java系列-HashMap遍历

1.遍历例子

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

public class HashMapTraversalExample {
    public static void main(String[] args) {
        HashMap<String, Integer> hashMap = new HashMap<>();
        hashMap.put("A", 1);
        hashMap.put("B", 2);
        hashMap.put("C", 3);

        // 获取HashMap的迭代器
        Iterator<Map.Entry<String, Integer>> iterator = hashMap.entrySet().iterator();

        // 遍历HashMap
        while (iterator.hasNext()) {
            Map.Entry<String, Integer> entry = iterator.next();
            System.out.println("Key: " + entry.getKey() + ", Value: " + entry.getValue());
        }
    }
}

2.遍历相关源码

2.1.entrySet

获取全局变量entrySet，如果为null，新建一个EntrySet

public class HashMap<K,V> extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable {

    transient Set<Map.Entry<K,V>> entrySet;

    public Set<Map.Entry<K,V>> entrySet() {
        Set<Map.Entry<K,V>> es;
        //如果为null, 则创建一个new EntrySet()
        return (es = entrySet) == null ? (entrySet = new EntrySet()) : es;
    }
}

2.2.iterator()

创建EntryIterator，构造方法里会给next赋值

public class HashMap<K,V> extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable {

    abstract class HashIterator {
        Node<K,V> next;        // next entry to return

        HashIterator() {
            expectedModCount = modCount;
            Node<K,V>[] t = table;
            current = next = null;
            index = 0;
            
            //一开始给next赋值
            if (t != null && size > 0) { // advance to first entry
                do {} while (index < t.length && (next = t[index++]) == null);
            }
        }
    }

    final class EntrySet extends AbstractSet<Map.Entry<K,V>> {

        public final Iterator<Map.Entry<K,V>> iterator() {
            return new EntryIterator();
        }
    }
}

2.3.hasNext

判断next是否为null

public class HashMap<K,V> extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable {

    final class EntryIterator extends HashIterator{

        public final boolean hasNext() {
            return next != null;
        }
    }
}

2.4.next

将这次要返回的节点赋值给e；

更新next

public class HashMap<K,V> extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable {

    final class EntryIterator extends HashIterator implements Iterator<Map.Entry<K,V>> {
        public final Map.Entry<K,V> next() { 
            return nextNode(); 
        }

        final Node<K,V> nextNode() {
            Node<K,V>[] t;
            Node<K,V> e = next;
            if (modCount != expectedModCount){
                throw new ConcurrentModificationException();
            }
            if (e == null){
                throw new NoSuchElementException();
            }
            //先看e.next是否为null，为null说明到了尾；需要找下一个桶不为null
            if ((next = (current = e).next) == null && (t = table) != null) {
                do {} while (index < t.length && (next = t[index++]) == null);
            }
            return e;
        }
    }
}