Java public variable is zero in private class

Question

This is my second day with java, and I can't figure out why public variable is 0 within private method. Code is as follows:

public class SomeClass {

    public int[][] gridArray; 

    private int N;

    public methodOne(int N){

        gridArray = new int[N][N];

    }

    private boolean isValid(int i, int j){
        if (i < 0 || i > N || j < 0 || j > N){
            StdOut.println(i + " and " + j + " and "+ N);
            throw new java.lang.IndexOutOfBoundsException();
        } else{return true;}
    }

    public static void main(String[] args) {
        int N = 5;

    // some code for getting indices

    }

So, I create an array, and input indices, and check is this indices are valid, or the are out of bounds. I always get exception, because within isValid method N is 0, despite declaration in main.

possible duplicate of [Java, What do curly braces mean by themselves?](http://stackoverflow.com/questions/241088/java-what-do-curly-braces-mean-by-themselves) — Damian Leszczyński - Vash, Jul 01 '14 at 08:42
You have tree issues here. You mix the static concept with object. If the main was not static then you will have issue that you hide variables. Last is that you use to simple names of variables. You should follow a simple tutorial rather. — Damian Leszczyński - Vash, Jul 01 '14 at 08:44
"This is my second day with java" SO is not a good resource for those new to programming or a programming language. Read and follow some tutorials and books. — Raedwald, Jul 01 '14 at 09:09

Jamie Cockburn · Accepted Answer · 2014-07-01T09:02:16.050

The N in main is not the same N as that in SomeClass.

This is to do with the concept of scope.

The N in main is local to that method. In other words, it doesn't exist outside that method.

N in the class SomeClass is an instance variable. If you create an instance of SomeClass, it gets its own version of N that goes along with it.

public class SomeClass {
    private static int static_member;
    private int instance_member;

    public void instance_method() {
        // this is completely different from the other
        // local_variable in static_method()
        int local_variable;

        static_member = 1; // legal
        SomeClass.static_member = 1; // legal
        instance_member = 1 // legal
        this.instance_member = 1 // legal
    }

    public static void static_method() {
        // this is completely different from the other
        // local_variable in instance_method()
        int local_variable;

        static_member = 1; // legal
        SomeClass.static_member = 1; // legal
        instance_member = 1 // not legal, in a static method, there is no instance
    }
}

Just because some variable has the same name, if it is in different scope, they are different variables.

Remember, that a class is like a template for objects. You make an object by instantiating the class.

You need to instantiate an instance of SomeClass in main, and then use it. You also need to some how setup the value of N. This is typically done with a constuctor (a special function that in in charge of making instances of classes). You also need to make isValid() a public method of your class, so that it can be called outside of your class.

public class SomeClass {

    public int[][] gridArray; 

    private int N;

    public SomeClass(int N){
        this.N = N;
        this.gridArray = new int[N][N];
    }

    public boolean isValid(int i, int j){
        if (i < 0 || i >= N || j < 0 || j >= N){
            System.out.println(i + " and " + j + " and "+ N);
            throw new IndexOutOfBoundsException();
        } else{return true;}
    }

    public static void main(String[] args) {
        SomeClass instance = new SomeClass(5);
        instance.isValid(4,4);
    }
}

Finally, your logic in is valid is wrong. With an N of 5, your upper bound checks will allow an index of 5, but the maximum index is 4. I've also corrected that above.

@SoumyaKumar `N`is not static, he need to create an object to access it — JohnnyAW, Jul 01 '14 at 08:45

paubo147 · Answer 2 · 2014-07-01T08:57:35.923

0

your problem is, that the scopes are different:

inside the constructor the int N argument is used. So the compiler uses this variable to initialize the array.
the private int N field on the class is not instantiated. Therefore the isValid method uses the default value of the class-field.
inside the main function you are instantiating another variable

The resulting problem is, that you have actually 3 different N inside the code.

if you put this.N = N inside the methodOne and give it a return-type (void), everything should be fine

edited Jul 01 '14 at 08:57

answered Jul 01 '14 at 08:46

paubo147

598
3
8

did not mean inside the main-method. Thought it would be the constructor, but apparently it is just another method. – paubo147 Jul 01 '14 at 08:55

score 0 · Answer 3 · answered Jul 01 '14 at 08:46

The difference is variable defined as instance variable and method variable.

private int N;

This variable N is defined as field memmber (directly inside class), it is a instance variable.

int N = 5;

This int N is defined under main method, it is a method variable.

Both variables have different scope, and can not share the same value.

Java public variable is zero in private class

3 Answers3