Java constructor issue

Question

I can not figure out why this code:

class HelloWorld
{
    HelloWorld()
    {
        System.out.println("1 cnstr ");
    }

    public static void main(String[] args)
    {
        HelloWorld a = new HelloWorld();
    }


    {
        System.out.println("2 cnstr ");
    }
}

gives me the output:

2 cnstr
1 cnstr

Why does line System.out.println("2 cnstr "); work? What kind of block is it? I used jdb and found that JVM starts block with this line in HelloWorld() constructor before any line in it.

Thanks for your help.

You can also use `static { }` to initialize `static` variables which are declared outside constructor. — giannis christofakis, Jan 06 '13 at 12:50

score 6 · Accepted Answer · edited Jan 06 '13 at 11:44

6

{
        System.out.println("2 cnstr ");
    }

Its an Instance Initialization block. It is run before constructor of the class is executed.

From Documentation:

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

edited Jan 06 '13 at 11:44

Karthik T

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answered Jan 06 '13 at 11:36

PermGenError

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score 5 · Answer 2 · answered Jan 06 '13 at 11:37

5

It's an instance initializer block, which gets executed before each constructor.

answered Jan 06 '13 at 11:37

assylias

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Initialization block of the constructor or of the whole class? – alexvassel Jan 06 '13 at 11:38
2

Such a block is used in every constructor of that class. http://docs.oracle.com/javase/tutorial/java/javaOO/initial.html – Jeroen Vannevel Jan 06 '13 at 11:40

Java constructor issue

2 Answers2