private Set<String> fields = new HashSet<>();
public void fields(String... s){
this.fields.addAll(s);
}
Doesn't compile, saying that:
addAll
(java.util.Collection<? extends java.lang.String>)
in Set cannot be applied
to
(java.lang.String[])
is there is a good way to get around this?
I think you can convert your array to a list first. Maybe something like
this.fields.addAll(Arrays.asList(s));
If you further don't want to modify the Set, you can use Set.of with Java-9+ as:
public void fields(String... s) {
this.fields = Set.of(s);
}
or else you can use Collections.addAll as in:
public void fields(String... s) {
Collections.addAll(fields, s);
}
You simply convert array to Set as below code.
String [] array = new String[] {"one","two","three","four","five"};
Set<String> set = new HashSet<String>(Arrays.asList(array));
If you can use google Guava the follow code may help you
public void fields(String... s) {
this.fields.addAll(Sets.newHashSet(s));
}
another method:
public void fields1(String... s) {
Sets.union(fields,Sets.newHashSet(s)).copyInto(fields);
}
Related
In my Java project, I have a need to work with a handful of strings (about 10-30 at a time). I want a data structure to hold them, with properties like so:
Can assign a unique name to each string
The unique names can be used in the code just as if they were variables, with support for IDE auto-complete, no calling getValue() or toString(), etc.
Can iterate over each value in the data structure
In practice, I'd want the code to look something like this:
MagicalDataStructure<String> mds = new MagicalDataStructure(
FirstString = "foo",
SecondString = "bar",
);
/*
This section would output:
foo
bar
*/
for (String value : mds) {
System.out.println(value);
}
/*
This section would output:
The first value is: foo
*/
System.out.println("The first value is: " + FirstString);
Things I've considered:
A class full of static finals. This satisfies #1 and #2, but I can't iterate over them -- at least not without resorting to dark-mojo reflection.
A dictionary. This satisfies #1 and #3, but the keys wouldn't be auto-completable, and there's additional syntax involved in accessing the values.
An enum. This also solves #1 and #3, but accessing the string value takes a little bit of extra code.
Is there a data structure, library, etc that will do what I want?
I would definitely favor a Map for this:
public enum PagePath {
PATH1,
PATH2,
// etc.
}
public static final Map<PagePath, String> ALL_PATHS;
static {
Map<PagePath, String> paths = new EnumMap<>(PagePath.class);
paths.put(PagePath.PATH1, "/html/div[0]/h1");
paths.put(PagePath.PATH2, "/html//form/input[id='firstname']");
// etc.
// Make sure no one breaks things by removing entries
// or by adding enum constants while forgetting to account
// for them in the above Map.
if (!paths.keySet().equals(EnumSet.allOf(PagePath.class))) {
throw new RuntimeException(
"Map does not have entries for all PagePath constants!");
}
ALL_PATHS = Collections.unmodifiableMap(paths);
}
Another possibility, as you’ve mentioned, is using String constants. You can place the initialization of those constants inside the initialization of the “all values” list, to make sure none of them are forgotten:
public static final String PATH1;
public static final String PATH2;
// etc.
public static final Collection<String> ALL_PATHS;
static {
ALL_PATHS = Collections.unmodifiableCollection(Arrays.asList(
PATH1 = "/html/div[0]/h1",
PATH2 = "/html//form/input[id='firstname']",
// etc.
));
}
If someone removes a constant, they’ll be forced to remove its initialization from the Arrays.asList call. If someone adds a constant, and keeps it consistent with the other constants’ declarations, they will be forced to add it to the ALL_PATHS List, since failing to do so would mean it never gets initialized, which compiler will catch.
If your strings are properties your may want to use RessourceBundle or Properties. This can be use to solve problem 1/3.
To solve problem 2, you may create Enum that are Keys to your HashMap so that you need to write hashMap.get(enum) that will auto-complete everything. This solution add words but benefit from auto-completion.
Can you just write a custom method to return the string values using enum?
public enum MagicalDataStructure {
FirstString("foo"),
SecondString("bar");
String value;
MagicalDataStructure(String value) {
this.value = value;
}
public static List<String> getMagicalStrings() {
List<String> strings = new ArrayList<String>();
for (MagicalDataStructure item : MagicalDataStructure.values()) {
strings.add(item.value);
}
return strings;
}
}
And call the function wherever you need to iterate:
public static void main(String[] args) {
for (String magicalString: MagicalDataStructure.getMagicalStrings()) {
System.out.println(magicalString);
}
}
How about this :) The main idea here is the following we use the EnumMap as a base for our CustomEnumMap. My understanding is that you don't need put methods so our first task is to actually throw Unsupported Operation for them. The second step is to define the different enums with the values they are actually representing. The third step is achieved through a static method that converts any Enumeration into our CustomEnumMap. How the map is later used you can see for yourself.
