using keyword this with multiple constructors in a Class

Question

I don't understand what happens when you create a Rational object with the constructor Rational(). My book says it will create a Rational object whose value is 0 but internally stored as 0/1. How does this(0) get stored as 0/1? Isn't the default value of the instance variables for both num and den 0?

public class Rational{

  public Rational(){
      this(0);
  }

  public Rational(int n){
      this(n,1);
  }

  public Rational(int x, int y){
     num = x; 
     den = y; 
  }

  private int num;
  private int den; 
}

EpicPandaForce · Accepted Answer · 2014-06-03 11:08:32Z

5

The term

Rational r = new Rational();

calls upon

public Rational(){
    this(0);
}

which calls upon

public Rational(int 0){
    this(0,1);
}

which calls upon

public Rational(int 0, int 1){
   num = 0; 
   den = 1; 
}

which means your final object will be Rational(0,1);

answered Jun 3, 2014 at 11:08

EpicPandaForce

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2 Comments

Jessica M. Over a year ago

Thanks! I was stumped for the longest time. It turned out to be a simple answer.

EpicPandaForce Over a year ago

No problem, it can be confusing if one isn't used to seeing the "this()" constructor call :)

Sanjay Rabari · Accepted Answer · 2014-06-03 11:22:20Z

this refers to the current object.

But

this() will call current class constructor that is default constructor.

this(val) will call class constructor with one argument...

this(val1,val2) call class constructor with two argument...

now if you call this() only but you have written you constructor in such way where you might have called other this(withArg). like below..so you can write nested way too.

public Rational(){
    this(0);
}

1) The this keyword can be used to refer current class instance variable.

public Rational(int num, int den){
     this.num = num; 
     this.den = deb; 
  }

2) this() can be used to invoked current class constructor.

   public Rational(int num, int den){
             this(num);
             this.den = deb; 
          }

3)The this keyword can be used to invoke current class method (implicitly).

public Rational(){
        this.someMethod();
    }

kmera · Accepted Answer · 2014-06-03 11:14:24Z

2

From within a constructor, you can also use the this keyword to call another constructor in the same class. Doing so is called an explicit constructor invocation. Source

If you do

new Rational()

the empty constructor will be called. This constructor will then call the constructor with one argument, i.e.

new Rational(0)

which again will call

new Rational(0,1)

That last constructor will then set the instance variables.

For further information look at this tutorial.

Also interesting: Java Language Specification

edited Jun 3, 2014 at 11:14

answered Jun 3, 2014 at 11:08

kmera

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3 Comments

Jessica M. Over a year ago

That makes a lot of sense. Thank you very much.

Jessica M. Over a year ago

What would be the disadvantage of having Rational() { this (0,1) } instead of passing 0 the constructor Rational(int n)? Is it bad practice?

kmera Over a year ago

the only difference is that 'Rational(0,1)' will be called immediately instead of the constructor in between. in this case it doesn't make a difference. but in another case the constructor with one argument could set up some other stuff which would then be skipped.

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