Java Question about Copy Constructor

Question

I hope you will help me understand this Copy Constructor I took more then 2 hours reading reading on websites and I didn't understand anything about it.

I know that a copy constructor is used to copy an object of a class. But I don't understand the code of the copy consturctor.

For Example:

public class Rectangle 
{
    private double length = 10.0;
    private double width = 10.0;


    public Rectangle(double length, double width){
        this.length = length;
        this.width = width;

    }
   //this is the copy constructor  what exactly the argument here? is it the object ref it self? please explain what happening here. and the use

    public Rectangle(Rectangle ref){
        this.length = ref.length;
        this.width = ref.width;
    }

this is what I see all the time. but I don't understand the code at all! ref is it going to be created in the main program?

let say this is the main program

public class rectangleTest{ 
public static void main(String[] args){
//is this new_ref object going to be created here?

Rectangle new_ref = new Rectangle(10.0 , 10.0);

This thing will not 100% clear to me unless if someone make a small class and main class showing me what's happening

Thank you.

Let's say this never compiles: Rectangle new ref = new ref(10.0 , 10.0); — Hyperboreus
– Hyperboreus, Commented Jun 11, 2011 at 21:32
I think he meant Rectangle new ref = new Rectangle (10.0 , 10.0); — Cosmin Cosmin
– Cosmin Cosmin, Commented Jun 11, 2011 at 21:35

Jon Skeet · Accepted Answer · 2011-06-11 21:31:36Z

3

ref isn't the name of a class; it's the name of the parameter to the second constructor. So the main method would actually look like this:

Rectangle foo = new Rectangle(10.0 , 10.0);

// Create another Rectangle with the same width and height
Rectangle bar = new Rectangle(foo);

Note that objects don't have names - variables do. Here the value of the foo variable becomes the value of the ref parameter in the second constructor, when that constructor is invoked in the last line above. Also note that the values of foo, bar and ref aren't objects... they're references to objects.

answered Jun 11, 2011 at 21:31

Jon Skeet

1.5m893 gold badges9.3k silver badges9.3k bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Stas Jaro Over a year ago

do you know anything about verlet physics? lol im asking because your the all time #1 user apparently and i have to a question that no one could answer yet

Mohammad Fadin Over a year ago

I should've asked SOF instead of wasting 2 hours. @Jon and everyone this is really helped me so so much! Thank you! and I'm sorry I made a mistake in the code and @stas fixed it for me.

Voo Over a year ago

@stas Yep maybe someone should ask a quantum mechanics question on SO - that way we'd finally get a deeper understanding of our universe: Either we find out more about physics or can finally decide if Jon is omniscient or not ;)

Hyperboreus · Accepted Answer · 2011-06-11 21:35:05Z

2

You can use your "copy constructor" like this

Rectangle a = new Rectangle (3.0, 4.0);
Rectangle b = new Rectangle (a);

NOTA BENE: Unlike in C++ where a copy ctor is part of the language and called implicitely, the following example in Java will NOT call your copy ctor, but just assign the reference.

Rectangle a = new Rectangle (3.0, 4.0);
Rectangle b = a;

In your case, I would prefer to implement a clone method.

answered Jun 11, 2011 at 21:35

Hyperboreus

32.5k9 gold badges50 silver badges88 bronze badges

2 Comments

Mohammad Fadin Over a year ago

Thank you!!! So this is what they call it a privacy leak? In the second part of your code. When I changed the value of b.getLength() the value of the length is also changed in object a .

Hyperboreus Over a year ago

Why should GETlength change something? But yes a and b refer in the 2nd example to the same instance. Evrything you do to a, you do it to b too.

Cosmin Cosmin · Accepted Answer · 2011-06-11 21:34:13Z

0

It's more like a clone(), it duplicates your object so that you have two different instances but which are equal.

answered Jun 11, 2011 at 21:34

Cosmin Cosmin

1,5561 gold badge17 silver badges35 bronze badges

Comments

Mxyk · Accepted Answer · 2012-11-19 00:52:08Z

0

The names don't matter. The name just refers to an object, so when you pass it into a method it just used a different name.

edited Nov 19, 2012 at 0:52

Mxyk

10.7k16 gold badges60 silver badges76 bronze badges

answered Jun 11, 2011 at 21:31

Stas Jaro

4,8955 gold badges33 silver badges53 bronze badges

Collectives™ on Stack Overflow

Java Question about Copy Constructor

4 Answers 4

3 Comments

2 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

3 Comments

2 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related