOfA,instanceOfB);
}
Regarding the issue about multiple return values in general I usually use a small helper class that wraps a single return value and is passed as parameter to the method:
public class ReturnParameter<T> {
private T value;
public ReturnParameter() { this.value = null; }
public ReturnParameter(T initialValue) { this.value = initialValue; }
public void set(T value) { this.value = value; }
public T get() { return this.value; }
}
(for primitive datatypes I use minor variations to directly store the value)
A method that wants to return multiple values would then be declared as follows:
public void methodThatReturnsTwoValues(ReturnParameter<ClassA> nameForFirstValueToReturn, ReturnParameter<ClassB> nameForSecondValueToReturn) {
//...
nameForFirstValueToReturn.set("...");
nameForSecondValueToReturn.set("...");
//...
}
Maybe the major drawback is that the caller has to prepare the return objects in advance in case he wants to use them (and the method should check for null pointers)
ReturnParameter<ClassA> nameForFirstValue = new ReturnParameter<ClassA>();
ReturnParameter<ClassB> nameForSecondValue = new ReturnParameter<ClassB>();
methodThatReturnsTwoValues(nameForFirstValue, nameForSecondValue);
Advantages (in comparison to other solutions proposed):
You do not have to create a special class declaration for individual methods and its return types
The parameters get a name and therefore are easier to differentiate when looking at the method signature
Type safety for each parameter
All possible solutions will be a kludge (like container objects, your HashMap idea, “multiple return values” as realized via arrays). I recommend regenerating the comma-separated list from the returned List. The code will end up being a lot cleaner.
Keep it simple and create a class for multiple result situation. This example accepts an ArrayList and a message text from a databasehelper getInfo.
Where you call the routine that returns multiple values you code:
multResult res = mydb.getInfo();
In the routine getInfo you code:
ArrayList<String> list= new ArrayList<String>();
add values to the list...
return new multResult("the message", list);
and define a class multResult with:
public class multResult {
public String message; // or create a getter if you don't like public
public ArrayList<String> list;
multResult(String m, ArrayList<String> l){
message = m;
list= l;
}
}
As I see it there are really three choices here and the solution depends on the context. You can choose to implement the construction of the name in the method that produces the list. This is the choice you've chosen, but I don't think it is the best one. You are creating a coupling in the producer method to the consuming method that doesn't need to exist. Other callers may not need the extra information and you would be calculating extra information for these callers.
Alternatively, you could have the calling method calculate the name. If there is only one caller that needs this information, you can stop there. You have no extra dependencies and while there is a little extra calculation involved, you've avoided making your construction method too specific. This is a good trade-off.
Lastly, you could have the list itself be responsible for creating the name. This is the route I would go if the calculation needs to be done by more than one caller. I think this puts the responsibility for the creation of the names with the class that is most closely related to the objects themselves.
In the latter case, my solution would be to create a specialized List class that returns a comma-separated string of the names of objects that it contains. Make the class smart enough that it constructs the name string on the fly as objects are added and removed from it. Then return an instance of this list and call the name generation method as needed. Although it may be almost as efficient (and simpler) to simply delay calculation of the names until the first time the method is called and store it then (lazy loading). If you add/remove an object, you need only remove the calculated value and have it get recalculated on the next call.
Can do some thing like a tuple in dynamic language (Python)
public class Tuple {
private Object[] multiReturns;
private Tuple(Object... multiReturns) {
this.multiReturns = multiReturns;
}
public static Tuple _t(Object... multiReturns){
return new Tuple(multiReturns);
}
public <T> T at(int index, Class<T> someClass) {
return someClass.cast(multiReturns[index]);
}
}
and use like this
public Tuple returnMultiValues(){
return Tuple._t(new ArrayList(),new HashMap())
}
Tuple t = returnMultiValues();
ArrayList list = t.at(0,ArrayList.class);
I followed a similar approach than the described in the other answers with a few tweaks based on the requirement I had, basically I created the following classes(Just in case, everything is Java):
public class Pair<L, R> {
final L left;
final R right;
public Pair(L left, R right) {
this.left = left;
this.right = right;
}
public <T> T get(Class<T> param) {
return (T) (param == this.left.getClass() ? this.left : this.right);
}
public static <L, R> Pair<L, R> of(L left, R right) {
return new Pair<L, R>(left, right);
}
}
Then, my requirement was simple, in the repository Class that reaches the DB, for the Get Methods than retrieve data from the DB, I need to check if it failed or succeed, then, if succeed, I needed to play with the returning list, if failed, stop the execution and notify the error.
So, for example, my methods are like this:
public Pair<ResultMessage, List<Customer>> getCustomers() {
List<Customer> list = new ArrayList<Customer>();
try {
/*
* Do some work to get the list of Customers from the DB
* */
} catch (SQLException e) {
return Pair.of(
new ResultMessage(e.getErrorCode(), e.getMessage()), // Left
null); // Right
}
return Pair.of(
new ResultMessage(0, "SUCCESS"), // Left
list); // Right
}
Where ResultMessage is just a class with two fields (code/message) and Customer is any class with a bunch of fields that comes from the DB.
Then, to check the result I just do this:
void doSomething(){
Pair<ResultMessage, List<Customer>> customerResult = _repository.getCustomers();
if (customerResult.get(ResultMessage.class).getCode() == 0) {
List<Customer> listOfCustomers = customerResult.get(List.class);
System.out.println("do SOMETHING with the list ;) ");
}else {
System.out.println("Raised Error... do nothing!");
}
}
In C++ (STL) there is a pair class for bundling two objects. In Java Generics a pair class isn't available, although there is some demand for it. You could easily implement it yourself though.
I agree however with some other answers that if you need to return two or more objects from a method, it would be better to encapsulate them in a class.
Why not create a WhateverFunctionResult object that contains your results, and the logic required to parse these results, iterate over then etc. It seems to me that either:
These results objects are intimately tied together/related and belong together, or:
they are unrelated, in which case your function isn't well defined in terms of what it's trying to do (i.e. doing two different things)
I see this sort of issue crop up again and again. Don't be afraid to create your own container/result classes that contain the data and the associated functionality to handle this. If you simply pass the stuff around in a HashMap or similar, then your clients have to pull this map apart and grok the contents each time they want to use the results.
public class MultipleReturnValues {
public MultipleReturnValues() {
}
public static void functionWithSeveralReturnValues(final String[] returnValues) {
returnValues[0] = "return value 1";
returnValues[1] = "return value 2";
}
public static void main(String[] args) {
String[] returnValues = new String[2];
functionWithSeveralReturnValues(returnValues);
System.out.println("returnValues[0] = " + returnValues[0]);
System.out.println("returnValues[1] = " + returnValues[1]);
}
}
This is not exactly answering the question, but since every of the solution given here has some drawbacks, I suggest to try to refactor your code a little bit so you need to return only one value.
Case one.
You need something inside as well as outside of your method. Why not calculate it outside and pass it to the method?
Instead of:
[thingA, thingB] = createThings(...); // just a conceptual syntax of method returning two values, not valid in Java
Try:
thingA = createThingA(...);
thingB = createThingB(thingA, ...);
This should cover most of your needs, since in most situations one value is created before the other and you can split creating them in two methods. The drawback is that method createThingsB has an extra parameter comparing to createThings, and possibly you are passing exactly the same list of parameters twice to different methods.
Case two.
Most obvious solution ever and a simplified version of case one. It's not always possible, but maybe both of the values can be created independently of each other?
