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A lambda is a short function value:

Function<Integer, Integer> square = n -> n * n;
square.apply(5);   // 25

Predicate<String> nonEmpty = s -> !s.isEmpty();
BiFunction<Integer, Integer, Integer> add = (a, b) -> a + b;
Consumer<String> print = s -> System.out.println(s);
Supplier<Double> rand = () -> Math.random();

A functional interface is any interface with exactly one abstract method. The standard ones live in java.util.function:

Method references are a shorter syntax for lambdas that just call an existing method:

// these are equivalent
list.forEach(s -> System.out.println(s));
list.forEach(System.out::println);

list.stream().map(s -> s.toUpperCase());
list.stream().map(String::toUpperCase);

Four kinds of method references:

• Class::staticMethod
• instance::method
• Class::instanceMethod (the receiver becomes the first arg)
• Class::new (constructor reference)