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package map; import com.alibaba.fastjson.JSONArray; import com.alibaba.fastjson.JSONObject; import org.junit.Test; import java.util.*; public class MapTest { /** * map 赋值到list属于引用传递，不可用clear清除对象信息 */ @Test public void mapClearTest(){ Map map1 = new HashMap(); List list = new ArrayList<>(); map1.put("name","bf"); map1.put("age","23"); map1.put("phone","110"); list.add(map1); // [{phone=110, name=bf, age=23}] System.out.println(list.toString()); map1.put("age","22"); System.out.println(list.toString()); map1.clear(); // [{}] System.out.println(list.toString()); } @Test public void JSONObjeactClearTest(){ JSONObject map1 = new JSONObject(); JSONArray l = new JSONArray(); map1.put("name","bf"); map1.put("age","23"); map1.put("phone","110"); l.add(map1); // [{"phone":"110","name":"bf","age":"23"}] System.out.println(JSONArray.toJSONString(l)); map1.clear(); // [{}] System.out.println(JSONArray.toJSONString(l)); } /** * 证明方法调用才会出现判断是否为空 */ @Test public void mapIsNullTest(){ Map map = new HashMap(); map.put("name","iceflow"); map.put("age",22); String name = map.get("name").toString(); System.out.println(name); // actual map,get("wife") is null Object wife = map.get("wife"); // 这里会发生 java.lang.NullPointerException System.out.println(wife.toString()); // 这里会发生 java.lang.NullPointerException String phone = map.get("phone").toString(); System.out.println(phone); } }