Is there a class that represents the concatenation of a collection with another collection? This class should be a Collection in itself, and should delegate all methods to the underlying (inner) collections - no extra memory should be allocated, nor any of the original collections modified.
Example usage:
Collection<String> foo = ...
Collection<String> bar = ...
// this should be O(1) memory and time
Collection<String> combined = concat(foo, bar);
if (combined.contains("Zee"))
...
for (String str : combined)
System.out.println(str);
As always for any collections stuff, look at google-collections. If you have Sets, specifically (not just a general collection), you want:
Set<String> combined = Sets.union(foo, bar);
which creates an unmodifiable view of the two sets. That is, changes in foo or bar will be reflected in combined (but combined.add() etc is not supported).
For the more generic case, you have Iterables.concat() but that merely lets you iterate over the joined item, the Iterable interface obviously doesn't include contains so you're a little hosed there.
The other collections utilities classes in google-collections (com.google.common.collect.Lists and com.google.common.collect.Collections2) don't contain any concatenation methods. Don't see why they couldn't, but at the moment they don't.
Your question is very vague. Especially "with another item another collection" is quite unclear.
You can at least add the contents of another Collection to the current Collection using Collection#addAll(). Here Collection can be anything of its subinterfaces/implementations, e.g. List or Set.
Example:
List<String> foos = Arrays.asList("foo1", "foo2", "foo3");
List<String> bars = Arrays.asList("bar1", "bar2", "bar3");
foos.addAll(bars); // Now foos contains everything.
Edit: Or do you actually want to create a new Collection based on an existing Collection and then add a new item to it? In this case just construct a new Collection with the existing Collection as constructor argument. E.g.:
List<String> foos = Arrays.asList("foo1", "foo2", "foo3");
List<String> bars = new ArrayList<String>(foos);
bars.add("bar"); // Now bars contains everything.
There is not, but writing it yourself should be straight forward
package ch.akuhn.util;
import java.util.Iterator;
import java.util.NoSuchElementException;
public class Concat {
public static <T> Iterable<T> all(final Iterable<T>... iterables) {
return new Iterable<T>() {
#Override
public Iterator<T> iterator() {
return new Iterator<T>() {
Iterator<Iterable<T>> more = Arrays.asList(iterables).iterator();
Iterator<T> current = more.hasNext() ? more.next().iterator() : null;
#Override
public boolean hasNext() {
if (current == null) return false;
if (current.hasNext()) return true;
current = more.hasNext() ? more.next().iterator() : null;
return this.hasNext();
}
#Override
public T next() {
if (!hasNext()) throw new NoSuchElementException();
return current.next();
}
#Override
public void remove() {
throw new UnsupportedOperationException();
}
};
}
};
}
}
And then
for (Object each: Concat.all(collection,whatever,etcetera,...)) {
// ...
}
Just wrote this code here, compile at your own risk!
PS, if you gonna write unit tests for this class, send 'em to me.
I think what you're asking for is a Java construct that allows you to put collections together without modifying the original collections. In other words, you have collections A and B, both of size N and M respectively. After the concat call, you still have collections A and B and their sizes are still N and M, however you have collection C as well which points to A and B, making its size N+M.
The answer is no, Java doesn't have anything out of the box that does this... However you could write a quick wrapper that wraps a series of collections and add those collections to it. (All it would do is maintain references to each of the collections) and you could expose get/insert methods as needed.
Apache Commons Collections also has a more general CompositeCollection class which can be used as an interface to an arbitrary number of Collections.
I'm not sure what your asking. My interpretation of your question is that your looking for the add method on the Collection. I don't think that's what you're asking though.
Try InterleavingEnumeration or apache's commons collections' ListUtils (ListUtils.union())
Related
I have a class (A.java) that contains two private fields of type ArrayList and HashMap.
I also have another class (B.java) that should have access to their data. I could make two getters, but I don't want to return my collections as is. Class B.java should only have access to data, not to add(), isEmpty(), containsKey() etc.
Can I return my collections in such way, so I could somehow use it with foreach in class B somehow but without giving the possibility to modify them?
Don't return a collection, return a Stream. That way it is easy for the user to know that they are getting a stream of objects, not a collection. And it's easy to change the implementation of the collection without changing the way it's used. It's trivial for the user to filter, map, reduce collect etc.
