Question

While going through one of the library, I found the following construct in java which is really new for me. Assume there is a class Point in java.

class Point {
    int x;
    int y;
    public Point() {}
    public Point(int x, int y) {
    this.x = x;
    this.y = y;
    }
}

While creating instance for Point, they initialize the variables x and y in the instance creation itself as below :

Point inst = new Point() {
            {
                this.x = 10;
                this.y = 20;
            }
        };

Is this related to instance block in java or something different?

Answer 1

Your second example:

1. creates an anonymous class derived from Point
2. uses an initializer block to intialise values

The syntax:

new Object() { ... }

looks like an Object, but is in fact creating a new object derived from Object. The inner braces then declare the initialiser block.

It’s a practise occassionally used to initialise collections e.g.

new ArrayList<String>(){{
   add("1");
   add("2");
}};

etc. One thing to note is that it’s an inner class and consequently there’s an implict reference to the outer (surrounding) class. Not normally a problem unless (say) you come to serialise this.