Java: Lambda Cheat Sheet

Lambdas

() -> "Hello"
() -> System.out.println("Hello")
(String str) -> str.length()
(str) -> str.length()
str -> str.length()
(int i, int j) -> i + j
(i, j) -> i + j

() -> {
    System.out.println("Hello");
    System.out.println("World");
}

(int i) -> {
    System.out.println("Hello");
    return i;
}

Method References

// Static methods
Supplier<Thread> runtimeSup = Thread::currentThread;

// Bound instance methods
Supplier<String> helloSup = "hello"::toUpperCase;
Consumer<String> printer = System.out::println;

// Unbound instance methods
Function<String, String> lower = String::toLowerCase;

// Constructors
Supplier<String> stringSup = String::new;
Function<Integer, int[]> arrSup = int[]::new;

Standard Functional Interfaces

Custom Functional Interfaces

Declared like:

@FunctionalInterface
interface MyInterface {
    String method(String str);
}

Used like:

MyInterface doubler = str -> str + str;
String abab = doubler.method("ab");

• Functional interface: Any interface with a single abstract method
• Can have additional default methods
• The @FunctionalInterface annotation (which is optional) causes the compiler to complain if the interface is not a functional interface