Collection、Iterable、Iterator

一、概念

Collection是集合类的顶级接口，继承自Iterable

二、源码分析

1、Collection

//继承自Iterable<E>
public interface Collection<E> extends Iterable<E> {
int size();//返回collection中元素的个数

boolean isEmpty();//判断collection中元素是否为空，如果为空，则返回true

boolean contains(Object o);//判断collection中是否包含某个元素，如果包含，则返回true

Iterator<E> iterator();//迭代器

Object[] toArray();//将collection中的元素返回成Object数组

<T> T[] toArray(T[] a);//转换为特定类型数组，且数组为list或者list父级

boolean add(E e);//添加元素

boolean remove(Object o);//删除元素

boolean containsAll(Collection<?> c);//是否包含某个集合的所有元素

boolean addAll(Collection<? extends E> c);//添加某个集合的所有元素

boolean removeAll(Collection<?> c);//删除某个集合的所有元素

default boolean removeIf(Predicate<? super E> filter) {//验证一个参数是否符合要求
    Objects.requireNonNull(filter);
    boolean removed = false;
    final Iterator<E> each = iterator();
    while (each.hasNext()) {
        if (filter.test(each.next())) {
            each.remove();
            removed = true;
        }
    }
    return removed;
}

boolean retainAll(Collection<?> c);//判断list是否包含某个集合的全部

void clear();//清空集合

boolean equals(Object o);//比较某个元素

int hashCode();//返回hashCode

@Override
default Spliterator<E> spliterator() {//分割迭代器，分段对集合进行迭代，提高程序的并行能力，在多线程情形之下，每个线程处理集合一部分，线程安全
    return Spliterators.spliterator(this, 0);
}

default Stream<E> stream() {//返回非并行连续的流对象
    return StreamSupport.stream(spliterator(), false);
}

default Stream<E> parallelStream() {//返回并行的流对象
    return StreamSupport.stream(spliterator(), true);
}
}

2、Iterable

public interface Iterable<T> {
    /**
     * Returns an iterator over elements of type {@code T}.
     * 返回一个对应泛型的迭代器
     * @return an Iterator.
     */
    Iterator<T> iterator();

    /**
     * Performs the given action for each element of the {@code Iterable}
     * until all elements have been processed or the action throws an
     * exception.  Unless otherwise specified by the implementing class,
     * actions are performed in the order of iteration (if an iteration order
     * is specified).  Exceptions thrown by the action are relayed to the
     * caller.
     *
     * @implSpec
     * <p>The default implementation behaves as if:
     * <pre>{@code
     *     for (T t : this)
     *         action.accept(t);
     * }</pre>
     * 执行每个元素给定的行为，直到所有元素被处理或者处理过程中抛出异常
     * @param action The action to be performed for each element
     * @throws NullPointerException if the specified action is null
     * @since 1.8
     */
    default void forEach(Consumer<? super T> action) {
        Objects.requireNonNull(action);
        for (T t : this) {
            action.accept(t);
        }
    }

    /**
     * Creates a {@link Spliterator} over the elements described by this
     * {@code Iterable}.
     *
     * @implSpec
     * The default implementation creates an
     * <em><a href="Spliterator.html#binding">early-binding</a></em>
     * spliterator from the iterable's {@code Iterator}.  The spliterator
     * inherits the <em>fail-fast</em> properties of the iterable's iterator.
     *
     * @implNote
     * The default implementation should usually be overridden.  The
     * spliterator returned by the default implementation has poor splitting
     * capabilities, is unsized, and does not report any spliterator
     * characteristics. Implementing classes can nearly always provide a
     * better implementation.
     * 分割迭代器，分段对集合进行迭代，提高程序的并行能力，在多线程情形之下，每个线程处理集合一部分，线程安全
     * @return a {@code Spliterator} over the elements described by this
     * {@code Iterable}.
     * @since 1.8
     */
    default Spliterator<T> spliterator() {
        return Spliterators.spliteratorUnknownSize(iterator(), 0);
    }
}

3、Iterator

public interface Iterator<E> {
    /**
     * Returns {@code true} if the iteration has more elements.
     * (In other words, returns {@code true} if {@link #next} would
     * return an element rather than throwing an exception.)
     * 判断是否有下一个元素
     * @return {@code true} if the iteration has more elements
     */
    boolean hasNext();

    /**
     * Returns the next element in the iteration.
     * 迭代过程返回下个元素
     * @return the next element in the iteration
     * @throws NoSuchElementException if the iteration has no more elements
     */
    E next();

    /**
     * Removes from the underlying collection the last element returned
     * by this iterator (optional operation).  This method can be called
     * only once per call to {@link #next}.  The behavior of an iterator
     * is unspecified if the underlying collection is modified while the
     * iteration is in progress in any way other than by calling this
     * method.
     *
     * @implSpec
     * The default implementation throws an instance of
     * {@link UnsupportedOperationException} and performs no other action.
     *
     * @throws UnsupportedOperationException if the {@code remove}
     *         operation is not supported by this iterator
     * 默认删除方法
     * @throws IllegalStateException if the {@code next} method has not
     *         yet been called, or the {@code remove} method has already
     *         been called after the last call to the {@code next}
     *         method
     */
    default void remove() {
        throw new UnsupportedOperationException("remove");
    }

    /**
     * Performs the given action for each remaining element until all elements
     * have been processed or the action throws an exception.  Actions are
     * performed in the order of iteration, if that order is specified.
     * Exceptions thrown by the action are relayed to the caller.
     *
     * @implSpec
     * <p>The default implementation behaves as if:
     * <pre>{@code
     *     while (hasNext())
     *         action.accept(next());
     * }</pre>
     * 执行每个元素给定的行为，直到所有元素被处理或者处理过程中抛出异常
     * @param action The action to be performed for each element
     * @throws NullPointerException if the specified action is null
     * @since 1.8
     */
    default void forEachRemaining(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        while (hasNext())
            action.accept(next());
    }
}