java迭代器原理及迭代map的四种方式

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

    default void forEach(Consumer<? super T> action) {
        Objects.requireNonNull(action);
        for (T t : this) {
            action.accept(t);
        }
    }
    default Spliterator<T> spliterator() {
        return Spliterators.spliteratorUnknownSize(iterator(), 0);
    }
}

public interface Iterator<E> {
    /**
    * 如果迭代拥有更多元素，那么返回true
    */
    boolean hasNext();
    /**
    * 返回iteration中的下一个元素
    */
    E next();
    /**
    * 如果删除一个集合中的元素没有调用这个方法，二十直接中集合中删除，那么这个迭代器的行为没有被指定
    */
    default void remove() {
        throw new UnsupportedOperationException("remove");
    }
    /**
    * 遍历集合中的剩余元素（如果之前调用了两次next()那么只会遍历集合中剩余元素
    * 使用案例：
    * Iterator<Integer> it = map.keySet().iterator();
        it.next();
        it.next();
        it.forEachRemaining(System.out::println);
    */
    default void forEachRemaining(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        while (hasNext())
            action.accept(next());
    }
}

        final Node<K,V> nextNode() {
            Node<K,V>[] t;
            Node<K,V> e = next; // 对象属性中的next是下一个值
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            if (e == null)
                throw new NoSuchElementException();
            // next = e.next 如果e.next为null，那么继续找数组下一个不为null的值
            if ((next = (current = e).next) == null && (t = table) != null) {
                do {} while (index < t.length && (next = t[index++]) == null);
            }
            return e;
        }

        public final void remove() {
            Node<K,V> p = current;
            if (p == null)
                throw new IllegalStateException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            current = null;
            K key = p.key;
            removeNode(hash(key), key, null, false, false);
            expectedModCount = modCount;

import java.util.*;
import java.util.concurrent.ConcurrentHashMap;
import java.util.HashMap;

public class MapTest {
    public static void main(String[] args) {
        HashMap<Integer,Integer> map = new HashMap();
        map.put(0,1);
        map.put(2,2);
        map.put(1,2);
        map.put(4,5);
        map.put(3,4);
        // 遍历hashmap entry foreach
        Set<Map.Entry<Integer,Integer>> ent = map.entrySet();
        for(Map.Entry<Integer,Integer> entry:ent){
            System.out.println(entry.getKey()+"  :  "+entry.getValue());
            //map.remove(0);
        }
        System.out.println();
        // 通过keySet或者values()遍历
        Set<Integer> set = map.keySet();
        for(Integer key:set){
            System.out.println(key+"  --  "+map.get(key));
        }
        Collection<Integer> set1 = map.values();
        for(Integer val:set1){
            System.out.println(val);
        }
        System.out.println();
        // iterator原理是什么 通过iterator遍历map
        Iterator<Map.Entry<Integer,Integer>> iter = map.entrySet().iterator();
        while(iter.hasNext()){
            Map.Entry entry = iter.next();
            System.out.println(entry.getKey()+"  :  "+entry.getValue());
            iter.remove();
        }
        System.out.println();
        Iterator<Integer> keys = map.keySet().iterator();
        while(keys.hasNext()){
            int k = keys.next();
            System.out.println(k+" -- "+ map.get(k));
        }

    }
}