2016-7-20 getKeyByValue in Map

Map中通过KEY获取VALUE是很容易,但是通过VALUE来获取KEY,则可能得到多个相同VALUE的KEY,所以Map本身并没有其对应的函数.

public class MapUtils {
        private static Map<Integer, Object> map = null;

        public static Map<Integer, Object> setMap(Map<Integer, Object> _map) {
            return map = _map;
        }
        
        public static Object getKeyByValue(Object value) {
            Object o = null;
            ArrayList<Object> matchList = new ArrayList<Object>(); // 声明顺序表来存放符合条件的KEY

            /*
             * 这里关键是:
             * entrySet() : 它返回一个包含Map.Entry集的Set对象.
             * Map.Entry对象有getValue和getKey的方法,利用这两个方法就可以达到从值取键的目的了
             **/
            Set<?> set = map.entrySet();
            Iterator<?> it = set.iterator();
            while (it.hasNext()) {
                @SuppressWarnings("rawtypes")
                Map.Entry entry = (Map.Entry) it.next();
                if (entry.getValue().equals(value)) {
                    o = entry.getKey();
                    matchList.add(o);    // 把符合条件的KEY先放到容器中
                }
            }
            return matchList;
        }
}

调用:

import java.util.*;

class Elemt {
    public int a;
    String b = "";

    public Elemt(int a, String string) {
        this.a = a;
        b = string;
    }
public Object findElem() {
        return MapUtils.getKeyByValue(this);
    }
}

public class test_find_Key {

    public static void main(String[] args) {
        Map<Integer, Object> map = new HashMap<>();

        Elemt e1 = new Elemt(1, "1");
        Elemt e2 = new Elemt(1, "1");
        Elemt e3 = new Elemt(1, "1");
        Elemt e4 = e3;

        map.put(6, e1);
        map.put(7, e2);
        map.put(8, e3);
        MapUtils.setMap(map);
        
        @SuppressWarnings("unchecked")
        ArrayList<Object> list = (ArrayList<Object>)e4.findElem();
        
        for (Object object : list) {
            System.out.println(object);
        }
    }
}

结果: