All 15 Functional Interfaces & Method References questions
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What are the four kinds of method references in Java (static, instance-bound, instance-unbound, and constructor), and how does the compiler determine which form applies? How do the default methods andThen() and compose() on Function work, and what's the difference between them for chaining transformations? What is the difference between UnaryOperator<T> and Function<T,T>, and why does UnaryOperator exist as a separate, more specific interface? How do the primitive-specialized functional interfaces (IntFunction, ToIntFunction, IntPredicate, IntUnaryOperator, etc.) avoid the performance cost of autoboxing, and when should you prefer them over the generic equivalents? How does the Predicate interface's default methods and(), or(), and negate() enable composing complex boolean conditions from simpler ones? What is the difference between a Consumer<T> and a BiConsumer<T,U>, and how would you use BiConsumer for operations needing two inputs but no return value, such as Map.forEach()? How would you convert a checked-exception-throwing method into a lambda usable with a standard functional interface (which doesn't declare any checked exceptions), given standard interfaces like Function don't permit checked exceptions in their abstract method signature? How does a lambda expression capture variables from its enclosing scope, and why must captured local variables be effectively final? How would you implement a custom functional interface with generic type parameters, and what considerations apply when designing one instead of reusing a built-in java.util.function interface? How would you implement memoization (caching function results) as a higher-order function wrapping an existing Function, and what thread-safety considerations apply for a memoized function used concurrently?
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What is a functional interface, and what are the four core functional interfaces in java.util.function (Function, Predicate, Consumer, Supplier)?

Beginner

Answer

A functional interface has exactly one abstract method, making it eligible for lambda expression or method reference assignment -- Function<T,R> transforms an input of type T into an output of type R (apply()); Predicate<T> tests a condition, returning boolean (test()); Consumer<T> accepts an input and performs a side effect, returning nothing (accept()); Supplier<T> takes no input and produces a value (get()) -- these four generic interfaces cover the vast majority of common functional programming needs without requiring a custom interface for each specific case.
Function<String, Integer> length = String::length;
Predicate<String> isEmpty = String::isEmpty;
Consumer<String> printer = System.out::println;
Supplier<String> greeting = () -> "Hello";

System.out.println(length.apply("hello"));  // 5
System.out.println(isEmpty.test(""));         // true
Real-world example A validation pipeline uses Predicate<Order> to check business rules, Function<Order, Invoice> to transform an order into an invoice, and Consumer<Invoice> to send the resulting invoice via email, composing a complete order-processing flow entirely out of these standard functional interfaces without defining any custom ones.

Common follow-ups: What's the difference between Function and UnaryOperator, given both seem to take and return one value?;Why do these interfaces exist as generic interfaces rather than the JDK using raw Object types?

Streams & Lambdas;Generics
All 15 Functional Interfaces & Method References questions