BiFunction & Supplier (Java 8)

BiFunction<T, U, R>

• What: A functional interface that takes two inputs and returns one result.
• Signature: R apply(T t, U u)
• Extras: default <V> BiFunction<T, U, V> andThen(Function<? super R, ? extends V> after)
• When to use: combine two values, compute/merge/map from two inputs.
• Related:
  • BinaryOperator<T> = BiFunction<T,T,T> (same type for both inputs & result)
  • Primitive specials: ToIntBiFunction<T,U>, ToLongBiFunction<T,U>, ToDoubleBiFunction<T,U>

Examples

// combine two inputs
BiFunction<Integer, Double, Double> total = (qty, price) -> qty * price;

// chain result
BiFunction<Integer, Double, String> label =
    total.andThen(t -> "Total: " + t);

// Map.merge uses a BiFunction
Map<String,Integer> counts = new HashMap<>();
counts.merge("apple", 1, Integer::sum);  // (oldVal, newVal) -> oldVal + newVal

Supplier

• What: A functional interface that supplies a value; takes no input, returns T.
• Signature: T get()
• When to use: lazy creation, factories, fallbacks, stream generators.
• Related: Primitive specials: IntSupplier, LongSupplier, DoubleSupplier, BooleanSupplier

Examples

// constructor/factory
Supplier<List<String>> listFactory = ArrayList::new;
List<String> list = listFactory.get();

// lazy fallback (only runs if null)
String value = Optional.ofNullable(System.getenv("CFG"))
                       .orElseGet(() -> loadFromFile());

// infinite stream source
Stream.generate(UUID::randomUUID)
      .limit(3)
      .forEach(System.out::println);

Constructor method references

class User { User(String name, int age) { /*...*/ } }

BiFunction<String,Integer,User> userFactory = User::new; // two args → BiFunction
Supplier<User> emptyUserFactory = () -> new User("guest", 0); // no args → Supplier

Rule of thumb

• Need to combine two inputs into one result? → BiFunction
• Need to defer or centralize creation of a value with no inputs? → Supplier