What does this 'static' mean and why is it like that

public class tt {
static{
    System.out.println("cl开发者_JS百科ass tt");
    }
}

It the first time ive come across it and im wondering what it is and what it's used for

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It is the static initialiser of the class. When the class is loaded, the static initialiser is run. It is like the constructor, but for the class rather than for individual objects.

Multiple static initialisers can appear in a class, as well as direct initialisers for static variables. These will be combined into one initialiser in the order in which they are declared. For example, the following will print "foo" to stdout whenever the class is loaded (usually once per application).

public class Foo {

  static String a;

  static {
    a = "foo";
  }

  static String b = a;

  static {
    System.println(b);
  }

}

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Its initilizer block

A static initialization block is a normal block of code enclosed in braces, { }, and preceded by the static keyword. Here is an example:

static {
    // whatever code is needed for initialization goes here
}

A class can have any number of static initialization blocks, and they can appear anywhere in the class body. The runtime system guarantees that static initialization blocks are called in the order that they appear in the source code. There is an alternative to static blocks —you can write a private static method:

class Whatever {
    public static varType myVar = initializeClassVariable();

    private static varType initializeClassVariable() {

        //initialization code goes here
    }
}

The advantage of private static methods is that they can be reused later if you need to reinitialize the class variable.

• Resource

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here is the static initializer tutorial http://download.oracle.com/javase/tutorial/java/javaOO/initial.html

It runs when the class is loaded before the initialization.

public class A {

    static {
        System.out.println("A from static initializer"); // first 
    }

    public A(){
        System.out.println("A"); // second
    }

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

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It is a static initializer. The code inside that block runs when the JVM loads the class, which is immediately before the first time the program needs to do anything with that class (e.g. look up a static field, call a static method, instantiate an object,...).

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It's a static initializer block. It will be executed once when the class is first loaded, along with static field initializers like this:

private static int staticField = someMethod();

The difference is that an initializer block can contain control flow structures like try/catch blocks.