So:
class A {
private List<C> cs = new ArrayList<>();
public Stream<C> getCs() {
return cs.stream();
}
}
class B {
public void processCs(A a) {
a.getCs().filter(C::hasFooness).forEach(...);
}
}
Create a getter method that returns a "Collections.unmodifiableList()" like this:
List<String> strings = new ArrayList<String>();
List<String> unmodifiable = Collections.unmodifiableList(strings);
unmodifiable.add("New string"); // will fail at runtime
strings.add("Aha!"); // will succeed
System.out.println(unmodifiable);
Example code:
modifyMyList(myList);
public void modifyMyList(List someList){
someList.add(someObject);
}
or:
List myList = modifyMyList(myList);
public List modifyMyList(List someList){
someList.add(someObject)
return someList;
}
There is also a 3rd option I believe: You can create a new List in modifyMyList method and return this new List...
( 3rd option is here, I was too lazy but someone already added it in the answers: )
List myList = modifyMyList(myList);
public List modifyMyList(List someList){
List returnList = new ArrayList();
returnList.addAll(someList);
returnList.add(someObject);
return Collections.unmodifiableList(returnList);
}
Is there any reason why I should choose one over another? What should be considered in such case?
I have a (self imposed) rule which is "Never mutate a method parameter in a public method". So, in a private method, it's ok to mutate a parameter (I even try to avoid this case too). But when calling a public method, the parameters should never be mutated and should be considered immutable.
I think that mutating method arguments is a bit hacky and can lead to bugs that are harder to see.
I have been known to make exceptions to this rule but I need a really good reason.
Actually there is no functional difference.
You'll come to know the difference when you want the returned list
List someNewList = someInstnace.modifyMyList(list);
The second is probably confusing as it implies a new value is being created and returned - and it isn't.
An exception would be if the method was part of a 'fluent' API, where the method was an instance method and was modifying its instance, and then returning the instance to allow method chaining: the Java StringBuilder class is an example of this.
In general, however, I wouldn't use either.
I'd go for your third option: I write a method that creates and returns a new list with the appropriate change. This is a bit artificial in the case of your example, as the example is really just reproducing List.add(), but...
/** Creates a copy of the list, with val appended. */
public static <T> List<T> modifyMyList(List<T> list, T val) {
List<T> xs = new ArrayList<T>(list);
xs.add(val);
return xs;
}
Aside: I wouldn't, as suggested by Saket return an immutable list. His argument for immutability and parallelism is valid. But most of the time Java programmers expect to be able to modify a collection, except in special circumstances. By making you method return an immutable collection, you limit it's reusability to such circumstances. (The caller can always make the list immutable if they want to: they know the returned value is a copy and won't be touched by anything else.) Put another way: Java is not Clojure. Also, if parallelism is a concern, look at Java 8 and streams (the new kind - not I/O streams).
Here's a different example:
/** Returns a copy of a list sans-nulls. */
public static <T> List<T> compact(Iterable<T> it) {
List<T> xs = new ArrayList<T>();
for(T x : it)
if(x!=null) xs.add(x);
return xs;
}
Note that I've genercized the method and made it more widely applicable to taking an Iterable instead of a list. In real code, I'd have two overloaded versions, one taking an Iterable and one an Iterator. (The first would be implemented by calling the second, with the iterable's iterator.) Also, I've made it static as there was no reason for your method to be an instance method (it does not depend on state from the instance).
Sometimes, though, if I'm writing library code, and if it is not clear whether a mutating or non-mutating implementation is more generally useful, I create both. Here's a fuller example:
/** Returns a copy of the elements from an Iterable, as a List, sans-nulls. */
public static <T> List<T> compact(Iterable<T> it) {
return compact(it.iterator());
}
public static <T> List<T> compact(Iterator<T> iter) {
List<T> xs = new ArrayList<T>();
while(iter.hasNext()) {
T x = iter.next();
if(x!=null) xs.add(x);
}
return xs;
}
/** In-place, mutating version of compact(). */
public static <T> void compactIn(Iterable<T> it) {
// Note: for a 'fluent' version of this API, have this return 'it'.
compactIn(it.iterator());
}
public static <T> void compactIn(Iterator<T> iter) {
while(iter.hasNext()) {
T x = iter.next();
if(x==null) iter.remove();
}
}
If this was in a real API I'd check the arguments for null and throw IllegalArgumentException. (NOT NullPointerException - though it is often used for this purpose. NullPointerException happens for other reasons as well, e.g. buggy code. IllegalArgumentException is better for invalid parameters.)
