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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)? 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? 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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How do the default methods andThen() and compose() on Function work, and what's the difference between them for chaining transformations?

Intermediate

Answer

andThen(after) creates a composed function that first applies the original function, then applies the 'after' function to that result (f.andThen(g) means g(f(x))); compose(before) instead creates a composed function that first applies the 'before' function, then applies the original (f.compose(g) means f(g(x))) -- the key distinction is the ORDER of application: andThen reads left-to-right matching the chain's declaration order, while compose reads right-to-left, and choosing the wrong one for your intended order produces a function that runs its steps in the opposite sequence from what you intended.
Function<Integer, Integer> addOne = x -> x + 1;
Function<Integer, Integer> multiplyByTwo = x -> x * 2;

Function<Integer, Integer> addThenMultiply = addOne.andThen(multiplyByTwo);
System.out.println(addThenMultiply.apply(3));  // (3+1)*2 = 8

Function<Integer, Integer> multiplyThenAdd = addOne.compose(multiplyByTwo);
System.out.println(multiplyThenAdd.apply(3));  // (3*2)+1 = 7 -- compose applies multiplyByTwo FIRST
Real-world example A data-transformation pipeline needing to first sanitize input then apply a business calculation chains sanitize.andThen(calculate) to express this order explicitly and readably, choosing andThen specifically because its left-to-right reading matches the intended sanitize-then-calculate sequence.

Common follow-ups: How would you chain more than two functions together using andThen() repeatedly?;Does Predicate have similar default methods (and(), or(), negate()) for combining boolean conditions?

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