Instead of:
[thingA, thingB] = createThings(...); // see above
Try:
thingA = createThingA(...);
thingB = createThingB(...);
To make it more useful, these two methods can share some common logic:
public ThingA createThingA(...) {
doCommonThings(); // common logic
// create thing A
}
public ThingB createThingB(...) {
doCommonThings(); // common logic
// create thing B
}
Pass a list to your method and populate it, then return the String with the names, like this:
public String buildList(List<?> list) {
list.add(1);
list.add(2);
list.add(3);
return "something,something,something,dark side";
}
Then call it like this:
List<?> values = new ArrayList<?>();
String names = buildList(values);
You can utilize a HashMap<String, Object> as follows
public HashMap<String, Object> yourMethod()
{
.... different logic here
HashMap<String, Object> returnHashMap = new HashMap<String, Object>();
returnHashMap.put("objectA", objectAValue);
returnHashMap.put("myString", myStringValue);
returnHashMap.put("myBoolean", myBooleanValue);
return returnHashMap;
}
Then when calling the method in a different scope, you can cast each object back to its initial type:
// call the method
HashMap<String, Object> resultMap = yourMethod();
// fetch the results and cast them
ObjectA objectA = (ObjectA) resultMap.get("objectA");
String myString = (String) resultMap.get("myString");
Boolean myBoolean = (Boolean) resultMap.get("myBoolean");
I noticed there is no no-custom class, n-length, no-cast, type-safe answers yet to returning multiple values.
Here is my go:
import java.util.Objects;
public final class NTuple<V, T extends NTuple<?, ?>> {
private final V value;
private final T next;
private NTuple(V value, T next) {
this.value = value;
this.next = next;
}
public static <V> NTuple<V, ?> of(V value) {
return new NTuple<>(value, null);
}
public static <V, T extends NTuple<?, ?>> NTuple<V, T> of(V value, T next) {
return new NTuple<>(value, next);
}
public V value() {
return value;
}
public T next() {
return next;
}
public static <V> V unpack0(NTuple<V, ?> tuple) {
return Objects.requireNonNull(tuple, "0").value();
}
public static <V, T extends NTuple<V, ?>> V unpack1(NTuple<?, T> tuple) {
NTuple<?, T> tuple0 = Objects.requireNonNull(tuple, "0");
NTuple<V, ?> tuple1 = Objects.requireNonNull(tuple0.next(), "1");
return tuple1.value();
}
public static <V, T extends NTuple<?, NTuple<V, ?>>> V unpack2(NTuple<?, T> tuple) {
NTuple<?, T> tuple0 = Objects.requireNonNull(tuple, "0");
NTuple<?, NTuple<V, ?>> tuple1 = Objects.requireNonNull(tuple0.next(), "1");
NTuple<V, ?> tuple2 = Objects.requireNonNull(tuple1.next(), "2");
return tuple2.value();
}
}
Sample use:
public static void main(String[] args) {
// pre-java 10 without lombok - use lombok's var or java 10's var if you can
NTuple<String, NTuple<Integer, NTuple<Integer, ?>>> multiple = wordCount("hello world");
String original = NTuple.unpack0(multiple);
Integer wordCount = NTuple.unpack1(multiple);
Integer characterCount = NTuple.unpack2(multiple);
System.out.println(original + ": " + wordCount + " words " + characterCount + " chars");
}
private static NTuple<String, NTuple<Integer, NTuple<Integer, ?>>> wordCount(String s) {
int nWords = s.split(" ").length;
int nChars = s.length();
return NTuple.of(s, NTuple.of(nWords, NTuple.of(nChars)));
}
Pros:
no-custom container class - no need to write a class just for a return type
n-length - can handle any number of return values
no-cast - no need to cast from Object
type-safe - the types are checked via Java's generics
Cons:
inefficient for large numbers of return values
according to my experience with python's multiple return values, this should not happen in practice
heavy type declarations
can be alleviated by lombok/Java 10 var
In C, you would do it by passing pointers to placeholders for the results as arguments:
void getShoeAndWaistSizes(int *shoeSize, int *waistSize) {
*shoeSize = 36;
*waistSize = 45;
}
...
int shoeSize, waistSize;
getShoeAndWaistSize(&shoeSize, &waistSize);
int i = shoeSize + waistSize;
Let's try something similar, in Java.
void getShoeAndWaistSizes(List<Integer> shoeSize, List<Integer> waistSize) {
shoeSize.add(36);
waistSize.add(45);
}
...
