15. Initialization Blocks

 

Initialization Blocks

• Initialization blocks are the third place (other than methods and constructors) in a Java program where operations can be performed.
• They don't have names, they can't take arguments, and they don't return anything.
• A static initialization block runs once, when the class is first loaded.
• An instance initialization block runs once every time a new instance is created.
• Instance initialization block code runs right after the class to super() in a constructor and before the constructor's code has run.
• When it's time for initialization blocks to run, if a class has more than one, they will run in the order in which they appear in the class file.
class A {
    A() {
        super();
        System.out.println("4. A's Constructor");
    }


    static {
        System.out.println("1. A's Static Init Block");
    }


    {
        System.out.println("3. A's Instance Init Block");
    }
}


class ShellClass extends A {


    ShellClass() {
        super();
        System.out.println("7. ShellClass's Constructor");
    }


    {
        System.out.println("5. ShellClass's 1st Instance Init Block");
    }


    static {
        System.out.println("2. ShellClass's Static Init Block");
    }


    {
        System.out.println("6. ShellClass's 2nd Instance Init Block");
    }


    public static void main(String[] args) {
        new ShellClass();
        System.out.println("\nCreating Another Instance of ShellClass...\n");
        new ShellClass();
    }
}



• Instance init blocks are often used as a place to put code that all the constructors in a class should share.
• If we make a mistake in our static init blocks, the JVM can throw an exception.
class ShellClass {
    static int x[] = new int[4];
    static {
        x[4] = 5;
    }


    public static void main(String[] args) {
        new ShellClass();
    }
}



class ShellClass {
    int x[] = new int[4];
    {
        x[4] = 5;
    }


    public static void main(String[] args) {
        new ShellClass();
    }
}