(There'd also be more Javadoc than actual code too!)
The first and second solution are very similar, The advantage of the second is to permit chaining. The question of "is it a good practise" is subjected to debate as we can see here:
Method Chaining in Java
So the real question is between the first solution with mutable list and the third with a unmutable list, and again, there is not a unique response, it is the same debate between returning String, which are immutable and using Stringbuffer, which are mutable but permits better performance.
If you need reliablility of your API , and if you don't have performance issues use immutable (the third solution). Use it if your lists are always small.
If you need only performance use a mutable list (the first solution)
I will recommend creating a new list in the method and returning an immutable list. That way your code will work even when you are passed in an Immutable list. It is generally a good practice to create immutable objects as we generally move towards functional programming and try to scale across multiple processor architectures.
List myList = modifyMyList(myList);
public List modifyMyList(List someList){
List returnList = new ArrayList();
returnList.addAll(someList);
returnList.add(someObject);
return Collections.unmodifiableList(returnList);
}
As I said in my other answer, I don't think you should mutate the list parameter. But there are times where you also don't want to take a copy of the original list and mutate the copy.
The original list might be large so the copy is expensive
You want the copy to be kept up-to-date with any updates to the original list.
In these scenarios, you could create a MergedList which is a view over two (or perhaps more) lists
import java.util.*;
public class MergedList<T> extends AbstractList<T> {
private final List<T> list1;
private final List<T> list2;
public MergedList(List<T> list1, List<T> list2) {
this.list1 = list1;
this.list2 = list2;
}
#Override
public Iterator<T> iterator() {
return new Iterator<T>() {
Iterator<T> it1 = list1.iterator();
Iterator<T> it2 = list1.iterator();
#Override
public boolean hasNext() {
return it1.hasNext() || it2.hasNext();
}
#Override
public T next() {
return it1.hasNext() ? it1.next() : it2.next();
}
};
}
#Override
public T get(int index) {
int size1 = list1.size();
return index < size1 ? list1.get(index) : list2.get(index - size1);
}
#Override
public int size() {
return list1.size() + list2.size();
}
}
The you could do
public List<String> modifyMyList(List<String> someList){
return new MergedList(someList, List.of("foo", "bar", "baz"));
}
Both ways will work because in this case java works with the reference of the List but i prefer the secound way because this solution works for pass by value too, not only for pass by reference.
Functionally both are same.
However when you expose your method as an API, second method may give an impression that it returns a new modified list other than the original passed list.
While the first method would make it clear (of-course based on method naming convention) that it will modify the original list (Same object).
Also, the second method returns a list, so ideally the caller should check for a null return value even if the passed list is non null (The method can potentially return a null instead of modified list).
Considering this I generally prefer to use method one over second.
I have a list of MyObjects which I need to divide into three groups:
Known good (keep)
Known bad (reject)
Unrecognized (raise alert)
MyObject contains various properties which must be examined to determine which of the 3 groups to put the object in.
My initial implementation (Java) just takes a List in its constructor and does the triage there. Pseudocode:
class MyObjectFilterer {
public MyObjectFilterer(List<MyObject> list) {
// triage items here
}
public List<MyObject> getGood() {
// return sub-list of good items
}
public List<MyObject> getBad() {
// return sub-list of bad items
}
public List<MyObject> getUnrecognized() {
// return sub-list of unrecognized items
}
}
Any issues with this implementation? Is there a better OO choice?
I would probably prefer a static factory method to do the filtering, that then calls a private constructor that takes the three filtered lists, following the good code practice of never doing any serious work in a constructor. Other than that, this looks fine.
There may be multiple approachs. If the problem is generic / repetitive enough, you could define an interface with a method to classify the objects.
interface Selector {
public boolean isGood(MyObject myObject);
public boolean isBad(MyObject myObject);
public boolean isUnknown(MyObject myObject);
}
That way you could change the logic implementation easily.
