Why am I getting a nullpointer exception?

Question about the task

Constructor

Java Syntax, Level 5, Lesson 9

Under discussion

The class inside of a class is confusing me. Thanks in advance.

package com.codegym.task.task05.task0523;

/*
Constructor

*/

public class Circle {
    public Color color;

    public static void main(String[] args) {
        Circle circle = new Circle();
        circle.color.setDescription("Red");
        System.out.println(circle.color.getDescription());
    }

    public void Circle() {
        Color color1 = new Color();
        this.color = color1;
    }

    public class Color {
        String description;

        public String getDescription() {
            return this.description;
        }

        public void setDescription(String description) {
            this.description = description;
        }
    }
}

package com.codegym.task.task05.task0523; /* Constructor */ public class Circle { public Color color; public static void main(String[] args) { Circle circle = new Circle(); circle.color.setDescription("Red"); System.out.println(circle.color.getDescription()); } public void Circle() { Color color1 = new Color(); this.color = color1; } public class Color { String description; public String getDescription() { return this.description; } public void setDescription(String description) { this.description = description; } } }

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DavidJugaad

Level 10

29 July 2021, 18:33

Read carefully the differences between declaration of an initializer and a constructor here: https://codegym.cc/quests/lectures/questsyntax.level05.lecture08 Come back to the default constructor of Circle () as Ali said, and check it out. It´s a common mistake... Good luck

Ali

Level 41 , Nashville, United States

15 March 2021, 22:35

public void Circle() should be corrected to ....... and then only one line in the body

Gellert Varga

Level 23 , Szekesfehervar, Hungary

15 March 2021, 02:17

When you run this program, you get the error message that nullpointer in line 13. This means that you call a method (setDescription) instead of a real object on a "null" reference: circle.color.setDescription ("Red"); So either the circle or color variable has not been given an initial value yet, one of them is still null. Line 12 creates a Circle object. The Circle constructor theoretically gives an initial value to the color variable of the circle object. But according to the error message, does not. As CodeGym hint says, take a very close look at that Circle constructor: why not give the initial value a color variable? There is something wrong with it, some typo...

Guadalupe Gagnon

Level 37 , Tampa, United States

15 March 2021, 13:33

The problem is that the variable 'color' is never initialized. The reason is because I believe you're intentions were to have the constructor of the Circle class initialize 'color', however there is no constructor. Here is how to implement a constructor: AccessLevel ClassName (Params) { } Where AccessLevel is either public, protected, private, or blank; the ClassName is the exact name of the class; and then Params are the parameters are what arguments should be used when calling the constructor. What is in the code here, on line 17, is a method. A method looks like this: [optional: static] [AccessLevel] [ReturnType] [MethodName] (Params) { } The MAIN difference is that constructors do not have a return type, while methods MUST have one included. If the method does not return anything then it still needs to have 'void' as its return type. Constructors are implicitly static, but methods don't have to be, its optional for them to be static; AccessLevel can be declared exactly like a constructors, and means the same in both cases; MethodName can be anything only restricted by Java's default naming rules; and Params word in the same way as a constructors params. tldr:

// constructor
Cirlce() {
}

// method (because it has a return type)
void Cirlce() {
}

Gellert Varga

Level 23 , Szekesfehervar, Hungary

15 March 2021, 21:50

Thank You!:) i know this is not a real constructor, just it seems like it would be that:

public void Circle() {
    Color color1 = new Color();
    this.color = color1;
}

I think this method was originally planned to be a constructor, just it got this mistake (return-type: "void"). If this method will be corrected to a right constructor again, the whole code will work well.