在创建时向集合添加元素

Question

How can I create a Set in java, and then add objects to it when it is constructed. I want to do something like:

testCollision(getObject(), new HashSet<MazeState>(){add(thing);});

But that doesn't seem quite right.

Answer 1

Since Java 7 , to instantiate a single-element , immutable Set, you can use:

Collections.singleton(thing);

Returns an immutable set containing only the specified object. The returned set is serializable.

— Javadoc reference: Collections.singleton(T)

---

In Java 8 you can instantiate a Set containing any number of your objects with the following, which is an adaptation of this answer :

Stream.of(thing, thingToo).collect(Collectors.toSet());

Answer 2

In Java 5

new HashSet<MazeState>(Arrays.asList(thing));

Arrays.asList(thing) converts your thing to the list of one element, and from that list set is created.

For the reference:
http://download.oracle.com/javase/6/docs/api/java/util/Arrays.html#asList(T...)

Answer 3

Since Java 9 you can also do it like this:

 Set<String> immutableSet = Set.of("value1", "value2");
 Set<Integer> immutableInt = Set.of(1, 2);

 List<String> immutableList = List.of("item1", "item2");

 Map<String, String> immutableMap = Map.of("key1", "value1", "key2", "value2", "key3", "value3");

Observe that any Sets/Maps/Lists created this way will be immutable (if my naming convention didn't convince you ;)

Answer 4

You can use double-braces:

testCollision(getObject(), new HashSet<MazeState>(){{ add(obj1); add(obj2);}};

or:

Set<String> set = new HashSet<String>(){{
  add("hello");
  add("goodbye");
}};

This is called double-brace initialization, and it's one of the lesser known features of Java. What it does is cause the compiler to create an anonymous inner class that does the creation and manipulation for you (So, for example, if your class was final, you couldn't use it.)

Now, having said that - I'd encourage you only to use it in cases where you really need the brevity. It's almost always better to be more explicit, so that it's easier to understand your code.

Answer 5

如果您不介意不变性，那么您可以使用Google Guava的ImmutableSet类：

ImmutableSet.of(new MazeState(), new MazeState());

Answer 6

Other answers are correct but want to add one more way .using initializer block

new HashSet<MazeState>() {

            {
                add(new MazeState());
                add(new MazeState());
            }
        };

Answer 7

您可以使用com.google.common.collect的util方法，这是一个非常好的方法： Sets.newHashSet("your value1", "your valuse2");