An other idea would be using the Chain of responsibility.
Your MyObjectFilterer contains a reference to three Objects GoodFilterer, BadFilterer and UnrecognizedFilterer. Each of them contains the following methods: addMethod(MyObject object), getObjects() and addFilter(). Of course they have to implement an interface Filterer.
With the addFilter method you can build the chain. so that the GoodFilterer contains a reference to the BadFilterer and this one contains a reference to the UnrecognizedFilterer
Now you go through your list of MyObjects and call the add method on the GoodFilterer (first one in this chain). Inside the add method you decide if this is good, than you keep it and finish the work, if not pass it on to the BadFilterer.
You keep your three methods for getting the good/bad and unrecognized, but you will pass this to the getObjects() method of the corresponding Filterer
The Benefit is that the logic if this is a good/bad or Unrecognized one is now seperated.
The Downside you would need 3 new classes and 1 Interface.
But like i said, this is just an other idea what you could do.
You should simplify as it's possible. Just make static method in MyObjectFilter with following signature:
public static List filterMyObjects(List data, Group group).
Group is enumeration with three values and it can be used as attribute of MyObject class
I might try something like:
enum MyObjectStatus {
GOOD, BAD, UNRECOGNIZED;
}
class MyObjectFilterer {
private MyObjectStatus getStatus(MyObject obj) {
// classify logic here, returns appropriate enum value
}
// ListMultimap return type below is from Google Guava
public ListMultimap<MyObjectStatus, MyObject> classify(List<MyObject> objects) {
ListMultimap<MyObjectStatus, MyObject> map = ArrayListMultimap.create();
for(MyObject obj: objects) {
map.put(getStatus(obj), obj);
}
}
}
Call classify() to get a Multimap, and extract each category as needed with something like:
List<MyObject> good = map.get(GOOD);
List<MyObject> bad = map.get(BAD);
List<MyObject> unknown = map.get(UNRECOGNIZED);
A nice thing about this solution is you don't have to create/publish accessor methods for each category (unless you want to), and if new categories are created, you also don't add new accessors -- just the new enum and the additional classifier logic.
How do I write a static method in Java that will take a List, perform an action on each element, and return the result (without affecting the original of course)?
For example, if I want to add 2 to each element what goes in the ... here? The concrete return type must be the same, e.g. if my List is a LinkedList with values 1,2,3 I should get back a LinkedList with values 3,4,5. Similarly for ArrayList, Vector, Stack etc, which are all Lists.
I can see how to do this using multiple if (lst instanceof LinkedList) ... etc... any better way?
import java.util.List;
public class ListAdd {
static List<Integer> add2 (List<Integer> lst) {
...
return result;
}
}
There are already many answers, but I'd like to show you a different way to think of this problem.
The operation you want to perform is known as map in the world of functional programming. It is something we do really all the time in functional languages.
Let M<A> be some kind of container (in your case, M would be List, and A would be Integer; however, the container can be lots of other things). Suppose you have a function that transforms As into Bs, that is, f: A -> B. Let's write this function as of type F<A, B>, to use a notation closer to Java. Note that you can have A = B, as in the example you give (in which A = B = Integer).
Then, the operation map is defined as follows:
M<B> map(M<A>, F<A, B>)
That is, the operation will return a M<B>, presumably by applying F<A, B> to each A in M<A>.
In practice...
There's a brilliant library developed by Google, called Guava, which brings lot's of functional idioms to Java.
In Guava, the map operation is called transform, and it can operate on any Iterable. It has also more specific implementations that work directly on lists, sets, etc.
Using Guava, the code you want to write would look like this:
static List<Integer> add2(List<Integer> ns) {
return Lists.transform(ns, new Function<Integer, Integer>() {
#Override Integer apply(Integer n) { return n + 2; }
}
}
Simple as that.
This code won't touch the original list, it will simply provide a new list that calculates its values as needed (that is, the values of the newly created list won't be calculated unless needed -- it's called a lazy operation).
As a final consideration, it is not possible for you to be absolutely sure that you will be able to return exactly the same implementation of List. And as many others pointed out, unless there's a very specific reason for this, you shouldn't really care. That's why List is an interface, you don't care about the implementation.
Fundamentally, the List interface doesn't make any guarantees that you'll have a way to duplicate it.
