What is the rationale behind this code block in java?

Question

One point is that it will execute whichever constructor is called. If you have several constructors and they don't call each other (for whatever reason, e.g. each wanting to call a directly-corresponding superclass constructor) this is one way of making sure the same code is executed for all constructors, without putting it in a method which could be called elsewhere.

It's also potentially useful when you're writing an anonymous class - you can't write a constructor, but you can write an initializer block. I've seen this used for JMock tests, for example.

It's called an initializer block.

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

It called init block. In such block you can perform logic that are same for all constructions also you can separate declaration and initialization of same fields.

upd and of course double brace initialization, like

List<Integer> answers = new ArrayList<Integer>(){{add(42);}}

This is an initialization block. As mentioned by Matt Ball, they are copied into each constructor.

You might be interested to know about static initialization blocks (also in Matt's link):

public class Foo {
    static {
        System.out.println("class Foo just got initialized!");
    }

    {
        System.out.println("an instance of Foo just got initialized!");
    }
}