为什么此引用不起作用？

Question

What I Have: A static ArrayList which stores Integers. I add an Integer via reference to it and then I change the value but the value is not updated.

import java.util.ArrayList;

public class Main {

    public static final ArrayList<Integer> integers = new ArrayList<>();
    public Integer integer;

    void addInteger(Integer i){
        integers.add(i);
    }

    public static void main(String[] args) {

        Main main = new Main();
        main.integer = 3;

        integers.add(main.integer);

        main.integer = 4;

        System.out.println(integers.get(0));

    }
}

Output: 3

Question: Why is the output not 4 ?

Further Question based on follow ups: What is really stored within the ArrayList ?

EDIT BASED ON THE ACCTPTED ANSWER:

Since integer is an Integer (and not an int ) the 3 is autoboxed. The ArrayList stores actually Integer.valueOf(3) .

Answer 1

When executing main.integer = 4 you're reassigning the value of main.integer , but the old reference was already added to the List<Integer> . Here's what happens in the code:

//commented this line
//main.integer = 3;
//this line below is what really happens
main.integer = Integer.valueOf(3);
integers.add(main.integer);
//main.integer = 4;
main.integer = Integer.valueOf(4);

Answer 2

Here's what happens:

• You create a reference to 3 (and store that reference in main.integer ).
• This reference is then stored in the list (by integers.add(main.integer) ). The reference to 3 is passed by value, so the reference is copied into the list.
• You then update main.integer to contain a reference to 4 . This does not affect what is stored in the list.

Compare this with the following snippet:

public AtomicInteger integer;

Main main = new Main();
main.integer = new AtomicInteger(3);  // Reference to a mutable object

integers.add(main.integer);           // Add reference to list

main.integer.set(4);                  // Change mutable object

System.out.println(integers.get(0));  // Prints 4

Answer 3

From your current code you only assign the first element of your list a value (3). If you are expecting to output 4 you need to do 2 things. First make sure you add a new value to your ArrayList. Add another integers.add(main.integer); after you change the value of main.integer. After you add another element to your ArrayList remember to index the new correct element. ie: System.out.println(integers.get(1));