I can't understand the task

Domanda sul compito

Calling a constructor from a constructor

Risolta

Good day. I understand calling a constructors with an other one mean when i am invoking a method with an other. I don't understand the purpose of it. And i have read i use this (). But i want understand what i am doing. Like this( x,y,10) invoking constructor with 3 parameter?

Figure out what the program does.
Correct the constructor with two parameters so that it calls another constructor with a radius of 10.
Think about which constructor you need to call.

Hint:
carefully study the implementation of the default constructor.

Requisiti:

The class must have 3 constructors.
The constructor with two parameters must initialize the variables x and y to the passed values.
The constructor with two parameters must initialize the variable radius to a value of 10.
The constructor with two parameters must call another constructor, passing the correct arguments.
Don't change the main method.
Don't change the default constructor.

package com.codegym.task.task05.task0521; /* Calling a constructor from a constructor */ public class Circle { public double x; public double y; public double radius; public Circle(double x, double y, double radius) { this.x = x; this.y = y; this.radius = radius; } public Circle(double x, double y) { //write your code here this(x, y,10.00); this.x=x; this.y= y; this.radius = radius; } public Circle() { this(5, 5, 1); } public static void main(String[] args) { Circle circle = new Circle(); System.out.println(circle.x + " " + circle.y + " " + circle.radius); Circle anotherCircle = new Circle(10, 5); System.out.println(anotherCircle.x + " " + anotherCircle.y + " " + anotherCircle.radius); } }

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Thomas

Livello 41 , Bayreuth, Germany

15 ottobre 2022, 07:14

The purpose is, that you can eg. use default parameters. Like here with the circle example. You can create circles with a position (x, y) and a radius. If you do not pass the radius to the constructor (just the coords x, y) then the x, y constructor calls the one x, y, radius constructor and setting the default radius. If you know have a lot of code in your constructor to create the object, then you do not have to duplicate that.

public class Circle {

    public Circle(double x, double y) {
        // instead of duplication the code of the Circle(double, double, double) constructor, we call it with a
        // default value for radius
        this(x, y, 1);
    }

    public Circle(double x, double y, double radius) {
        // a
        // bunch
        // of
        // code
        // ...
        // very
        // complicated one
        // really
        // complicated
    }
}

When you compare that to your code... you call the constructor with three args from the two args one but still do all the 'calculation'. So you missed the purpose. In addition to that you use the radius variable here that isn't declared at all. Look at the default constructor how it has been done there... no further code executed after calling the 3 args constructor. Of course you can do that, too. But you should have a reason for that. And here there is no.

EvelynSt

Livello 10 , Italy

15 ottobre 2022, 10:51

But even like that is not working public Circle(double x, double y) { //write your code here this(x, y,10.00); //this.x=x; //this.y= y; //this.radius = radius; } What i am missing?

Thomas

Livello 41 , Bayreuth, Germany

15 ottobre 2022, 11:11

It seems correct to me. You could try to pass the required 10 as written in the conditions (int) and not 10.00 (double).

EvelynSt

Livello 10 , Italy

15 ottobre 2022, 13:31

It works thanks so much!