Java: Initialising complex types at declaration

One of the many things I like about Python is it’s expressiveness, especially around initialising complex types. This made me look into ways of achieving the same thing in Java. Most programmers will be aware of initialising arrays at declaration:

    String[] anArray = {"One", "Two", "Three"};

Using the java.util.Arrays class, you can quickly do the same thing for Lists too:

    List<String> aList = Arrays.asList( 
            new String[]{"One", "Two", "Three"} 
    );

But what about more complex types like Maps and Sets?

Well, first you define an anonymous inner subclass of the complex type, and then put your initialisation code in the instance constructor:

    Map<String, String> aMap = new HashMap<String, String>() {
       {
           put("One", "1");
           put("Two", "2");
           put("Three", "3");
       }
   };

Note that while your variable can be an abstract type (‘Map’ in this case), your anonymous declaration must extend a concrete implementation (‘HashMap’ in this case).

Using this, we can also initialise a Set -

    Set<String> aSet = new HashSet<String>() {
       {
           add("One");
           add("Two");
           add("Three");
       }
   };

And finally, if you don’t want to use the Arrays.asList(…) detour to initialise your List, you can do it using the same approach -

    List<String> anotherList = new Vector<String>() {
       {
           add("One");
           add("Two");
           add("Three");
       }
   };

And that’s how I initialise complex types at declaration in Java.