Java 7 introduced a new operator - <> - referred to as the “diamond” operator. This new keystroke-saving syntax allows you to maintain the benefits of compile-time generics without typing out long strings of redundant type parameters. Prior to Java 7, creating and initializing a variable with nested type parameters was an arduous task:

HashMap<TreeSet<TreeMap<String, Integer>>, Double> myTerribleDataStructure =
    new HashMap<TreeSet<TreeMap<String, Integer>>, Double>();

The diamond operator greatly simplifies this unnecessary complexity. The following is semantically identical to the above example:

HashMap<TreeSet<TreeMap<String, Integer>>, Double> myTerribleDataStructure = new HashMap<>();

Note that this is not the same as:

HashMap<TreeSet<TreeMap<String, Integer>>, Double> myTerribleDataStructure = new HashMap();

Without the diamond operator, myTerribleDataStructure is initialized to the raw HashMap type instead of its type-specific counterpart.