java iterator和Iterable的区别

iterator为Java中的迭代器对象

Iterable是对iterator的一个封装， 以支持for each循环。

 

 

Iterator<Tuple2<String, String>> el = elements.iterator();

// iterator为Java中的迭代器对象，是能够对List这样的集合进行迭代遍历的底层依赖
// 而Iterable接口里定义了iterator的方法，相当于对iterator的封装,支持for each循环

// iterator 不支持for each循环
for (Tuple2<String, String> e: el){

}

// Iterable 支持for each循环
Tuple2<String, String> ret = Tuple2.of("", "");
for(Tuple2<String, String> element: elements){
    ret = element;
}


// iterator 循环遍历
while (el.hasNext()){
    Tuple2<String, String> data = el.next();
    System.out.println(data);
}

 

Iterable.java

 

Iterable接口支持for each循环, 然而实现Iterable接口，就必需为foreach语句提供一个迭代器

这个迭代器是用接口定义的 iterator方法提供的。也就是iterator方法需要返回一个Iterator对象。

public interface Iterable<T> {
    /**
     * Returns an iterator over elements of type {@code T}.
     *
     * @return an Iterator.
     */
    Iterator<T> iterator();
}

 

Iterator.java

1、每次在迭代前   ，先调用hasNext()探测是否迭代到终点（本次还能再迭代吗？）。

2、next方法不仅要返回当前元素，还要后移游标cursor

3、remove()方法用来删除最近一次已经迭代出的元素

4、 迭代出的元素是原集合中元素的拷贝（重要）

5、配合foreach使用

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.)
     *
     * @return {@code true} if the iteration has more elements
     */
    boolean hasNext();

    /**
     * Returns the next element in the iteration.
     *
     * @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
     *
     * @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>
     *
     * @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());
    }
}

参考： https://www.cnblogs.com/keyi/p/5821285.html