java variables and parameters

Question

public class Main {

    public static void main(String[] args){

        Class2 class2Object = new Class2();

        //class2Object
        //num1
        class2Object.setNumber(class2Object.number1, 1) ;

        class2Object.getNumber(class2Object.number1);
    }   

}

public class Class2 {
    public int number1;

    public void setNumber(int x, int value){
        x = value;

    }
    public void getNumber(int number){
        System.out.println("Class2, x = "+number);
    }
}

I have 2 class: Class2 and Main. I assign an instance variable to 1 in Main class. Why is class2Object.number1 not assign to a value of 1? Output is 0.

Mark Byers · Accepted Answer · 2012-04-07 03:18:17Z

3

Your setter doesn't do anything:

public void setNumber(int x, int value){
    x = value;   // This just overwrites the value of x!
}

A setter should have only one parameter. You need to assign the received value to the member field:

// The field should be private.
private int number;

public void setNumber(int value){
    this.number = value;
}

edited Apr 7, 2012 at 3:18

answered Apr 7, 2012 at 3:06

Mark Byers

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3 Comments

user1050548 Over a year ago

What if i have a lot of integers like number1, number2... I need a setter for every variables?

sarwar026 Over a year ago

You need array of List in that case, i guess.

user12345613 Over a year ago

Most people have a separate getter and setter for each variable. It makes it much more clear what variable is being set by the method.

user12345613 · Accepted Answer · 2012-04-07 03:57:52Z

2

In Java, primitive types (like int) are passed by value, not by reference.

So saying:

Class2 class2Object = new Class2();
class2Object.setNumber(class2Object.number1, 1) ;

passes class2object.number1 by value, not by reference. So the parameter x in setNumber is a whole new int and is not pointing to the same location as number1 in class2object. The method just overwrites the value of the new int x and does not modify the value pointed to by number1.

In order to set the value it should be:

void setNumber(int newNumber) { this.number1 = newNumber; }

answered Apr 7, 2012 at 3:57

user12345613

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Comments

sarwar026 · Accepted Answer · 2012-04-07 03:20:53Z

1

Since you should assign like this:

private int number1;

public void setNumber(int value){
    this.number1 = value;
}

edited Apr 7, 2012 at 3:20

answered Apr 7, 2012 at 3:08

sarwar026

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2 Comments

twain249 Over a year ago

you forgot to remove the unnecessary parameter.

sarwar026 Over a year ago

ohh sorry, yes @twain249. you are right. I am updating my codes.

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