HashMap对象的深层克隆

1.java.util.HashMap 的 clone 方法是浅层copy，clone出来的对象，仅仅是原来对象的一个引用，并且对克隆出来的对象进行操作是无效的。

下面是个例子：

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

    /**
     * @author hzp
     *
     */
    public class Test {  

        /**
         * @param args
         */
        public static void main(String[] args) {
            // TODO Auto-generated method stub  

            HashMap source = new HashMap();
            source.put("key1","value1");
            source.put("key2","value2");  

            for(Iterator keyItr = source.keySet().iterator();keyItr.hasNext();) {
                Object key = keyItr.next();
                System.out.println(key + " : "+source.get(key));
            }
            System.out.println("----------------- 1 ----------------");  

            Map targetMap = (HashMap)source.clone();  

            for(Iterator keyItr = targetMap.keySet().iterator();keyItr.hasNext();){
                Object key = keyItr.next();
                System.out.println(key + " : "+source.get(key));
            }  

            System.out.println("---------------- 2 ----------------");  

            Object temp = targetMap.put("key1","modify");
            System.out.println("temp : "+temp);  

            for(Iterator keyItr = source.keySet().iterator();keyItr.hasNext();){
                Object key = keyItr.next();
                System.out.println(key + " : "+source.get(key));
            }
        }  

    }  

输出结果为：

    key1 : value1
    key2 : value2
    ----------------- 1 ----------------
    key1 : value1
    key2 : value2
    ---------------- 2 ----------------
    temp : value1
    key1 : value1
    key2 : value2  

若想实现深层copy，则需要自己重写clone方法。

如下面的例子：

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

    /**
     * @author hzp
     *
     */
    public class Test {  

        class customHashMap extends HashMap {  

            public customHashMap() {
                super();
            }  

            public customHashMap(int initialCapacity) {
                super(initialCapacity);
            }  

            public Object clone() {
                Map target = new HashMap();
                for (Iterator keyIt = this.keySet().iterator(); keyIt.hasNext();) {
                    Object key = keyIt.next();
                    target.put(key, this.get(key));
                }
                return target;
            }
        }  

        /**
         * @param args
         */
        public static void main(String[] args) {
            // TODO Auto-generated method stub  

            customHashMap source = (new Test()).new customHashMap();
            source.put("key1", "value1");
            source.put("key2", "value2");  

            for (Iterator keyItr = source.keySet().iterator(); keyItr.hasNext();) {
                Object key = keyItr.next();
                System.out.println(key + " : " + source.get(key));
            }  

            System.out.println("----------------- 1 ----------------");  

            Map target = (Map) source.clone();
            target.put("key1", "modify");  

            System.out.println("----------------- 2 the souce map print----------------");
            for (Iterator keyItr = source.keySet().iterator(); keyItr.hasNext();) {
                Object key = keyItr.next();
                System.out.println(key + " : " + source.get(key));
            }  

            System.out.println("----------------- 3 the target map print----------------");
            for (Iterator keyItr = target.keySet().iterator(); keyItr.hasNext();) {
                Object key = keyItr.next();
                System.out.println(key + " : " + target.get(key));
            }  

        }  

    }  

输出结果：

    key1 : value1
    key2 : value2
    ----------------- 1 ----------------
    ----------------- 2 the souce map ----------------
    key1 : value1
    key2 : value2
    ----------------- 3 the target map ----------------
    key1 : modify
    key2 : value2