List<Integer> shoeSize = new List<>();
List<Integer> waistSize = new List<>();
getShoeAndWaistSizes(shoeSize, waistSize);
int i = shoeSize.get(0) + waistSize.get(0);
PASS A HASH INTO THE METHOD AND POPULATE IT......
public void buildResponse(String data, Map response);
UPDATE: The answer is actually in the documentation:
Note: Great care must be exercised if mutable objects are used as set
elements. The behavior of a set is not specified if the value of an
object is changed in a manner that affects equals comparisons while
the object is an element in the set.
Case closed, thanks everyone!
Edit: The referenced topic about duplicates in a hash sets does have the same point, however it does not answer my question: Why is the documentation not saying anything about that a set is only guaraneteed to work with immutable objects?
edit2: I do understand what happens. The set of course cannot know when the hashcode of the entities change after they have been added. But the point is that imo the documentation should clearly state that sets only work properly with immutable objects.
I've been working with Java for more than 5 years now, and don't laugh, but only now I realized something about the Sets. I thought I understood what a set is, namely what the doc says:
A collection that contains no duplicate elements. More formally, sets
* contain no pair of elements e1 and e2 such that * e1.equals(e2), and at most one null element.
But, this is totally not true?! See here:
public static void main(String[] args) {
Set<Entity> entitySet = new HashSet<>();
Entity e1 = new Entity("One");
Entity e2 = new Entity("Two");
entitySet.add(e1);
entitySet.add(e2);
e2.name = "One"; // !
System.out.println("Objects equal:" + e1.equals(e2));
Iterator<Entity> iterator = entitySet.iterator();
while (iterator.hasNext()) {
System.out.println(iterator.next());
}
}
static class Entity {
String name;
Entity(String name) {
this.name = name;
}
#Override
public boolean equals(Object obj) {
if (!(obj instanceof Entity)) {
return false;
}
return name.equals(((Entity) obj).name);
}
#Override
public int hashCode() {
return name.hashCode();
}
#Override
public String toString() {
return "Entity[name=" + name + "]";
}
Output:
Objects equal:true
Entity[name=One]
Entity[name=One]
So, I guess the thing about sets not containing duplicates is only true when we deal with immutable entries? But why is the doc not saying anything about it? I was never really aware of this. The problem with this of course is that the entites could contain any number of further fields that are not part of the equality definition; and they might be different in those fields. I'm thinking about something like this:
public static void main(String[] args) {
Set<Entity> entitySet = new HashSet<>();
Entity e1 = new Entity("Public", true);
Entity e2 = new Entity("Secret", false);
entitySet.add(e1);
entitySet.add(e2);
e2.name = "Public";
Iterator<Entity> iterator = entitySet.iterator();
// print only public entity (e1)
while (iterator.hasNext()) {
Entity e = iterator.next();
if (e.equals(e1)) {
System.out.println(e);
}
}
}
static class Entity {
String name;
boolean mayBeDisplayedToUser;
Entity(String name, boolean mayBeDisplayedToUser) {
this.name = name;
this.mayBeDisplayedToUser = mayBeDisplayedToUser;
}
#Override
public boolean equals(Object obj) {
if (!(obj instanceof Entity)) {
return false;
}
return name.equals(((Entity) obj).name);
}
#Override
public int hashCode() {
return name.hashCode();
}
#Override
public String toString() {
return "Entity[name=" + name + ", may be displayed:" + mayBeDisplayedToUser + "]";
}
}
Output:
Entity[name=Public, may be displayed:false] Entity[name=Public, may be
displayed:true]
So.. I'm quite puzzled right now. Am I the only one that was not aware of this?
You are adding the items to the HashSet while they are unique and then mutating the items after the fact. The containing HashSet has no idea that you broke the set contract by changing obj.name.
I have created a Vector object to store data in Table object as Vector<Table>. Vector<Table> contains components as below.
[Vector<Record> records, String tableName, String keyColumnName, int recordCount, int columnCount]
I need to sort tableName in above Vector to my own order and return Vector<Table> with sorted tableNames for other processes.