You may have some luck with various techniques:
Using clone() on the passed in List, although it may throw, or (since it is protected in Object) simply not be accessible
Use reflection to look for a public no-argument constructor on the passed-in List
Try to serialize and deserialize it in order to perform a "deep clone"
Create some sort of factory and build in knowledge of how to duplicate each different kind of List your code may encounter (What if it's a wrapper created by unmodifiableList(), or some oddball custom implementation backed by a RandomAccessFile?)
If all else fails, either throw, or return an ArrayList or a Vector for lack of better options
You could use reflection to look for a public zero-arg constructor on the result of lst.getClass() and then invoke() it to obtain the List into which you'll place your results. The Java Collections Framework recommends that any derivative of Collection offer a zero-arg constructor. That way, your results we be of the same runtime class as the argument.
Here is a variant which does neither copies nor modifies the original list. Instead, it wraps the original list by another object.
public List<Integer> add2(final List<Integer> lst) {
return new AbstractList<Integer>() {
public int size() {
return lst.size();
}
public Integer get(int index) {
return 2 + lst.get(index);
}
};
}
The returned list is not modifiable, but will change whenever the original list changes.
(This implements the iterator based on index access, thus it will be slow for a linked list. Then better implement it based on AbstractSequentialList.)
Of course, the resulting list will obviously not be of the same class as the original list.
Use this solution only if you really only need a read-only two added view of your original list, not if you want a modified copy with similar properties.
The whole point of using an interface, in this case List, is to abstract the fact that the implementation is hidden behind the interface.
Your intention is clear to me, however: the Clonable interface supports creating a new instance with the same state. This interface might not be defined on your List.
Often it's a good idea to rethink this situation: why do you need to clone the List in this place, this class? Shouldn't your list-creator be responsible for cloning the list? Or shouldn't the caller, who knows the type, make sure he passes in a clone of his list?
Probably, if you look for the semantics as you defined it, you can implement all your supported Lists:
static Vector<Integer> addTwo(Vector<Integer> vector) {
Vector<Integer> copy = null; // TODO: copy the vector
return addTwo_mutable(copy);
}
static ArrayList<Integer> addTwo(ArrayList<Integer> aList) {
ArrayList<Integer> copy = null; // TODO: copy the array list
return addTwo_mutable(copy);
}
static LinkedList<Integer> addTwo(LinkedList<Integer> lList) {
LinkedList<Integer> copy = null; // TODO: copy the linked list
return addTwo_mutable(copy);
}
private <T extends List<Integer>> static T addTwo_mutable(T list) {
return list; // TODO: implement
}
Even, when you don't support a data-type, you'll get a nice compiler error that the specified method does not exists.
(code not tested)
Just to show you that what you want to do is not possible in the general case, consider the following class:
final class MyList extends ArrayList<Integer> {
private MyList() {
super.add(1);
super.add(2);
super.add(3);
}
private static class SingletonHolder {
private static final MyList instance = new MyList();
}
public static MyList getInstance() {
return SingletonHolder.instance;
}
}
It is a singleton (also, a lazy, thread-safe singleton by the way), it's only instance can be obtained from MyList.getInstance(). You cannot use reflection reliably (because the constructor is private; for you to use reflection, you'd have to rely on proprietary, non-standard, non-portable APIs, or on code that could break due to a SecurityManager). So, there's no way for you to return a new instance of this list, with different values.
It's final as well, so that you cannot return a child of it.
Also, it would be possible to override every method of ArrayList that would modify the list, so that it would be really an immutable singleton.
Now, why would you want to return the exact same implementation of List?
OK well someone mentioned reflection. It seems to be an elegant solution:
import java.util.*;
public class ListAdd {
static List<Integer> add2 (List<Integer> lst) throws Exception {
List<Integer> result = lst.getClass().newInstance();
for (Integer i : lst) result.add(i + 2);
return result;
}
}
Concise, but it thows an checked exception, which is not nice.
