CODEWITHSUNDEEP
java
Mohammad Fadin
Mohammad Fadin

Reputation: 579

Java Question about Copy Constructor

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.

Upvotes: 1

Views: 1376

Answers (4)

Stas Jaro
Stas Jaro

Reputation: 4885

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.

Upvotes: 0

Hyperboreus
Hyperboreus

Reputation: 32429

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.

Upvotes: 2

Cosmin Cosmin
Cosmin Cosmin

Reputation: 1556

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

Upvotes: 0

Jon Skeet
Jon Skeet

Reputation: 1500525

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.

Upvotes: 3

Related Questions