I have wrote method as below.
private Vector<Table> orderTables(Vector<Table> loadTables) {
List<String> tableNames = new ArrayList<String>();
for (Table table : loadTables) {
String tblName = table.getTableName();
tableNames.add(tblName);
}
Collections.sort(tableNames, new MyComparable());
return null;
}
But I have no idea about how to write Comparator to this. My own sort order is stored in .properties file. I can read it and get value. But I have no idea about how to compare it.
How could I do it?
Before clarification
You need to write a Comparator for Table objects that delegates to the tableName's comparator:
new Comparator<Table>() {
#Override public int compare(Table one, Table two) {
return one.getTableName().compareTo(two.getTableName());
}
}
Note that this will consider Tables that have the same name to be equal. This can mess things up if you put these tables in a HashMap or HashSet. To avoid this, you can detect this case and return one.hashCode() - two.hashCode() if the table names are the same.
Guava's ComparisonChain is a convenient way to write such multi-stage comparisons:
new Comparator<Table>() {
#Override public int compare(Table one, Table two) {
return ComparisonChain.start()
.compare(one.getTableName(), two.getTableName())
.compare(one.hashCode(), two.hashCode())
.result();
}
}
After clarification
Okay, the question is to impose a predefined sorting order rather than sorting the Tables by name. In that case, you need to make a Comparator that is aware of the ordering defined in the .properties file.
One way to achieve this is to initialize a mapping of table names to sorting order indices, and refer that mapping during the comparison. Given the property value:
SORT_ORDER = SALES,SALE_PRODUCTS,EXPENSES,EXPENSES_ITEMS
The mapping should look like:
{
SALES: 0,
SALE_PRODUCTS: 1,
EXPENSES: 2,
EXPENSES_ITEMS: 3
}
Here's what the comparator would look like:
private static class PredefinedOrderComparator implements Comparator<Table> {
public PredefinedOrderComparator() {
// Initialize orderIndex here
}
private final Map<String, Integer> orderIndex;
#Override public int compare(Table one, Table two) {
return orderIndex.get(one.getTableName()) - orderIndex.get(two.getTableName());
}
}
To populate orderIndex from the property value, you need to:
Get the comma-separated list using getProperty() as you mentioned
Split that value on comma (I recommend using Guava's Splitter, but String.split or others will work too)
Initialize a new HashMap<String, Integer> and an int index = 0
Iterate through the split tokens, map the current token to index and increment index
Note the implicit assumption that none of the table names have a comma in it.
public class MyComparable implements Comparator<Table>{
#Override
public int compare(Table table1, Table table2) {
return (table1.getTableName().compareTo(table2.getTableName());
}
}
make sure that you have overridden the hashcode and equals in Table class to achieve this.
I wrote you a very simple example on how to work with a Comparator. If you create a class called Main, copy paste below contents in it, compile and run it, you can see what's going on.
A comparator just needs to implement an interface. For this it needs to implement one method (public int compare(T arg0, T arg1). There you specify how a collection will get sorted; in this case according to the alfabet.
I hope this helps you.
import java.util.*;
public class Main {
public static void main(String[] args) {
System.out.println("Start\n");
List<Item> items = new ArrayList<Item>();
for(String s : new String[]{"mzeaez", "xcxv", "hjkhk", "azasq", "iopiop"}) {
items.add(createItem(s));
}
System.out.println("Items before sort:");
System.out.println(Item.toString(items));
Collections.sort(items, new ItemComparator());
System.out.println("Items after sort:");
System.out.println(Item.toString(items));
System.out.println("End");
}
private static Item createItem(String s) {
Item item = new Item();
item.setS(s);
return item;
}
}
class Item {
private String s;
public String getS() {
return s;
}
public void setS(String s) {
this.s = s;
}
#Override
public String toString() {
return "Item: " + s;
}
public static String toString(Collection<Item> items) {
String s = "";
for(Item item : items) {
s += item + "\n";
}
return s;
}
}
class ItemComparator implements Comparator<Item> {
#Override
public int compare(Item item1, Item item2) {
return item1.getS().compareTo(item2.getS());
}
}