java.util · java.base · since Java 1.2

Collection

declaration

public interface Collection<E> extends Iterable<E>

The root container contract: add/remove/contains/size/iterate. List, Set, and Queue refine it; algorithms accept it for maximum generality.

Key methods

`boolean add(E) / remove(Object) / contains(Object)`	Core membership operations.
`int size() / boolean isEmpty()`	Cardinality.
`void clear()`	Remove every element.
`Iterator<E> iterator()`	The cursor behind for-each; the only safe way to remove while iterating most collections.
`boolean addAll(Collection<? extends E>)`	Bulk insert — union when the target starts empty.
`boolean removeAll(Collection<?>)`	Bulk removal — set difference.
`boolean retainAll(Collection<?>)`	Bulk removal — set intersection.
`boolean containsAll(Collection<?>)`	Subset test.
`boolean removeIf(Predicate<? super E>)`	Safe bulk removal — no CME.
`Stream<E> stream() / parallelStream()`	Bridge into the Stream API.
`Object[] toArray() / <T> T[] toArray(IntFunction<T[]>)`	Array snapshots — pass String[]::new.
`<T> T[] toArray(T[] a)`	Legacy array overload — pass a correctly-sized array to skip a second allocation.