public void add(int index,E element) Inserts the specified element at the specified position in this list. Shifts the element currently at that position (if any) and any subsequent elements to the right (adds one to their indices).
package com.iot.test;
import java.util.ArrayList;
import java.util.List;
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello world");
List list = new ArrayList<Integer>();
for (int i = 0; i < 10; i++) {
list.add(i,0);
}
System.out.println("before ,size:"+list.size());
System.out.println(list);
for (int i = 0; i < 10; i++) {
list.add(i, (Integer)list.get(i)+1);
System.out.println("end ("+i+" <=> "+list.get(i)+")");
}
System.out.println("after ,size:"+list.size());
System.out.println(list);
}
}
------------------输出----------------：
Hello world
before ,size:10
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
end (0 <=> 1)
end (1 <=> 1)
end (2 <=> 1)
end (3 <=> 1)
end (4 <=> 1)
end (5 <=> 1)
end (6 <=> 1)
end (7 <=> 1)
end (8 <=> 1)
end (9 <=> 1)
after ,size:20
[1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Process finished with exit code 0
0 
3
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Class AbstractList<E>中 add 方法的文档:List.add 方法是用来增加元素的，不是用来增加元素的值的，你完全没搞清楚就用。
修改元素的值要用 List.set 方法
我写了一段代码,展示了
add的效果，希望对你能知道自己哪错了------------------输出----------------：