What is an initialization block?

Question

We can put code in a constructor or a method or an initialization block. What is the use of initialization block? Is it necessary that every java program must have it?

Answer 1

would like to add to @aioobe's answer

Order of execution:

1. static initialization blocks of super classes

2. static initialization blocks of the class

3. instance initialization blocks of super classes

4. constructors of super classes

5. instance initialization blocks of the class

6. constructor of the class.

A couple of additional points to keep in mind (point 1 is reiteration of @aioobe's answer):

1. The code in static initialization block will be executed at class load time (and yes, that means only once per class load), before any instances of the class are constructed and before any static methods are called.

2. The instance initialization block is actually copied by the Java compiler into every constructor the class has. So every time the code in instance initialization block is executed exactly before the code in constructor.

Answer 2

Initializer block contains the code that is always executed whenever an instance is created. It is used to declare/initialise the common part of various constructors of a class.

The order of initialization constructors and initializer block doesn’t matter, initializer block is always executed before constructor.

What if we want to execute some code once for all objects of a class?

We use Static Block in Java.

Answer 3

The sample code, which is approved as an answer here is correct, but I disagree with it. It does not shows what is happening and I'm going to show you a good example to understand how actually the JVM works:

package test;

    class A {
        A() {
            print();
        }

        void print() {
            System.out.println("A");
        }
    }

    class B extends A {
        static int staticVariable2 = 123456;
        static int staticVariable;

        static
        {
            System.out.println(staticVariable2);
            System.out.println("Static Initialization block");
            staticVariable = Math.round(3.5f);
        }

        int instanceVariable;

        {
            System.out.println("Initialization block");
            instanceVariable = Math.round(3.5f);
            staticVariable = Math.round(3.5f);
        }

        B() {
            System.out.println("Constructor");
        }

        public static void main(String[] args) {
            A a = new B();
            a.print();
            System.out.println("main");
        }

        void print() {
            System.out.println(instanceVariable);
        }

        static void somethingElse() {
            System.out.println("Static method");
        }
    }

Before to start commenting on the source code, I'll give you a short explanation of static variables of a class:

First thing is that they are called class variables, they belong to the class not to particular instance of the class. All instances of the class share this static(class) variable. Each and every variable has a default value, depending on primitive or reference type. Another thing is when you reassign the static variable in some of the members of the class (initialization blocks, constructors, methods, properties) and doing so you are changing the value of the static variable not for particular instance, you are changing it for all instances. To conclude static part I will say that the static variables of a class are created not when you instantiate for first time the class, they are created when you define your class, they exist in JVM without the need of any instances. Therefor the correct access of static members from external class (class in which they are not defined) is by using the class name following by dot and then the static member, which you want to access (template: <CLASS_NAME>.<STATIC_VARIABLE_NAME>).

Now let's look at the code above:

The entry point is the main method - there are just three lines of code. I want to refer to the example which is currently approved. According to it the first thing which must be printed after printing "Static Initialization block" is "Initialization block" and here is my disagreement, the non-static initialization block is not called before the constructor, it is called before any initializations of the constructors of the class in which the initialization block is defined. The constructor of the class is the first thing involved when you create an object (instance of the class) and then when you enter the constructor the first part called is either implicit (default) super constructor or explicit super constructor or explicit call to another overloaded constructor (but at some point if there is a chain of overloaded constructors, the last one calls a super constructor, implicitly or explicitly).

There is polymorphic creation of an object, but before to enter the class B and its main method, the JVM initializes all class(static) variables, then goes through the static initialization blocks if any exist and then enters the class B and starts with the execution of the main method. It goes to the constructor of class B then immediately (implicitly) calls constructor of class A, using polymorphism the method(overridden method) called in the body of the constructor of class A is the one which is defined in class B and in this case the variable named instanceVariable is used before reinitialization. After closing the constructor of class B the thread is returned to constructor of class B but it goes first to the non-static initialization block before printing "Constructor". For better understanding debug it with some IDE, I prefer Eclipse.

Answer 4

Initialization blocks are executed whenever the class is initialized and before constructors are invoked. They are typically placed above the constructors within braces. It is not at all necessary to include them in your classes.

They are typically used to initialize reference variables. This page gives a good explanation