Java集合框架之迭代器(Iterator)

1. Iterator 接口

public interface Iterator<E> {

    boolean hasNext();

    E next();

    default void remove() {
        throw new UnsupportedOperationException("remove");
    }

    default void forEachRemaining(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        while (hasNext())
            action.accept(next());
    }
}

1.1 Iterator遍历

@Test
public void test() {
    List<String> list = new ArrayList<>();
    list.add("111");
    list.add("222");
    list.add("333");

    Iterator<String> iterator = list.iterator();
    while (iterator.hasNext()) {
        String next = iterator.next();
    }
}

多层遍历

@Test
public void test() {
    Map<Long, Set<String>> map = new HashMap<>();
    map.put(1001L, new HashSet<>(Arrays.asList("aaa", "bbb")));
    map.put(1002L, new HashSet<>(Arrays.asList("ccc")));

    // Iterator<Set<String>> iterator = map.values().iterator();
    String next = map.values().iterator().next().iterator().next();
    // 输出 next: aaa
}

1.2 遍历时注意事项

迭代过程中不能进行删除、添加操作，会抛异常java.util.ConcurrentModificationException

@Test
public void test() {
    List<String> list = new ArrayList<>();
    list.add("111");
    list.add("222");
    list.add("333");

    for (String str : list) {
        if ("111".equals(str)) {
            list.remove("111");
            // list.add("444");
        }
    }
}

报错原因：

private class Itr implements Iterator<E> {
    final void checkForComodification() {
        if (modCount != expectedModCount)
            throw new ConcurrentModificationException();
    }
}

改为iterator.remove()

Iterator<String> iterator = list.iterator();
while (iterator.hasNext()) {
    if ("111".equals(iterator.next())) {
        iterator.remove();
    }
}

2. Iterable 接口

public interface Iterable<T> {

    Iterator<T> iterator();

    default void forEach(Consumer<? super T> action) {
        Objects.requireNonNull(action);
        for (T t : this) {
            action.accept(t);
        }
    }
}

实现 Iterable 接口，就拥有了获取迭代器的能力

2.1 集合里的迭代器

Collection 继承了 Iterable 接口

public interface Collection<E> extends Iterable<E> {
    Iterator<E> iterator();
}

public interface List<E> extends Collection<E> {
    Iterator<E> iterator();
}

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable {

    public Iterator<E> iterator() {
        return new Itr();
    }

    // 内部类实现了 Iterator
    private class Itr implements Iterator<E> {
        public boolean hasNext() {}

        public E next() {}

        public void remove() {}
    }

}

2.2 Iterable 存在的意义

解耦，有些集合有多个 Iterator 内部类，可以获取不同的 Iterator 执行不一样的操作，比如ArrayList，LinkedList

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable {

    public ListIterator<E> listIterator() {
        return new ListItr(0);
    }

    public Iterator<E> iterator() {
        return new Itr();
    }

    private class Itr implements Iterator<E> {}

    private class ListItr extends Itr implements ListIterator<E> {}
}