There is one place for improvement though and it is the implementation of the static method. Unfortunately I am just learning java 8 lambdas so I was not able to implement it fast in a good way. But I will work on that and will give you the final implementation of this method later. Or is someone wants to help me out with it is welcome.
public static class CustomEnumMap<K extends Enum<K>,V> extends EnumMap<K, V> {
public CustomEnumMap(EnumMap<K, ? extends V> m) {
super(m);
}
#Override
public V put(K key, V value) {
throw new UnsupportedOperationException();
}
#Override
public void putAll(Map<? extends K, ? extends V> m) {
throw new UnsupportedOperationException();
}
}
public static enum EnumA {
FIRST("value1"),SECOND("value2"),THREE("value3");
private String value;
private EnumA(String value) {
this.value = value;
}
public String toString() {
return value;
}
}
public static enum EnumB {
FIRST("value1"),SECOND("value2");
private String value;
private EnumB(String value) {
this.value = value;
}
public String toString() {
return value;
}
}
public static <T extends Enum<T>> CustomEnumMap<T, String> toMap(T[] myenum) {
return new CustomEnumMap<T,String>(new EnumMap<T,String>( Arrays.stream(myenum).collect(Collectors.toMap(t->(T)t, t->t.toString()))));
}
public static void main(String args[]) {
CustomEnumMap<EnumA, String> enumA = toMap(EnumA.values());
CustomEnumMap<EnumA, String> enumB = toMap(EnumA.values());
for (String stringA : enumA.values()) {
System.out.print(stringA);
}
System.out.println("");
for (String stringB : enumB.values()) {
System.out.print(stringB);
}
}
How can I convert values here:
List<String> values = new ArrayList<String>
to :
ArrayList<Custom>
EDIT:
public class Custom {
public Custom Parse(String input) {
// What should I do here?
}
}
You could use:
List<Custom> customList = new ArrayList<Custom>();
for (String value: values) {
customList.add(new Custom(value));
}
Although it would be better just to add a constructor with a String argument:
class Custom {
private final String input;
public Custom(String input) {
this.input = input;
}
// not needed but implemented for completeness
public static Custom parse(String input) {
return new Custom(input);
}
}
Assuming your list of Custom objects has the same size with values list.
With one enhanced for-loop, set the appropriate fields of your objects like this:
int i=0;
for(String str:values)
customList.get(i++).setSomeProperty(str);
you can find a solution using Google Collections libraries on this thread Converting a List<String> to a List<Integer> (or any class that extends Number)
I have a list of objects which all have a enum value called AssetType, is it possible to use the retainAll() method to sort the list so that only objects with AssetType.BANK_ACCOUNT is kept?
Thx in advance for any help.
(Disclosure: I contribute to Guava.)
A somewhat more intuitive Guava-based implementation would be
Iterables.removeIf(allAssets, new Predicate<MyObject>() {
public boolean apply(MyObject asset) {
return asset.getAssetType() != AssetType.BANK_ACCOUNT;
}
});
...That said, honestly I would prefer the dumb, plain-Java implementation:
Iterator<MyObject> itr = allAssets.iterator();
while (itr.hasNext()) {
if (itr.next().getAssetType() != AssetType.BANK_ACCOUNT) {
itr.remove();
}
}
If you pull in Guava, you can make a live transform between the object and its AssetType, and then call retainAll on that:
Lists.transform(allAssets, assetTypeFn).retainAll(
Collections.singleton(AssetType.BANK_ACCOUNT));
//...elsewhere...
public static final Function<MyObject, AssetType> assetTypeFn =
new Function<MyObject, AssetType>() {
public AssetType apply(MyObject object) {
return object.getAssetType();
}
};
Similarly you can use the filter() method if you don't want to change the original list:
List<MyObject> bankAccounts = Lists.newArrayList(
Iterables.filter(allAssets, isBankAccount));
public static final Predicate<MyObject> isBankAccount = new Predicate<MyObject>() {
public boolean apply(MyObject asset) {
return asset.getAssetType() == AssetType.BANK_ACCOUNT;
}
}
I'm obviously missing something here, as this sound basic enough but yet...
