Copying the entire object in memory

Question

I have this:

import java.util.ArrayList;


public class MyClass {
    ArrayList list = new ArrayList();

    public MyClass(){

    }

    public MyClass(MyClass aClass){
        this.list = aClass.getList();
    }

    public void add(int number){
        list.add(number);
    }

    public void set(int number){
        list.set(0, number);
    }

    public int get(){
        return (Integer)list.get(0);
    }

    public ArrayList getList(){
        return list;
    }
}



MyClass aName = new MyClass();
aName.add(5);
System.out.println("aName: "+aName.get());

MyClass aName2 = new MyClass(aName);
System.out.println("aName2: "+aName2.get());

aName2.set(1);
System.out.println("aName2: "+aName2.get());
System.out.println("aName: "+aName.get());

This will print: aName: 5 aName2: 5 aName2: 1 aName: 1

I don't want my second object changing the values in my first object. Is there any way to stop this happening but still be able to copy properties from another object?

You need to make a copy of the internal ArrayList, not just assign the reference. e.g. `this.list = aClass.getList().clone()` — nhahtdh, Jul 26 '12 at 19:37
Hmm, you're right. He's homerolling a stack with an arraylist. Does inserting to the beginning of an arraylist operate in O(N) time? — Wug, Jul 26 '12 at 19:44

SJuan76 · Accepted Answer · 2012-07-26T19:45:20.630

1

Then don't pass the List from the original object to the new object, in

public MyClass(MyClass aClass){ 
    this.list = aClass.getList(); 
}

It makes both object have a reference to the same list (there is only one list shared between both objects).

You should do

public MyClass(MyClass aClass){ 
    this.list = (List) aClass.getList().clone(); 
}

edited Jul 26 '12 at 19:45

answered Jul 26 '12 at 19:39

SJuan76

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Check whether it is necessary to cast, since clone() returns Object. – nhahtdh Jul 26 '12 at 19:40

score 1 · Answer 2 · answered Jul 26 '12 at 19:39

Yes you can stop this. You need to make defensive copy of the list in your copy constructor.

public MyClass(MyClass aClass){
    this.list = new ArrayList(aClass.getList());
}

You could use clone(), but as Josh Bloch states in Effective Java item "copy constructors have many advantages over clone" - they're preferable in practice.

score 1 · Answer 3 · answered Jul 26 '12 at 19:41

1

You could clone() or take a copy, but since you have created a list already, rather than discarding it you could use it. ;)

public MyClass(MyClass aClass){
    this.list.addAll(aClass.getList());
}

answered Jul 26 '12 at 19:41

Peter Lawrey

525,659
79
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1,130

2

That's true, he's creating one on declaration. Another reason why `final` variables are useful - that would have detected this bug on compile! – corsiKa Jul 26 '12 at 19:44

score 0 · Answer 4 · answered Jul 26 '12 at 19:44

You're copying the reference value of the ArrayList into your new object. You now have two references to the same list.

You need to create a new ArrayList when you construct your second object:

public MyClass(MyClass aClass){
    this.list = new ArrayList(aClass.getList());
}

Be aware, however, that both lists now contain the same objects. Again, this only creates a new list and copies the reference values of the objects inside the original list. If you needed two complete and separate lists containing unique objects you would need to write a method that did a deep copy, duplicating the objects in the list as well.

Copying the entire object in memory

4 Answers4