The data stored in the following two pieces of code are different
public static void main(String[] args) {
List list = new ArrayList();
list.add("a");
list.add("b");
list.add("c");
// 第一组
List list1 = new ArrayList();
Map map = new HashMap<String, String>();
for (int i = 0; i < list.size(); i++) {
map.put("测试", list.get(i));
list1.add(map);
}
for (int j = 0; j < list1.size(); j++) {
System.out.println(list1.get(j)+"---");
}
// 第二组
List list2 = new ArrayList();
for (int i = 0; i < list.size(); i++) {
Map map2 = new HashMap<String, String>();
map2.put("测试", list.get(i));
list2.add(map2);
}
for (int j = 0; j < list2.size(); j++) {
System.out.println(list2.get(j)+"***");
}
}The output of the first group is:
{Test=c}---
{Test=c}---{Test=c}---
The output of the second group is:
{Test=a}***
{Test=b}***
{Test=c}***
We found that the output data of the two groups is not connected, just because the location of the map is different.
In the first set of codes, the list stores a map object, and the map points to the same address in the heap memory. In this case, data duplication or overwriting will occur;
In the second group, we put the map into the loop, so that a map object will be re-instantiated every time the loop is looped, so that the map object points to a different address in the heap memory, so the output data is also different of.