constructor invoke another constructor using this KEYWORD in java

public class immutable {

    private double  radius;

  

public immutable(double radius)

{

    this.radius=radius;

}

public immutable()

{

    this(4.0);

}

public double getradius()

{

    return this.radius*this.radius*Math.PI;

}

}

...................................................................................................................................................................

public class TestImmutable {

    public static void main(String []arg)

    {

     immutable imm=new immutable();

     System.out.println(imm.getradius());

  

      

  

    }

  

}