What is an initialization block?

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.

In addition to what was said in previous answers, blocks can be synchronized .. never felt I need to use it, however,it's there

Just to add to the excellent answers from @aioobe and @Biman Tripathy.

A static initializer is the equivalent of a constructor in the static context. which is needed to setup the static environment. A instance initializer is best for anonymous inner classes.

• It is also possible to have multiple initializer blocks in class
• When we have multiple initializer blocks they are executed (actually copied to constructors by JVM) in the order they appear
• Order of initializer blocks matters, but order of initializer blocks mixed with Constructors doesn't
• Abstract classes can also have both static and instance initializer blocks.

Code Demo -

abstract class Aircraft {

    protected Integer seatCapacity;

    {   // Initial block 1, Before Constructor
        System.out.println("Executing: Initial Block 1");
    }

    Aircraft() {
        System.out.println("Executing: Aircraft constructor");
    }

    {   // Initial block 2, After Constructor
        System.out.println("Executing: Initial Block 2");
    }

}

class SupersonicAircraft extends Aircraft {

    {   // Initial block 3, Internalizing a instance variable
        seatCapacity = 300;
        System.out.println("Executing: Initial Block 3");
    }

    {   // Initial block 4
        System.out.println("Executing: Initial Block 4");
    }

    SupersonicAircraft() {
        System.out.println("Executing: SupersonicAircraft constructor");
    }
}

An instance creation of SupersonicAircraft will produce logs in below order

Executing: Initial Block 1
Executing: Initial Block 2
Executing: Aircraft constructor
Executing: Initial Block 3
Executing: Initial Block 4
Executing: SupersonicAircraft constructor
Seat Capacity - 300