FUNCTIONAL PROGRAMMING

Lambda & Functional Interfaces

Interface	Method	Use Case
Function<T, R>	R apply(T t)	Transform an input into an output
Predicate<T>	boolean test(T t)	Boolean-valued condition, e.g. filtering
Consumer<T>	void accept(T t)	Perform an action, no return value
Supplier<T>	T get()	Provide/produce a value, no input
BiFunction<T, U, R>	R apply(T t, U u)	Transform two inputs into an output
UnaryOperator<T>	T apply(T t)	Function<T, T> specialisation

Method References

Form	Example	Equivalent Lambda
Static method	Integer::parseInt	s -> Integer.parseInt(s)
Instance method (particular object)	list::add	x -> list.add(x)
Instance method (arbitrary object of a type)	String::toUpperCase	s -> s.toUpperCase()
Constructor	ArrayList::new	() -> new ArrayList<>()

Streams Basics

List<String> names = List.of("Bilbo", "Frodo", "Gandalf", "Aragorn");

List<String> longNames = names.stream()
    .filter(n -> n.length() > 5)
    .map(String::toUpperCase)
    .sorted()
    .toList(); // Java 16+ terminal shortcut for Collectors.toList()

Exam tip: Streams are lazily evaluated — intermediate operations (filter, map) only execute once a terminal operation (collect, toList, forEach, reduce) is invoked.