Five Lambdas: Name the Interface Each One Implements

Five lambdas in context. For each, name the functional interface it lands in and the method it supplies. All five before the reveal.

java

// 1
names.removeIf(n -> n.isBlank());
// 2
names.replaceAll(n -> n.trim());
// 3
names.forEach(n -> System.out.println(n));
// 4
names.sort((a, b) -> a.length() - b.length());
// 5
Optional.ofNullable(cached).orElseGet(() -> load());

The five interfaces, and the method each lambda supplies

Each lambda, its interface, and its method
	Interface	Method	Shape
1	`Predicate<String>`	`test`	Takes one, returns a boolean
2	`UnaryOperator<String>`	`apply`	Takes one, returns the same type
3	`Consumer<String>`	`accept`	Takes one, returns nothing
4	`Comparator<String>`	`compare`	Takes two, returns an `int`
5	`Supplier<String>`	`get`	Takes nothing, returns one

None of the five said what it was. Each one's type came from the parameter it was handed to, which is the thing worth being able to read off a call site.

Number four has a bug in it as well as an interface: subtracting two int values in a comparator can overflow. Comparator.comparingInt(String::length) is the version that cannot.