Static Block in Java [duplicate]

Question

I was looking over some code the other day and I came across:

static {     ... } 

Coming from C++, I had no idea why that was there. Its not an error because the code compiled fine. What is this "static" block of code?

Answer 1

It's a static initializer. It's executed when the class is loaded (or initialized, to be precise, but you usually don't notice the difference).

It can be thought of as a "class constructor".

Note that there are also instance initializers, which look the same, except that they don't have the static keyword. Those are run in addition to the code in the constructor when a new instance of the object is created.

Answer 2

It is a static initializer. It's executed when the class is loaded and a good place to put initialization of static variables.

From http://java.sun.com/docs/books/tutorial/java/javaOO/initial.html

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.

If you have a class with a static look-up map it could look like this

class MyClass {     static Map<Double, String> labels;     static {         labels = new HashMap<Double, String>();         labels.put(5.5, "five and a half");         labels.put(7.1, "seven point 1");     }     //... } 

It's useful since the above static field could not have been initialized using labels = .... It needs to call the put-method somehow.