ArrayLists 未按预期工作

Question

import java.util.ArrayList;

public class bugs 
{
    public ArrayList<Integer> elements1;
    public ArrayList<Integer> elements2;

    public bugs(ArrayList<Integer> args)
    {
        elements1 = args;
        elements2 = args;
    }

    public void change(int index, int value)
    {
        elements1.set(index, value);
    }

    public void reset()
    {
        elements1 = elements2;
    }

    public static void main(String[] a)
    {
        ArrayList<Integer> stuff = new ArrayList<Integer>();
        stuff.add(1);
        stuff.add(1);
        stuff.add(1);
        stuff.add(1);

        bugs b = new bugs(stuff);
        b.change(2, 999);
        b.reset();

        System.out.println(b.elements2);
    }
}

This outputs:

[1, 1, 999, 1]

The second arraylist elements2 is there to reset the arraylist elements1 to its original position. However, for some reason elements1 is being copied to elements2 , printing

[1, 1, 999, 1]

and not

[1, 1, 1, 1]

Answer 1

You are passing the same ArrayList reference to both variables.

What you meant to do is:

public bugs(ArrayList<Integer> args)
{
    elements1 = new ArrayList<Integer>(args);
    elements2 = new ArrayList<Integer>(args);
}

EDIT:

Note that this is only a temporary fix. Calling reset() will pass the reference of elements2 to elements1 and then you have the same situation. If you create a new arraylist and you pass another list as argument, you create a new reference with the same contents. This means you must also adjust your reset() method to create a new list and pass elements2 as argument.

Answer 2

In java you can pass by reference or by value. If you are using primitive data types, you are passing by value. So:

public void method(int num)
{
   int num1 = num;
   int num2 = num;
}

This method pass num value to num1 and num2 primitive data types. If you add something to num1 it will not change num2 to the same value. But if you are using non primitive data types like ArrayList:

public bugs(ArrayList<Integer> args)
{
    elements1 = args;
    elements2 = args;
}

You should expect that change in elements1 array will change elements2 array also. In this example you are passing the same ArrayList reference to both variables.

The solution for your problem is create copy of your arrays:

public bugs(ArrayList<Integer> args)
{
    elements1 = new ArrayList<>(args);
    elements2 = new ArrayList<>(args);
}

public void reset()
{
    elements1 = new ArrayList<>(elements2);
}