Java : AbstractCollection with Examples

class MyCollection extends AbstractCollection<String> {
    private final String[] values;

    MyCollection(String[] values) {
        this.values = Objects.requireNonNull(values);
    }

    @Override
    public Iterator<String> iterator() {
        return new Iterator<>() {
            private int index;

            @Override
            public boolean hasNext() {
                return values.length != index;
            }

            @Override
            public String next() {
                final var value = values[index];
                index++;
                return value;
            }
        };
    }

    @Override
    public int size() {
        return values.length;
    }
}

final var collection = new MyCollection(new String[]{"aaa", "bbb", "ccc"});

System.out.println(collection); // [aaa, bbb, ccc]
System.out.println(collection.size()); // 3
System.out.println(collection.isEmpty()); // false

System.out.println("-- values --");
for (final var value : collection) {
    System.out.println(value);
}

// Result
// ↓
//-- values --
//aaa
//bbb
//ccc

AbstractCollection ()

Sole constructor.

class MyCollection extends AbstractCollection<String> {
    private final String[] values;

    MyCollection(String[] values) {
        this.values = Objects.requireNonNull(values);
    }

    @Override
    public Iterator<String> iterator() {
        return new Iterator<>() {
            private int index;

            @Override
            public boolean hasNext() {
                return values.length != index;
            }

            @Override
            public String next() {
                final var value = values[index];
                index++;
                return value;
            }
        };
    }

    @Override
    public int size() {
        return values.length;
    }
}

final var collection = new MyCollection(new String[]{"aaa", "bbb", "ccc"});

System.out.println(collection); // [aaa, bbb, ccc]
System.out.println(collection.size()); // 3
System.out.println(collection.isEmpty()); // false

System.out.println("-- values --");
for (final var value : collection) {
    System.out.println(value);
}

// Result
// ↓
//-- values --
//aaa
//bbb
//ccc

boolean add (E e)

Ensures that this collection contains the specified element (optional operation).

final AbstractCollection<String> collection = new ArrayList<>();
System.out.println(collection.add("aaa")); // true
System.out.println(collection.add("bbb")); // true
System.out.println(collection.add("bbb")); // true

System.out.println(collection); // [aaa, bbb, bbb]

final AbstractCollection<String> collection = new HashSet<>();
System.out.println(collection.add("aaa")); // true
System.out.println(collection.add("bbb")); // true
System.out.println(collection.add("bbb")); // false

System.out.println(collection); // [aaa, bbb]

boolean addAll (Collection<? extends E> c)

Adds all of the elements in the specified collection to this collection (optional operation).

final AbstractCollection<String> collection = new ArrayList<>();

final var list1 = List.of("aaa", "bbb");
final var list2 = List.of("bbb", "ccc");
final var list3 = List.of("ccc");

System.out.println(collection.addAll(list1)); // true
System.out.println(collection); // [aaa, bbb]

System.out.println(collection.addAll(list2)); // true
System.out.println(collection); // [aaa, bbb, bbb, ccc]

System.out.println(collection.addAll(list3)); // true
System.out.println(collection); // [aaa, bbb, bbb, ccc, ccc]

final AbstractCollection<String> collection = new HashSet<>();

final var set1 = Set.of("aaa", "bbb");
final var set2 = Set.of("bbb", "ccc");
final var set3 = Set.of("ccc");

System.out.println(collection.addAll(set1)); // true
System.out.println(collection); // [aaa, bbb]

System.out.println(collection.addAll(set2)); // true
System.out.println(collection); // [aaa, ccc, bbb]

System.out.println(collection.addAll(set3)); // false
System.out.println(collection); // [aaa, ccc, bbb]

void clear ()

Removes all of the elements from this collection (optional operation).

final AbstractCollection<String> collection = new ArrayList<>();
collection.add("aaa");
collection.add("bbb");
collection.add("ccc");

System.out.println(collection); // [aaa, bbb, ccc]

collection.clear();

System.out.println(collection); // []
System.out.println(collection.isEmpty()); // true

boolean contains (Object o)

Returns true if this collection contains the specified element.

final AbstractCollection<String> collection = new ArrayList<>();
Collections.addAll(collection, "aaa", "bbb", "ccc");
System.out.println(collection); // [aaa, bbb, ccc]

System.out.println(collection.contains("aaa")); // true
System.out.println(collection.contains("bbb")); // true
System.out.println(collection.contains("XXX")); // false

boolean containsAll (Collection<?> c)

Returns true if this collection contains all of the elements in the specified collection.

final AbstractCollection<String> collection = new ArrayList<>();
Collections.addAll(collection, "aaa", "bbb", "ccc");
System.out.println(collection); // [aaa, bbb, ccc]

final var list1 = List.of("aaa", "bbb");
System.out.println(collection.containsAll(list1)); // true

final var list2 = List.of("bbb", "XXX");
System.out.println(collection.containsAll(list2)); // false

final var list3 = List.of("XXX", "YYY");
System.out.println(collection.containsAll(list3)); // false

boolean isEmpty ()

Returns true if this collection contains no elements.

final AbstractCollection<String> collection = new ArrayList<>();

