Diamond with anonymous classes

Code Comparison

Map<String, List<String>> map =
    new HashMap<String, List<String>>();
// anonymous class: no diamond
Predicate<String> p =
    new Predicate<String>() {
        public boolean test(String s) {..}
    };

Map<String, List<String>> map =
    new HashMap<>();
// Java 9: diamond with anonymous classes
Predicate<String> p =
    new Predicate<>() {
        public boolean test(String s) {..}
    };

Why the modern way wins

📏

Consistent rules

Diamond works everywhere — constructors and anonymous classes alike.

🧹

Less redundancy

Type arguments are stated once on the left, never repeated.

🔧

DRY principle

The compiler already knows the type — why write it twice?

Old Approach

Repeat Type Args

Modern Approach

Diamond <>

JDK Support

Diamond with anonymous classes

Available

Diamond with anonymous classes since JDK 9 (Sept 2017).

How it works

Java 7 introduced <> but it didn't work with anonymous inner classes. Java 9 fixed this, so you never need to repeat type arguments on the right-hand side.