I have a collection of objects . I need to use each one of them as parameter in constructor for a new object and return each new object to the caller method, one by one.
But -if I loop over the collection obviously the loop only runs once, and only returns the 1st object.
Edit : Returning the whole collection or some new collection will not work because :
The caller method [not mine to change] runs inside a start() method of a Runnable ThingProvider, which returns a single Thing whenever a request is submitted to it. So, returning List is not possible.
Thanks :)
public List<T> loop(Collection<? extends U> coll) {
List<T> a = new ArrayList<T>();
for (U u : coll){
a.add(new T(u));
}
return a;
}
Return a custom Iterator. Assumming your new objects are of class MyObject and the constructor accepts an Object:
public Iterator<MyObject> myObjectsIterator(final Iterator<? extends Object> it) {
return new Iterator<MyObject>() {
public boolean hasNext() {
return it.hasNext();
}
public MyObject next() {
return new MyObject(it.next());
}
public void remove() {
it.remove();
}
};
}
And you would call it like this:
...
Iterator<MyObject> myIt = myObjectsIterator(myListOfObjects.iterator());
// Now you can pass myIt around as a normal object. It will remember
// which one is the next Object with which to construct a MyObject
// and will generate it on the fly
...
while (myIt.hasNext()) { // is there any MyObject remaining?
MyObject myObj = myIt.next(); // gets the next MyObject
// do something with myObj
}
...
This is a poorly worded question and I think as others have noted, just returning a new list of the objects is fine. But if you really want to process them one at a time while you're looping through it, you can use the command pattern.
public interface Command {
void execute(NewType object);
}
Now in your caller method, you can do the following:
public void doSomething() {
processList(myList, new Command() {
void execute(NewType object) {
// Do whatever you want with this object
}
});
}
And, in the method that will actually go through the list:
public void processList(Iterable<OldType> values, Command command) {
for(OldType v : values) {
NewType newType = new NewType(v);
command.execute(newType);
}
}
In java you can return only once. So if you want to get some informations from your methods either you wrap them into a "Big" Object (here a List) or you give to the method the means to put informations in your parameters.
You could have something like this :
public static void main(String... args){
List<Parameter> parameters = methodToGetParameters();
List<Result> results = generateObjectsFromList(parameters);
for(Result result : results){
handleAResult(result);
}
}
public List<Result> generateObjectsFromList(List<Parameter> parameters){
List<Result> results = new ArrayList<Result>();
for(Parameter parameter : parameters){
results.add(new Result(parameter));
}
return results;
}
Or like this :
public static void main(String... args){
List<Parameter> parameters = methodToGetParameters();
List<Result> results = new ArrayList<Result>();
generateObjectsFromList(parameters, results);
for(Result result : results){
handleAResult(result);
}
}
public void generateObjectsFromList(List<Parameter> parameters, List<Result> results){
for(Parameter parameter : parameters){
results.add(new Result(parameter));
}
}
A third way to do this would be to use fields, but it's not really good to have a lot of fields if they're not really used (or only by one method).
On the same topic :
Java Object Oriented Design Question: Returning multiple objects in java(Updated)
Using a java method to return multiple values?
Return a collection from the method and in the collection implement a custom iterator to transform the input collection to the new collection. The following code shows how to do it using the Google Guava library:
import java.util.Arrays;
import java.util.Collection;
import java.util.List;
import com.google.common.base.Function;
import com.google.common.collect.Collections2;
public class Test {
static class Person {
public final String name;
public Person(String name) {
this.name = name;
}
}
public static Collection<Person> peopleFromNames(Collection<String> names) {
return Collections2.transform(names, new Function<String, Person>() {
public Person apply(String name) {
return new Person(name);
}});
}
public static void main(String[] args) {
List<String> names = Arrays.asList("Brian", "Albert", "Roger");
for (Person person : peopleFromNames(names)) {
System.out.println(person.name);
}
}
}
do you mean using of delegates something like below
public class Test {
private static class Person{
private final String name;
Person(String name){
this.name = name;
}
#Override
public String toString() {
return return name;
}
}
private interface Printer {
void print(Object object);
}
public static void main(String[] args) {
final String[] names = {"one", "two", "three"};
final ArrayList<Person> people = construct(names, new Printer() {
#Override
public void print(Object object) {
System.out.println(object.toString());
}
});
}
private static ArrayList<Person> construct(String[] names, Printer printer) {
ArrayList<Person> people = new ArrayList<Person>();
for (String name : names) {
printer.print(new Person(name));
}
return people;
}
}
It's Possible.
