CODEWITHSUNDEEP
javaclass
Vishvendra Singh
Vishvendra Singh

Reputation: 187

trying to reassign value to class instance variables

I am being beginner trying to learn java basics and here in the this program I am confused why we can't reassign the value of class instance variable.enter image description here this is error in this program. Please guys help me out to figure it out. thanks

class AddInsideClassVar{
    int a = 3;
    int c;
    c = a + a;
    public static void main(String args[]){
        System.out.println();
    }
}

Upvotes: 1

Views: 365

Answers (5)

MadSeb
MadSeb

Reputation: 8254

Explanation : int c = a + a is a declaration whereas " c = a + a ; " ( alone ) is a statement ; You have a point, it does not make much sense ; 

class MyClass {
    int a = 3;
    int c = a + a; // Correct
}

or

class MyClass {
    int a = 3;
    int c;
    public void addition () {
        c = a + a; // Correct
    }
}

but not 

class MyClass {
 int a = 3;
 int c;
 c = a + a; // Incorrect
}

NOTE : On the other, the Scala programming language ( compiles to JVM ) allows you to do the following : 

scala> class MyClass { val a:Int = 3; 
var c:Int = _; 
c = a + a ;  // Correct
}
defined class MyClass

Upvotes: 0

ErstwhileIII
ErstwhileIII

Reputation: 4863

You may define fields within a class, but you are not allowed to put calculation statements outside of a method definition. A field declaration is of the form type; or type = value;

For example (from your code);

class AddInsideClassVar{
    static int a = 3;        // ok this is a declaration for a field (variable)
    static int c;            // ok, this is too
    //c = a + a;        // this is a statement and not a declaration. A field may be 
                      // declared only once
    static int d = a + a;    // this will work since it is part of a declaration.

    public static void main(String args[]){
        System.out.println("a=" + a + ", c=" + c + ", d=" + d);

    }
}

Upvotes: 2

Ali Cheaito
Ali Cheaito

Reputation: 3856

You may be mixing your statics with instance variables. Here's how I would write this to not confuse myself:

public class AddInsideClassVar{
    int a;
    int c;

    public void doStuff() {
        a = 3;              
        c = a + a;
    }

    public static void main(String args[]){
        AddInsideClassVar instance = new AddInsideClassVar();
        instance.doStuff(); 
        System.out.println(c);
    }
}

a and c are instance variables. They are manipulated by a non-static method which requires an instance of the class to operate on. So, I create the instance in main(), then call the function to manipulate instance variables.

Upvotes: 0

sybear
sybear

Reputation: 7784

You can try this (just an example of workaround):

class AddInsideClassVar{
    static {
        int a = 3;
        int c;
        c = a + a;
        System.out.println(c);
    }

    public static void main(String args[]){

    }
}

Upvotes: 1

user695992
user695992

Reputation:

You cannot execute c = a + a in that section. If anything you'd need to do 

int a = 3;
int c = a + a;

If you make these variables static then you could do

private static int a = 3;
private static int c;
static {
    c = a + a;
}

Upvotes: 1

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