Question

I have come across with this syntax while reading some others code

Map<String, String> map = new HashMap<String, String>() {
    { 
         put("a", "b");
    }
};

I know how to use anonymous inner class but this seems something different. Could somebody explain me how exactly the above works and how it is different from Map<String, String> map = new HashMap<String, String>(); map.put("a", "b"); ?

Answer 1

You are basically creating a anonymous class instance and specifying an instance initializer. Think of it in terms of a normal class, e.g.:

class Person {

  String age, name;

  List<String> hobbies;

  {
    hobbies = new ArrayList<String>();
  }

  public Person(String name, String age) {
    this.name = name;
    this.age = age;
  } 

}

What do you think the above is doing? Your anonymous class is doing something similar. 🙂