What is the use of {} block in Java

Question

Hello Friends what is the use of having just a block in Java class

public class StaticExample {
  {
    System.out.println("I m here...");
  }
}

Other than *instance initialiser*, the magic phrase you may be looking for is *double-brace initialisation idiom*. Yeah, `{` or `{{` is difficult to google. — Tom Hawtin - tackline, Aug 09 '13 at 02:53
@texasbruce: but this is not an example of a static initializer. This is clearly a non-static initializer. — scottb, Aug 09 '13 at 03:59

score 7 · Answer 1 · answered Aug 09 '13 at 02:30

7

That's an initialization block. It gets executed when a new instance is created. If you think that that's a job for the constructor, it is a place where you can put code that is executed no matter which constructor is used. They are executed in the order they appear, before the constructor. If you put static in front of an initialization block, it becomes a static initialization block, which is executed as soon as the class is loaded.

answered Aug 09 '13 at 02:30

tbodt

16,609
6
58
83

2

Technically they're executed after the super constructor call and before the rest of the body. This is actually important if the superclass ctor calls a virtual method. – Antimony Aug 16 '13 at 02:18

Reimeus · Answer 2 · 2013-08-09T03:25:55.170

6

It's called an initializer block and is invoked every time an instance of the class is created.

The Java compiler copies initializer blocks into every constructor. Therefore, this approach can be used to share a block of code between multiple constructors.

The code is invoked before the code in the constructor and doesnt depend on the latter.

public class InitializerExample {

    public InitializerExample() {
        System.out.println("InitializerExample");
    }

    {
        System.out.println("I'm here...");
    }

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

will produce

I'm here...
InitializerExample

It is documented in the official tutorial here

edited Aug 09 '13 at 03:25

answered Aug 09 '13 at 02:30

Reimeus

158,255
15
216
276

1

Are these copied into the constructor at the beginning? The end? Undefined behavior? – nanofarad Aug 09 '13 at 02:33
1

The code is inserted before the constructor – Reimeus Aug 09 '13 at 02:35
1

The initialization block is executed before the constructor body. http://stackoverflow.com/questions/10689224/inline-initialization-blocks-in-java – Jeroen Vannevel Aug 09 '13 at 02:37
5

`super()` first then `{}` then your constructor block code – nachokk Aug 09 '13 at 02:40

What is the use of {} block in Java

2 Answers2