public static void main(String[] args) {
//list賦值
List list = new ArrayList();
Map map = new HashMap();
map.put("id", 1);
list.add(map);
Map map2 = new HashMap();
map2.put("id", 2);
list.add(map2);
System.out.println(list);
//newList = list*2
List newList = new ArrayList();
newList.add(list.get(0));
newList.add(list.get(1));
newList.add(list.get(0));
newList.add(list.get(1));
System.out.println(newList);
//改變第3個map的id值
for(int i=0;i<newList.size();i++){
if(i==2){
Map m = (Map)newList.get(2);
m.put("id", 3);
}
}
System.out.println(newList);
}
雖然修改的的是newList的第三個對象,但第一個對象id值也改變。
因爲list.get(2)存的是引用,而list.get(2)和list.get(0)存放的是相同引用,內存裏面同一個區域,只要一個改變,其他都會改變。
<span style="color:#ff0000;">改乘這樣就行了
</span><span style="background-color: rgb(204, 204, 204);">import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
public class Dog {
public static void main(String[] args) {
List list = new ArrayList();
Map map = new HashMap();
map.put("id", 1);
list.add(map);
Map map2 = new HashMap();
map2.put("id", 2);
list.add(map2);
System.out.println(list);
// newList = list*2
List newList = new ArrayList();
newList.add(list.get(0));
newList.add(list.get(1));
newList.add(list.get(0));
newList.add(list.get(1));
System.out.println(newList);
for (int i = 0; i < newList.size(); i++) {
if (i == 2) {
Map m = new HashMap();
Map n = (Map) newList.get(2);
m.put(n.keySet().iterator().next(), 3);
newList.set(2, m);
}
}
System.out.println(newList);
}
}</span>