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Java Static Keyword

Java Static Keyword

In this tutorial, we will learn about the Java static keyword along with static methods, static variables, and static blocks with the help of examples.

What is a static keyword in Java?

In Java, if we want to access class members, we must first create an instance of the class. But there will be situations where we want to access class members without creating any variables.

In those situations, we can use the static keyword in Java. If we want to access class members without creating an instance of the class, we need to declare the class members static.

The Math class in Java has almost all of its members static. So, we can access its members without creating instances of the Math class. For example,

public class Main {
    public static void main( String[] args ) {

        // accessing the methods of the Math class
        System.out.println("Absolute value of -12 =  " + Math.abs(-12));
        System.out.println("Value of PI = " + Math.PI);
        System.out.println("Value of E = " + Math.E);
        System.out.println("2^2 = " + Math.pow(2,2));
    }
}

Output:

Absolute value of -12 = 12
Value of PI = 3.141592653589793
Value of E = 2.718281828459045
2^2 = 4.0

In the above example, we have not created any instances of the Math class. But we are able to access its methods: abs() and pow() and variables: PI and E.

It is possible because the methods and variables of the Math class are static.


Static Methods

Static methods are also called class methods. It is because a static method belongs to the class rather than the object of a class.

And we can invoke static methods directly using the class name. For example,

class Test {
    // static method inside the Test class
    public static void method() {...}
}

class Main {
    // invoking the static method
    Test.method();
}

Here, we can see that the static method can be accessed directly from other classes using the class name.

In every Java program, we have declared the main method static. It is because to run the program the JVM should be able to invoke the main method during the initial phase where no objects exist in the memory.

Example 1: Java static and non-static Methods

class StaticTest {

    // non-static method
    int multiply(int a, int b){
        return a * b;
    }

    // static method
    static int add(int a, int b){
        return a + b;
    }
}

public class Main {

   public static void main( String[] args ) {

        // create an instance of the StaticTest class
        StaticTest st = new StaticTest();

        // call the nonstatic method
        System.out.println(" 2 * 2 = " + st.multiply(2,2));

        // call the static method
        System.out.println(" 2 + 3 = " + StaticTest.add(2,3));
   }
}

Output:

2 * 2 = 4
2 + 3 = 5

In the above program, we have declared a non-static method named multiply() and a static method named add() inside the class StaticTest.

Inside the Main class, we can see that we are calling the non-static method using the object of the class (st.multiply(2, 2)). However, we are calling the static method by using the class name (StaticTest.add(2, 3)).


Static Variables

In Java, when we create objects of a class, then every object will have its own copy of all the variables of the class. For example,

class Test {
    // regular variable
   int age;
}

class Main {
    // create instances of Test
    Test test1 = new Test();
    Test test2 = new Test();
}

Here, both the objects test1 and test2 will have separate copies of the variable age. And, they are different from each other.

However, if we declare a variable static, all objects of the class share the same static variable. It is because like static methods, static variables are also associated with the class. And, we don't need to create objects of the class to access the static variables. For example,

class Test {
    // static variable
    static int age;
}
class Main {
    // access the static variable
    Test.age = 20;
}

Here, we can see that we are accessing the static variable from the other class using the class name.

Example 2: Java static and non-static Variables

class Test {

   // static variable
   static int max = 10;
  
   // non-static variable
   int min = 5;
}

public class Main {
   public static void main(String[] args) {
       Test obj = new Test();

       // access the non-static variable
       System.out.println("min + 1 = " + (obj.min + 1));

       // access the static variable
       System.out.println("max + 1 = " + (Test.max + 1));
   }
}

Output:

min + 1 = 6
max + 1 = 11

In the above program, we have declared a non-static variable named min and a static variable named max inside the class Test.

Inside the Main class, we can see that we are calling the non-static variable using the object of the class (obj.min + 1). However, we are calling the static variable by using the class name (Test.max + 1).

Note: Static variables are rarely used in Java. Instead, the static constants are used. These static constants are defined by static final keyword and represented in uppercase. This is why some people prefer to use uppercase for static variables as well.


Access static Variables and Methods within the Class

We are accessing the static variable from another class. Hence, we have used the class name to access it. However, if we want to access the static member from inside the class, it can be accessed directly. For example,

public class Main {

   // static variable
   static int age;
   // static method
   static void display() {
       System.out.println("Static Method");
   }
   public static void main(String[] args) {

       // access the static variable
       age = 30;
       System.out.println("Age is " + age);

       // access the static method
       display();
   }
}

Output:

Age is 30
Static Method

Here, we are able to access the static variable and method directly without using the class name. It is because static variables and methods are by default public. And, since we are accessing from the same class, we don't have to specify the class name.


Static Blocks

In Java, static blocks are used to initialize the static variables. For example,

class Test {
    // static variable
    static int age;

    // static block
    static {
        age = 23;
    }
}

Here we can see that we have used a static block with the syntax:

static {
    // variable initialization
}

The static block is executed only once when the class is loaded in memory. The class is loaded if either the object of the class is requested in code or the static members are requested in code.

A class can have multiple static blocks and each static block is executed in the same sequence in which they have been written in a program.

Example 3: Use of static block in java

class Main {

   // static variables
   static int a = 23;
   static int b;
   static int max;

   // static blocks
   static {
       System.out.println("First Static block.");
       b = a * 4;
   }
   static {
       System.out.println("Second Static block.");
       max = 30;
   }

   // static method
   static void display() {

       System.out.println("a = " + a);
       System.out.println("b = " + b);
       System.out.println("max = " + max);
   }

   public static void main(String args[]) {
       // calling the static method
       display();
   }
}

Output:

First Static block.
Second Static block.
a = 23
b = 92
max = 30

In the above program. as soon as the Main class is loaded,

  • The value of a is set to 23.
  • The first static block is executed. Hence, the string First Static block is printed and the value of b is set to a * 4.
  • The second static block is executed. Hence, the string Second Static block is printed and the value of max is set to 30.
  • And finally, the print statements inside the method display() are executed.

Nested Static Class

In Java, we can declare a class inside another class. Such classes are known as nested classes. Nested classes are of 2 types:

  • Static Nested Classes
  • Non-static Nested Classes

For example,

class OuterClass {
    // static nested class
    static class NestedClass {...}

    // non-static nested class
    class InnerClass {...}
}

To learn more, visit the Java Nested Class.