Static block in java

In this post, we will see about how to implement static block in java.

Here are some important points about static block in java.

1. Static block is used for static initializations of a class
2. Static block is executed only once when either you create an object of the class or access static member of the class
3. Static block is executed before constructor of object is called.

Syntax

static{
   // statements
}

Example

Let’s see with the help of example.
Create a class App.java

package org.arpit.java2blog;

public class App {

    static int a;
    int b;

    static {
        a = 100;
        System.out.println("Static block is called");
    }

    App(int b)
    {
        this.b=b;
        System.out.println("Constructor is called");
    }
}

Create a Main class named StaticBlockMain.java

package org.arpit.java2blog;

public class StaticBlockMain {

    public static void main(String[] args) {
        System.out.println(App.a);
    }
}

When you run above program, you will get below output:

As you can see, when we accessed static variable of class, static block was called.
Let’s see another example and change content of StaticBlockMain.java as below.

package org.arpit.java2blog;

public class StaticBlockMain {

    public static void main(String[] args) {
        App app=new App(300);
        System.out.println("Static variable: "+App.a);
        System.out.println("Instance variable: "+app.b);
    }
}

Output:

As you can see, static block is called before constructor of object is called.
You can not use instance variable in static block.
This is similar to static method, you can not use instance variable in static block otherwise you will get compilation error with Cannot make a static reference to the non-static field b.

Use multiple static blocks

You can use multiple static blocks in class and they will called from top to down.

Let’s understand with the help of example.

package org.arpit.java2blog;

public class App {

    static int a;
    static int b;
    static int c;

    static {
        a = 100;
        System.out.println("Static block 1 is called");
    }

    static {
        b = 200;
        System.out.println("Static block 2 is called");
    }

    static {
        c = 300;
        System.out.println("Static block 3 is called");
    }

    App()
    {
        System.out.println("Constructor is called");
    }
}

Create main class StaticBlockMain.java

package org.arpit.java2blog;

public class StaticBlockMain {

    public static void main(String[] args) {
        System.out.println("Static variable 1: "+App.a);
        System.out.println("Static variable 1: "+App.b);
        System.out.println("Static variable 1: "+App.c);
    }
}

Output:

When we first accessed static variable App.a,all static blocks were called in top to down sequence.
That’s all about static block in java.