How do I create static variables in Java?

Class variables or static variable is variable that declared with static modifier. A given class will have only one copy of each of its static variables, regardless of how many times the class has been instantiated.

If the value of a static variable is changed, the new value is available equally in all instances of the class. The final keyword could be added to indicate the value of static variable will never change.

If you try to assign a new value to final variable, you will get a compile error.

package org.kodejava.basic;

public class StaticDemo {
    // static variable with final value that never change
    final static int Y = 20;
    // static variable
    static int x = 12;
    // non-static variable
    int z;

    public static void main(String[] args) {
        StaticDemo sd0 = new StaticDemo();

        System.out.println("x before update = " + StaticDemo.x);
        System.out.println("y= " + StaticDemo.Y);

        sd0.z = StaticDemo.x + StaticDemo.Y;
        System.out.println("z= " + sd0.z);

        StaticDemo.x = 15;
        System.out.println("x after update = " + StaticDemo.x);

        StaticDemo sd1 = new StaticDemo();
        StaticDemo sd2 = new StaticDemo();
        StaticDemo.x = 20;

        System.out.println("StaticDemo.x = " + StaticDemo.x);
        System.out.println("sd0 = " + sd0.getX());
        System.out.println("sd1 = " + sd1.getX());
        System.out.println("sd2 = " + sd2.getX());

        //
        // try to assign value to final variable, it will cause a
        // compile time error
        //
        // StaticDemo.Y = 30;
    }

    public int getX() {
        return StaticDemo.x;
    }
}

Here is the output printed by the program:

x before update = 12
y= 20
z= 32
x after update = 15
StaticDemo.x = 20
sd0 = 20
sd1 = 20
sd2 = 20

How do I use static import feature?

In order to use a static member of a class in Java we have to qualify the reference with the class name where they came from. For instance to access the PI and abs() from the Math class we should write:

double circle = Math.PI * 10;
int absolute = Math.abs(-100);

For some time you might want to call the members without the class name. This is allowed in Java 5.0 by using a feature called static import. It’s an import statement that allows you to statically import static class member. A static import declaration enables you to refer to imported static members as if they were declared in the class that uses them, the class name and a dot (.) are not required to use an imported static member.

You can write something like the following to static import.

import static java.lang.Math.PI;
import static java.lang.Math.*;

For a clear code it is better to import each member separately and not using the “*” to import every static member in your code.

Let’s see a simple static import below:

package org.kodejava.basic;

import java.util.Date;

import static java.lang.Math.PI;
import static java.lang.Math.abs;
import static java.lang.System.out;

public class StaticImport {
    public static void main(String[] args) {
        // Using static field PI and static method abs() from the
        // java.lang.Math class.
        double circle = PI * 10;
        int absolute = abs(-100);

        // Using static field of the java.lang.System class to
        // print out the current date.
        out.println("Today: " + new Date());
    }
}