Collection接口

星辰_大海

于 2018-05-26 17:13:02 发布

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本文介绍了Java集合框架中的Collection接口，详细列举了其主要方法及其作用，包括添加、删除、查询等操作，并解释了如何通过Iterator接口遍历Collection中的元素。

Collection接口表示一组对象，这些对象也称为Collection接口的元素。一些Collection接口允许有重复的元素，而另一些则不允许；一些Collection接口是有序的，而另一些是无序的。

Collection类有众多的API。下面是官方提供的API。

Modifier and Type	Method	Description
`boolean`	`add(E e)`	Ensures that this collection contains the specified element (optional operation).
`boolean`	`addAll(Collection<? extends E> c)`	Adds all of the elements in the specified collection to this collection (optional operation).
`void`	`clear()`	Removes all of the elements from this collection (optional operation).
`boolean`	`contains(Object o)`	Returns `true` if this collection contains the specified element.
`boolean`	`containsAll(Collection<?> c)`	Returns `true` if this collection contains all of the elements in the specified collection.
`boolean`	`equals(Object o)`	Compares the specified object with this collection for equality.
`int`	`hashCode()`	Returns the hash code value for this collection.
`boolean`	`isEmpty()`	Returns `true` if this collection contains no elements.
`Iterator<E>`	`iterator()`	Returns an iterator over the elements in this collection.
`default Stream<E>`	`parallelStream()`	Returns a possibly parallel `Stream` with this collection as its source.
`boolean`	`remove(Object o)`	Removes a single instance of the specified element from this collection, if it is present (optional operation).
`boolean`	`removeAll(Collection<?> c)`	Removes all of this collection's elements that are also contained in the specified collection (optional operation).
`default boolean`	`removeIf(Predicate<? super E> filter)`	Removes all of the elements of this collection that satisfy the given predicate.
`boolean`	`retainAll(Collection<?> c)`	Retains only the elements in this collection that are contained in the specified collection (optional operation).
`int`	`size()`	Returns the number of elements in this collection.
`default Spliterator<E>`	`spliterator()`	Creates a `Spliterator` over the elements in this collection.
`default Stream<E>`	`stream()`	Returns a sequential `Stream` with this collection as its source.
`Object[]`	`toArray()`	Returns an array containing all of the elements in this collection.

在Collection对象中并为提供get()方法获取元素。如果要遍历Collection中的元素，一般采用Iterator遍历器。Collection接口继承了接口Iterable<E>，实现这个接口将允许对象成为foreach语句的目标。在Iterable中提供了迭代方法iterator（），所以Collection接口定义的子类集合上都有一个与容器类相对应的遍历器。可以通过遍历器遍历出容器类中的各个元素对象。

Iterator接口中定义的方法如下。

Modifier and Type	Method	Description
`default void`	`forEachRemaining(Consumer<? super E> action)`	Performs the given action for each remaining element until all elements have been processed or the action throws an exception.
`boolean`	`hasNext()`	Returns `true` if the iteration has more elements.
`E`	`next()`	Returns the next element in the iteration.
`default void`	`remove()`	Removes from the underlying collection the last element returned by this iterator (optional operation).