Also, wouldn't it be nicer if we could use the method on concrete types as well, e.g. if a is an ArrayList with values 1, 2, 3, we could call add2(a) and get an ArrayList back? So in an improved version, we could make the signature generic:
static <T extends List<Integer>> T add2 (T lst) {
T res;
try {
res = (T) lst.getClass().newInstance();
} catch (InstantiationException e) {
throw new IllegalArgumentException(e);
} catch (IllegalAccessException e) {
throw new RuntimeException(e);
}
for (Integer i : lst) res.add(i + 2);
return res;
}
I think throwing a runtime exception is the least worst option if a list without a nullary construcor is passed in. I don't see a way to ensure that it does. (Java 8 type annotations to the rescue maybe?) Returning null would be kind of useless.
The downside of using this signature is that we can't return an ArrayList etc as the default, as we could have done as an alternative to throwing an exception, since the return type is guaranteed to be the same type as that passed in. However, if the user actually wants an ArrayList (or some other default type) back, he can make an ArrayList copy and use the method on that.
If anyone with API design experience reads this, I would be interested to know your thoughts on which is the preferable option: 1) returning a List that needs to be explicity cast back into the original type, but enabling a return of a different concrete type, or 2) ensuring the return type is the same (using generics), but risking exceptions if (for example) a singleton object without a nullary constructor is passed in?
Given a starting List<Foo>, what is the most concise way to determine if a Foo element having a property bar (accessed by getBar()) has a value of "Baz"? The best answer I can come up with is a linear search:
List<Foo> listFoo;
for(Foo f:listFoo) {
if(f.getBar().equals("Baz")) {
// contains value
}
}
I looked into HashSet but there doesn't seem to be a way to use contains() without first instantiating a Foo to pass in (in my case, Foo is expensive to create). I also looked at HashMap, but there doesn't seem to be a way to populate without looping through the list and adding each Foo element one at a time. The list is small, so I'm not worried about performance as much as I am clarity of code.
Most of my development experience is with C# and Python, so I'm used to more concise statements like:
// C#
List<Foo> listFoo;
bool contains = listFoo.Count(f => f.getBar=="Baz")>0;
or
# Python
# list_foo = [Foo(), ...]
contains = "Baz" in (f.bar for f in list_foo)
Does Java have a way to pull this off?
Java does not support closures (yet), so your solution is one of the shortest. Another way would be to use, for example, google-collections Iterable's closure-like Predicate:
boolean contains = Iterables.any(iterableCollection, new Predicate<Foo>() {
#Override
public boolean apply(Foo foo) {
return foo != null && foo.getBar().equals("Baz");
}
}
In and of itself Java does not.
Also (just as an fyi) f.getBar == "Baz" won't work for string comparison, due to the fact that strings are objects. Then you use the == operator you are actually comparing objects (which are not equal because they are not at the same memory location and are individual objects). The equals method is the best way to do object comparisons. And specifically it is best to "Baz".equals(f.getBar()) as this also avoids nasty NullPointerExceptions.
Now to address your question. I can think of ways to do it, but it probably depends on the relationship of the parent object Foo to the child object Bar. Will it always be one to one or not? In other words could the Bar value of "Baz" be associated with more than one Foo object?
Where I'm going with this is the HashMap object that you talked about earlier. This is because there are the methods containsKey and containsValue. Since HashMap does allow duplicate values associated with different keys, you could put Bar as the value and Foo as the key. Then just use myHashMap.containsValue("Baz") to determine if it is in "the list". And since it is, then you can always retrieve the keys (the Foos) that are associate with it.
You can only emulate this in Java, e.g. using a "function object". But since this is a bit awkward and verbose in Java, it is only worth the trouble if you have several different predicates to select elements from a list:
interface Predicate<T> {
boolean isTrueFor(T item);
}
Foo getFirst(List<Foo> listFoo, Predicate<Foo> pred) {
for(Foo f:listFoo) {
if(pred.isTrueFor(f)) {
return f;
}
}
}
class FooPredicateBar implements Predicate<Foo> {
private final String expected;
FooPredicateBar(String expected) {
this.expected = expected;
}
public boolean isTrueFor(Foo item) {
return item != null && expected.equals(item.getBar());
}
}
...
List<Foo> listFoo;
Foo theItem = getFirst(listFoo, new FooPredicateBar("Baz"));
You can also use Apache Commons CollectionUtils:
boolean contains = CollectionUtils.exists(listFoo, new Predicate() {
public boolean evaluate(Object input) {
return "Baz".equals(((Foo)input).getBar());
}
});