Copying object fields

Question

What is the best way of copying classes when I need to have two independent variables? I have simple class:

public class MyClass 
{
boolean a = false;
String b= "not empty";
}

do I need make some method like :

assign(MyClass data )
{
a= data.a;
b= data.b;
}

Is there any automatic method to copy (duplicate) objects in java?

What you need is a deep-copy. I think u can do that by a copy constructor. — codeMan, Sep 24 '13 at 18:19

score 3 · Answer 1 · edited May 23 '17 at 12:05

do I need make some method like :

Pretty close. Instead of making it method, you should make it a constructor. Such constructors are called copy constructor, and you create them like this:

MyClass(MyClass data) {  
    a = data.a;
    b = data.b;
}

And then to create copy of instance, you use the constructor like this:

MyClass obj1 = new MyClass();
MyClass obj2 = new MyClass(obj1);

Copy constructor can be tedious:

Using copy constructor to create deep-copy can be tedious, when your class has mutable fields. In which case, assignment like those will just create a copy of the reference, and not the object itself. You have to create copy of those fields also (If you want a deep copy). This can go recursive.

A better way to create deep copy is to Serialize and then Deserialize your object.

Why not use clone()?

Addition of clone() method in Object class was a big mistake, IMO. You should avoid using it for cloning objects.

For one, to use that method, you have to implement Cloneable interface, which by surprise doesn't actually have that clone() method. In fact, it's just a marker interface.
Return type of Object#clone() method is Object. So, this means that you can actually return an instance of a completely unrelated class, thus leading your code to a potential ClassCastException at runtime.

See also:

wow this is really great.. been programming in java for some time and never seen that! — Kuba Spatny, Sep 24 '13 at 18:22
@KubaSpatny It's standard in all of the `java.util` collection classes. — arshajii, Sep 24 '13 at 18:29
@arshajii one never stops learning :) Thanks for pointing that out, I'll have to look more deeply in to that. I've always been using methods for deep-copies. — Kuba Spatny, Sep 24 '13 at 18:34

score 1 · Answer 2 · answered Sep 24 '13 at 18:21

You can manually do a deep-clone using a copy constructor or u can use a utility available for that purpose, such as this one: https://code.google.com/p/cloning/

If ur class has many non-primitive member variables the complexity of deepcloning increases, I suggest you use that utility.

Ahmed Adel Ismail · Answer 3 · 2013-09-24T19:09:28.650

these are 2 approaches, first using the Cloneable interface (which is not preferred), and the other is creating a method inside your class that returns a new instance with the current state

public class MyObject implements Cloneable {

    private int num = 0;

    // a method to show how to clone an object
    private void copyObject() throws CloneNotSupportedException {

        MyObject a = new MyObject();
        a.num = 10;

        MyObject b = (MyObject)a.clone();
        System.out.println(b.num);
        // result : 10
        // values are copied to the new object

        b.num = 20;
        System.out.println(a.num + " - " + b.num);
        // result : 10 - 20
        // 2 different objects, the new object didnt effect the original one
    }

    // a method to create a copy
    public MyObject copy() {
        MyObject copy = new MyObject();
        copy.num = this.num;
        // do the same to all your variables
        return copy;
    }

    // a method to show how to use the copy() method
    private void useCopy(){

        MyObject a = new MyObject();
        a.num = 100;

        MyObject b = a.copy();

        System.out.println(a.num+" - "+b.num); 
        //  result : 100 - 100
        //  value copied to the new Object

        a.num = 10;
        b.num = 20;
        System.out.println(a.num + " - " + b.num);
        //  result : 10 - 20
        //  although b was copied from a, but both are 2 different Objects
    }
}

there are also another ways to copy values using reflection package, but after all, are you sure you are on the right track concerning design ?

if you can tell us what you are thinking of, maybe a design pattern may help

Copying object fields

3 Answers3