Question

I have a class MyMap which extends java.util.HashMap, the following code works as a block of statements but I don’t understand the use of the extra curly braces

MyMap m = new MyMap() {
  {
      put("some key", "some value");
  }
};

Now why do I need the extra curly braces, can’t I just do this (but this raises compile error)

MyMap m = new MyMap() {
    put("some key", "some value");
};

Answer 1

This:

MyMap m = new MyMap() {
    ....
};

creates an anonymous inner class, which is a subclass of HashMap.

This:

{
    put("some key", "some value");
}

is an instance initializer. The code is executed when the instance of the anonymous subclass is created.