Static vs Instance Variables: Difference?

Question

What is the difference between a static and instance variable. The following sentence is what I cant get:

In certain cases, only one copy of a particular variable should be shared by all objects of a class- here a static variable is used.
A static variable represents class wide info.All objects of a class share the same data.

I thought that instance vars were used class wide whereas static variables only had scope within their own methods?

Foreshore

possible duplicate of What does the 'static' keyword do in a class? – Letters

You're confusing static and local. Variables declared inside a method are local and only exist while that method is invoked. Static variables are similar to instance variables except that they belong to the actual Class object rather than a specific instance of the class, and hence the SAME variable can be accessed from all instances of the class. – Mitsukomitt

Answer 1

In the context of class attributes, static has a different meaning. If you have a field like:

private static int sharedAttribute;

then, each and every instance of the class will share the same variable, so that if you change it in one instance, the change will reflect in all instances, created either before or after the change.

Thus said, you might understand that this is bad in many cases, because it can easiy turn into an undesired side-effect: changing object a also affects b and you might end up wondering why b changed with no apparent reasons. Anyway, there are cases where this behaviour is absolutely desirable:

1. class constants: since they are const, having all the classes access the same value will do no harm, because no one can change that. They can save memory too, if you have a lot of instances of that class. Not sure about concurrent access, though.
2. variables that are intended to be shared, such as reference counters &co.

static vars are instantiated before your program starts, so if you have too many of them, you could slow down startup.

A static method can only access static attributes, but think twice before trying this.

Rule of thumb: don't use static, unless it is necessary and you know what you are doing or you are declaring a class constant.

Sleek

thanks, So if declaring a static variable, i would only do so if i was interested in being able to change all of the values for each object of that class, for example if i declared a static var for rate_of_pay for employees I could then update all employees pay rate by doing something like employee.rate_of_pay? – Foreshore

Exactly. Obviously, that change change will affect only operation happening after it. So if you want to, say, recalculate wages after changing rate, you will have to call recalculateWage() or similar after the change. – Sleek

Answer 2

Say there is a test class:

class Test{
public static int a = 5;
public int b = 10;
}
// here t1 and t2 will have a separate copy of b
// while they will have same copy of a.
Test t1 = new test(); 
Test t2 = new test();

You can access a static variable with it's class Name like this

Test.a = 1//some value But you can not access instance variable like this
System.out.println(t1.a);
System.out.println(t2.a);

In both cases output will be 1 as a is share by all instances of the test class. while the instance variable will each have separate copy of b (instance variable) So

 t1.b = 15 // will not be reflected in t2.
 System.out.println(t1.b); // this will print 15
 System.out.println(t2.b); / this will still print 10; 

Hope that explains your query.

Curiel

Answer 3

Suppose we create a static variable K and in the main function we create three objects: ob1 ob2 ob3; All these objects can have the same value for variable K. In contrast if the variable K was an instance variable then it could have different values as: ob1.k ob2.k ob3.k

Rickard

Answer 4

I think you are thinking about the C/C++ definition of the static keyword. There, the static keyword has many uses. In Java, the static keyword's functionality is described in your post. Anyhow, you can try it for yourself:

public class Test_Static{
    static int x;
    public static void main(String[] argv){
        Test_Static a = new Test_Static();
        Test_Static b = new Test_Static();
        a.x = 1; // This will give an error, but still compile.
        b.x = 2;
        System.out.println(a.x); // Should print 2
    }
}

and similarly for non static variables:

public class Test_NonStatic{
     int x;
     public static void main(String [] argv){
         Test_NonStatic a = new Test_NonStatic();
         Test_NonStatic b = new Test_NonStatic();
         a.x = 1;
         b.x = 2;
         System.out.println(a.x); // Should print 1.
     }
}

Joachima

Answer 5

Consider a class MyClass, having one static and one non-static member:

public class MyClass {
    public static int STATICVARIABLE = 0;
    public int nonStaticVariable = 0;
}

Now, let's create a main() to create a couple of instances:

public class AnotherClass{  
    public static void main(String[] args) {    
        // Create two instances of MyClass
        MyClass obj1  = new MyClass();
        MyClass obj2  = new MyClass();
        obj1.nonStaticVariable = 30;  // Setting value for nonstatic varibale
        obj1.STATICVARIABLE = 40; //Setting value for static variable       
        obj2.nonStaticVariable = 50;
        obj2.STATICVARIABLE = 60;

        // Print the values actually set for static and non-static variables.
        System.out.println(obj1.STATICVARIABLE);
        System.out.println(obj1.nonStaticVariable);
        System.out.println(obj2.STATICVARIABLE);
        System.out.println(obj2.nonStaticVariable);
    }
}

Result:

60
30
60
50

Now you can see value of the static variable printed 60 both the times, as both obj1 and obj2 were referring to the same variable. With the non-static variable, the outputs differ, as each object when created keeps its own copy of non-static variable; changes made to them do not impact on the other copy of the variable created by another object.

Cutter

Answer 6

Instance Variables

• Any variable that is defined in class body and outside bodies of methods; and it should not be declared static, abstract, stricftp, synchronized, and native modifier.
• An instance variable cannot live without its object, and it is a part of the object.
• Every object has their own copies of instance variables.

Static Variables (class variables)

• Use static modifier

• Belong to the class (not to an object of the class)

• One copy of a static variable

• Initialize only once at the start of the execution.

• Enjoy the program’s lifetime

Rothschild