Creating a constructor with an object of the creating class

Question

public class Foo {
    private int field;

    public Foo(Foo foo) {
        this.field = foo.getField();
    }

    public int getField() {
        return field;
    }

    public void setField(int field) {
        this.field = field;
    }
}

Kindly can someone please help me understand this and how it will be initialized in the test class, what it does and if it is legal.

what don't you understand? You just create a second instance of the class, in a new object, with the same values. You will need to be able to instantiate the original one, though, so you'll need a second constructor. — Stultuske, Nov 25 '19 at 06:51
You might need to declare a no arg constructor too ,for your code to work. All you are doing is copying the attributes of one object to other ,by passing it to the constructor with args — Shubham Dixit, Nov 25 '19 at 06:54
You don't have to pass an actual `Foo` object to the `Foo(Foo foo)` constructor. You could pass `null` instead: `new Foo(null)`. Just take care with the design of the `Foo` class so that having `field == null` won't cause a problem. — Kevin Anderson, Nov 25 '19 at 07:04
@KevinAnderson Doing that is generally considered an antipattern as opposed to simply throwing NPE. (This assumes that the expected no-arg or int-arg constructor is also present.) — chrylis -cautiouslyoptimistic-, Nov 25 '19 at 08:02
thanks guys, I really appreciate the help. @Shubh the link has helped a lot. — Owaga_JR, Nov 25 '19 at 15:57

score 1 · Answer 1 · answered Nov 25 '19 at 07:00

This the working example of what you are doing .You need to define two constructors.One with no args ,as you have already defined a constructor with args.Also see copy constructor in java

 class Foo {
    private int field;

     public Foo(int arg) {
       this.field=arg;

    }


    public Foo(Foo foo) { // copy constructor
        this.field = foo.getField();
    }

    public int getField() {
        return field;
    }

    public void setField(int field) {
        this.field = field;
    }
}


public class Main {
  public static void main(String[] args) {
    Foo obj=new Foo(4); // Default constructor called
    Foo obj2=new Foo(obj);
    System.out.println(obj2.getField());
     System.out.println(obj.getField());
  }
}

Creating a constructor with an object of the creating class

1 Answers1