System.out.println(collection); // []
System.out.println(collection.size()); // 0
System.out.println(collection.isEmpty()); // true

collection.add("aaa");
System.out.println(collection); // [aaa]
System.out.println(collection.size()); // 1
System.out.println(collection.isEmpty()); // false

collection.add("bbb");
System.out.println(collection); // [aaa, bbb]
System.out.println(collection.size()); // 2
System.out.println(collection.isEmpty()); // false

abstract Iterator<E> iterator ()

Returns an iterator over the elements contained in this collection.

final AbstractCollection<String> collection = new ArrayList<>();
Collections.addAll(collection, "aaa", "bbb", "ccc");
System.out.println(collection); // [aaa, bbb, ccc]

final var iterator = collection.iterator();
iterator.forEachRemaining(System.out::println);

// Result
// ↓
//aaa
//bbb
//ccc

boolean remove (Object o)

Removes a single instance of the specified element from this collection, if it is present (optional operation).

final AbstractCollection<String> collection = new ArrayList<>();
collection.add("aaa");
collection.add("bbb");
collection.add("ccc");

System.out.println(collection); // [aaa, bbb, ccc]

System.out.println(collection.remove("aaa")); // true
System.out.println(collection); // [bbb, ccc]

System.out.println(collection.remove("bbb")); // true
System.out.println(collection); // [ccc]

System.out.println(collection.remove("XXX")); // false
System.out.println(collection); // [ccc]

boolean removeAll (Collection<?> c)

Removes all of this collection's elements that are also contained in the specified collection (optional operation).

final var src = List.of("aaa", "bbb", "ccc");

{
    final AbstractCollection<String> collection = new ArrayList<>(src);
    System.out.println(collection); // [aaa, bbb, ccc]

    final var list = List.of("aaa", "bbb");
    System.out.println(collection.removeAll(list)); // true
    System.out.println(collection); // [ccc]
}
{
    final AbstractCollection<String> collection = new ArrayList<>(src);
    System.out.println(collection); // [aaa, bbb, ccc]

    final var list = List.of("XXX", "YYY");
    System.out.println(collection.removeAll(list)); // false
    System.out.println(collection); // [aaa, bbb, ccc]
}
{
    final AbstractCollection<String> collection = new ArrayList<>(src);
    System.out.println(collection); // [aaa, bbb, ccc]

    final var list = List.of("aaa", "bbb", "ccc", "XXX");
    System.out.println(collection.removeAll(list)); // true
    System.out.println(collection); // []
}

boolean retainAll (Collection<?> c)

Retains only the elements in this collection that are contained in the specified collection (optional operation).

final var src = List.of("aaa", "bbb", "ccc");

{
    final AbstractCollection<String> collection = new ArrayList<>(src);
    System.out.println(collection); // [aaa, bbb, ccc]

    final var list = List.of("aaa", "bbb");
    System.out.println(collection.retainAll(list)); // true
    System.out.println(collection); // [aaa, bbb]
}
{
    final AbstractCollection<String> collection = new ArrayList<>(src);
    System.out.println(collection); // [aaa, bbb, ccc]

    final var list = List.of("XXX", "YYY");
    System.out.println(collection.retainAll(list)); // true
    System.out.println(collection); // []
}
{
    final AbstractCollection<String> collection = new ArrayList<>(src);
    System.out.println(collection); // [aaa, bbb, ccc]

    final var list = List.of("aaa", "bbb", "ccc", "XXX");
    System.out.println(collection.retainAll(list)); // false
    System.out.println(collection); // [aaa, bbb, ccc]
}

Object[] toArray ()

Returns an array containing all of the elements in this collection.

final AbstractCollection<String> collection = new ArrayList<>();
Collections.addAll(collection, "aaa", "bbb", "ccc");
System.out.println(collection); // [aaa, bbb, ccc]

final Object[] ret = collection.toArray();
System.out.println(Arrays.toString(ret)); // [aaa, bbb, ccc]

<T> T[] toArray (T[] a)

Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.

final AbstractCollection<String> collection = new ArrayList<>();
Collections.addAll(collection, "aaa", "bbb", "ccc");
System.out.println(collection); // [aaa, bbb, ccc]

{
    final String[] ret = collection.toArray(new String[0]);
    System.out.println(Arrays.toString(ret)); // [aaa, bbb, ccc]
}
{
    final String[] array = new String[collection.size()];
    System.out.println(Arrays.toString(array)); // [null, null, null]

    final var ret = collection.toArray(array);
    System.out.println(Arrays.toString(ret)); // [aaa, bbb, ccc]

    System.out.println(Arrays.toString(array)); // [aaa, bbb, ccc]
}

String toString ()

Returns a string representation of this collection.

final AbstractCollection<String> collection = new ArrayList<>();

final var str1 = collection.toString();
System.out.println(str1); // []

collection.add("aaa");

final var str2 = collection.toString();
System.out.println(str2); // [aaa]

collection.add("bbb");
collection.add("ccc");

final var str3 = collection.toString();
System.out.println(str3); // [aaa, bbb, ccc]

equals, hashCode, parallelStream, removeIf, size, spliterator, stream, toArray

Please see the link below.

• Java API Examples : Collection

forEach

Please see the link below.

• Java API Examples : Iterable