java－－List、Set、Map的遍历

一、Iterator接口

所有实现Collection接口的容器类都有一个iterator()方法用来返回一个实现了Iterator接口的对象

Iterator对象称作迭代器，用来方便的实现对容器内元素的遍历操作。

package java.util;

import java.util.function.Consumer;

public interface Iterator<E> {
   
     *
    boolean hasNext();　//返回是否有元素为被遍历

    
    E next(); //返回游标当前位置元素并将游标移动到下一个位置

  default void remove() {
    throw new UnsupportedOperationException("remove");
  } //删除游标左边的元素，因为在执行完next之后游标移到下一个元素，该操作只能执行一次

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

二、List的遍历方式

/*
* 测试List遍历的方式
* */
public class TestArrayList {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("aa");
        list.add("bb");
        list.add("cc");

        //方式一，for循环遍历,通过get()方法
        　　　for (int i = 0; i < list.size(); i++) {
            System.out.println(list.get(i));
        }

        //方式二,while()循环,用迭代器实现
       　　　 Iterator iterator = list.iterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next());
        }

        //方式三，for循环,用迭代器实现
       　　　 for (Iterator iterator1 = list.iterator(); iterator1.hasNext(); ) {
            System.out.println(iterator1.next());
        }

        //方式四,增强for循环  foreach
        for (String str : list) {
            System.out.println(str);
        }

        //方式五，转化数组
      　　　  String[] a = new String[3];
        String[] aa = list.toArray(a);
        for (int i = 0; i < aa.length; i++) {
            System.out.println(aa[i]);
        }
    }
}

ArrayList类中的定义内部类Itr，内部类实现Iterator接口

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

/**
 * An optimized version of AbstractList.Itr
 */
private class Itr implements Iterator<E> {
    int cursor;       // index of next element to return
    int lastRet = -1; // index of last element returned; -1 if no such
    int expectedModCount = modCount;

    Itr() {}

    public boolean hasNext() {
        return cursor != size;
    }

    @SuppressWarnings("unchecked")
    public E next() {
        checkForComodification();
        int i = cursor;
        if (i >= size)
            throw new NoSuchElementException();
        Object[] elementData = ArrayList.this.elementData;
        if (i >= elementData.length)
            throw new ConcurrentModificationException();
        cursor = i + 1;
        return (E) elementData[lastRet = i];
    }

    public void remove() {
        if (lastRet < 0)
            throw new IllegalStateException();
        checkForComodification();

        try {
            ArrayList.this.remove(lastRet);
            cursor = lastRet;
            lastRet = -1;
            expectedModCount = modCount;
        } catch (IndexOutOfBoundsException ex) {
            throw new ConcurrentModificationException();
        }
    }

    @Override
    @SuppressWarnings("unchecked")
    public void forEachRemaining(Consumer<? super E> consumer) {
        Objects.requireNonNull(consumer);
        final int size = ArrayList.this.size;
        int i = cursor;
        if (i >= size) {
            return;
        }
        final Object[] elementData = ArrayList.this.elementData;
        if (i >= elementData.length) {
            throw new ConcurrentModificationException();
        }
        while (i != size && modCount == expectedModCount) {
            consumer.accept((E) elementData[i++]);
        }
        // update once at end of iteration to reduce heap write traffic
        cursor = i;
        lastRet = i - 1;
        checkForComodification();
    }

    final void checkForComodification() {
        if (modCount != expectedModCount)
            throw new ConcurrentModificationException();
    }
}

三、Set遍历方式

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HashSet中获取Iterator对象的方式

public Iterator<E> iterator() {
    return map.keySet().iterator();
}

public class TestHashSet {
    public static void main(String[] args) {
        Set<String> set = new HashSet();
        set.add("aa");
        set.add("bb");
        set.add("cc");

        //方式一,用while循环遍历,迭代器实现
                  Iterator iterator = set.iterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next());
        }

        //方式二，用for循环遍历,迭代器实现
                  for (Iterator iterator1 = set.iterator(); iterator1.hasNext(); ) {
            System.out.println(iterator1.next());
        }

        //方式三,增强for循环
                  for (String str : set) {
            System.out.println(str);
        }
    }
}

四、Map的遍历方式

public class TestHashMap {
    public static void main(String[] args) {

        Map<String, String> map = new HashMap<>();
        map.put("a", "2000");
        map.put("b", "3000");

        //方式一、在for-each循环中使用entrySet来遍历
       　　　 Set<Map.Entry<String, String>> entrySet = map.entrySet();
        for (Map.Entry<String, String> entry : entrySet) {
            String key = entry.getKey();
            String value = entry.getValue();
            System.out.println("key=" +key+"  value="+value);
        }

        //方式二、在for-each循环中通过键找值遍历
        　　　for (String key : map.keySet()) {
            System.out.println("key=" + key+"  value="+map.get(key));
        }

        //只打印键值keys
        for (String key : map.keySet()) {
            System.out.println("key=" + key);
        }
        //只打印values
        for (String value : map.values()) {
            System.out.println("value=" + value);
        }

        //方式三,使用iterator遍历
                   Iterator<Map.Entry<String, String>> iterator = map.entrySet().iterator();
        while (iterator.hasNext()) {
            Map.Entry<String, String> entry = iterator.next();
            System.out.println("key=" + entry.getKey() + "  value=" + entry.getValue());
        }

        //不使用泛型
                  Iterator iterator1 = map.entrySet().iterator();
        while (iterator1.hasNext()) {
            Map.Entry entry1 = (Map.Entry) iterator1.next();
            System.out.println("key=" + entry1.getKey() + "  value=" + entry1.getValue());
        }

    }
}

public Set<Map.Entry<K,V>> entrySet() {
    Set<Map.Entry<K,V>> es;
    return (es = entrySet) == null ? (entrySet = new EntrySet()) : es;
}

public Set<K> keySet() {
    Set<K> ks = keySet;
    if (ks == null) {
        ks = new KeySet();
        keySet = ks;
    }
    return ks;
}