CODEWITHSUNDEEP
java
kemparaj565
kemparaj565

Reputation: 385

Confusion in Constructor in java

When trying to initialize the instance variable from constructor by keeping the parameter name as instance variable name. I am receiving the output as 0. instead of the value passed. Kindly explain? 

public class Circle
    {
         int x;
        int y;
        int radius;

    //Constructor with same parameters of field name

    public Circle(int x,int y,int radius)
    {
        x=x;
        y=y;
        radius=radius;
    }
    //Overridden to String()

    public String toString()
    {
        return "center("+x+" , "+y+") and radius ("+radius+")";
    }

    //Main method 

    public static void main(String[] args) 
    {
        System.out.println(new Circle(5,5,50));
    }


    }

When i pass values from constructor 5,5,50 to the parameters of constructor. Shouldn't they display the same value.

System.out.println(new Circle(5,5,50)); // Value sent to constructor

public Circle(int x,int y,int radius) // value to be taken by constructor parameters

Upvotes: 0

Views: 286

Answers (5)

shikjohari
shikjohari

Reputation: 2288

A Local Variable Shadows An Instance Variable

The local variable in your constructor shadows the instance variable Whereever you are assigning and reading an instance variable, use this keyword

Consider the below example

public class Employee{
    int id;
    String name;

public Employee(int id, String name){
    this.id = id; //sets the id to the Employee's id
    this.name = name; //sets the name to Employee's name
}
}

What you have done in your constructor will not have any effect. If you are using eclipse you can easily see a warning saying The assignment to variable id has no effect

When you are doing new Circle(5,5,50)), your constructor is just reassigning the local varaibles and not even touching the instance variable.

But when you are printing the value with return "center("+x+" , "+y+") and radius ("+radius+")"; you are referring to the instance variables. Because there was no shadowing in your toString method. Thats why you are getting 0s as all the instance variables are initialized with the default values

Upvotes: 0

Bathsheba
Bathsheba

Reputation: 234875

You need to use this.x = x etc.

The x passed as a parameter shadows the field. When this happens, you can use this.x to denote the field.

Upvotes: 5

AlexR
AlexR

Reputation: 115388

Line x=x; does not have any sense because it assigns value of variable x into the same variable.

If you have field named x you should refer to it using this keyword: this.x = x. In this case you will assign value of argument x to member x.

This however does not mean that you always have to use this to refer to members. You have to do this if you have local variable with the same identifier it your scope.

Upvotes: 0

TheLostMind
TheLostMind

Reputation: 36304

This is where you use this :

public Circle(int x,int y,int radius)
{
    this.x=x;
    this.y=y;
    this.radius=radius;
}

without this you are actually working only on local variables x,y etc.

Upvotes: 0

Eran
Eran

Reputation: 394126

The constructor's arguments hide the members of your class.

Change your constructor to:

public Circle(int x,int y,int radius)
{
    this.x=x;
    this.y=y;
    this.radius=radius;
}

Or use different names for the members and the arguments.

Upvotes: 1

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