Check these Project for Java-yield , yield4Java, infomancers
If you're using this just once in your entire code, You're better off choosing a method from the other answers.
Return a list of the new objects.
Can I contain two different types in a collection? For example, can I have List< String U Integer > ?
Short answer? No. You can (of course) have a List of Objects, but then you can put anything in it, not just String or Integer objects.
You could create a list of container objects, and that container object would contain either an Integer or String (perhaps via generics). A little more hassle.
public class Contained<T> {
T getContained();
}
and implement Contained<Integer> and Contained<String>.
Of course, the real question is why you want to do this? I would expect a collection to contain objects of the same type, and then I can iterate through and perform actions on these objects without worrying what they are. Perhaps your object hierarchy needs further thought?
Nope. You have a couple of alternatives, though:
You can use a List < Object > and stash whatever you like; or
You can use a List < Class-with-2-members > and put your data in one of those class members.
EDIT: Example.
class UnionHolder {
public String stringValue;
public int intValue;
}
List < UnionHolder > myList
...
Of course you'll need a bit of additional code to figure out which kind of data to pull out of the UnionHolder object you just got out of your list. One possibility would be to have a 3rd member which has different values depending on which it is, or you could, say, have a member function like
public boolean isItAString() { return (this.stringValue != null }
If you are doing something like functional programming in Java 8 or above, you may want to try JavaSealedUnions:
Union2.Factory<String, Integer> factory = GenericUnions.doubletFactory();
Union2<String, Integer> strElem = factory.first("hello");
Union2<String, Integer> intElem = factory.second(3);
List<Union2<String, Integer>> list = Array.asList(strElem, intElem);
for (Union2<String, Integer> elem : list) {
elem.continued(
strElem -> System.out.println("string: " + strElem),
intElem -> System.out.println("integer: " + intElem));
}
Haven't tested this, but I think you got the idea.
In addition to the nice answers already provided ...
Possibly, you have the two data types in your algorithm. But you may not have to put them in the same list...
Creating two typed lists could be the clearer for your algorithm, you would still keep the "type-safeness" and carry all your data. Two code samples follow, the second grouping the two lists in a MyData object.
public class Algorithm1 {
public void process(List<String> strings, List<Integer> integers) {
...
}
}
--------------------------------------
public class DataPair {
public List<String> strings;
public List<Integer> integers;
}
public class Algorithm2 {
public void process(DataPair dataPair) {
...
}
}
what you're decribing is the perfect use case for the Visitor pattern
100% statically type-checked
doesn't need Java 8 or above
usage:
List<UnionType> unionTypes = Arrays
.asList(new StringContainer("hello"), new IntegerContainer(4));
for (UnionType unionType : unionTypes) {
unionType.when(new UnionType.Cases<Integer>() {
#Override
public Integer is(StringContainer stringContainer) {
// type-specific handling code
}
#Override
public Integer is(IntegerContainer integerContainer) {
// type-specific handling code
}
});
}
boilerplate code:
interface UnionType {
<R> R when(Cases<R> c);
interface Cases<R> {
R is(StringContainer stringContainer);
R is(IntegerContainer integerContainer);
}
}
class StringContainer implements UnionType {
private final String value;
public StringContainer(String value) { this.value = value; }
public String getValue() { return value; }
#Override
public <R> R when(Cases<R> cases) {
return cases.is(this);
}
}
class IntegerContainer implements UnionType {
private final Integer value;
public IntegerContainer(Integer value) { this.value = value; }
public Integer getValue() { return value; }
#Override
public <R> R when(Cases<R> cases) {
return cases.is(this);
}
}
No. Think about it this way: with generics, the whole idea is to provide type safety. That would not be possible if you could put Objects of different types into it.
You can use the non-generic java.util.List for your purpose.
If you want to ensure that only String or Integer objects enter the list, you could create your own List implementation like so:
public class MySpecialList {
private List list= new LinkedList();
...
public void add(final String string) {
list.add(string);
}
public void add(final Integer integer) {
list.add(integer);
}
...
// add rest of List style methods
}
Drawback: you loose the List interface clarity...