2351. Java 8 - Tricks
Lamda Expression

New features in java 8

1. Overview

Verbose code for sorting.

Collections.sort(inventory, new Comparator<Apple>() {
    public int compare(Apple a1, Apple a2){
        return a1.getWeight().compareTo(a2.getWeight());
    }
});

With Lamda expression.

inventory.sort(comparing(Apple::getWeight));

2. Core Feature/Concepts

  • stream processing
  • behavior parameterization, functional programming
  • default methods in interface
  • method reference: :: syntax
  • anonymous functions, lambda expression
  • functional interface: an interface that specifies exactly one abstract method.

image

A lambda expression is composed of parameters, an arrow, and a body.

1 ()->{}
2 () -> "Raoul"
3 () -> { return "Mario"; }
4 (Integer i) -> return "Alan" + i;
5 (String s) -> { "Iron Man"; }

4 and 5 are invalid lambdas; the rest are valid. Details:

  • 1 This lambda has no parameters and returns void. It’s similar to a method with an empty body:publicvoidrun(){}. Fun fact: this is usually called the burger lambda. Take a look at it from the side, and you will see it has a burger shape with two buns.
  • 2 This lambda has no parameters and returns a String as an expression.
  • 3 This lambda has no parameters and returns a String (using an explicit return statement, within a block).
  • 4 return is a control-flow statement. To make this lambda valid, curly braces are required as follows:(Integeri)->{return”Alan”+i;}.
  • 5 “Iron Man” is an expression, not a statement. To make this lambda valid, you can remove the curly braces and semicolon as follows:(Strings)->”Iron Man”. Or if you prefer, you can use an explicit return statement as follows: (String s) -> { return “Iron Man”; }.
Use case Examples of lambdas
A boolean expression (List list) -> list.isEmpty()
Creating objects () -> new Apple(10)
Consuming from an object (Apple a) -> { System.out.println(a.getWeight()); }
Select/extract from an object (String s) -> s.length()
Combine two values (int a, int b) -> a * b
Compare two objects (Apple a1, Apple a2) -> a1.getWeight().compareTo(a2.getWeight())

Functional Interface.

Which of these interfaces are functional interfaces?

public interface Adder {
    int add(int a, int b);
}
public interface SmartAdder extends Adder {
    int add(double a, double b);
}
public interface Nothing {
}

Answer:

  • Only Adder is a functional interface.
  • SmartAdder isn’t a functional interface because it specifies two abstract methods called add (one is inherited from Adder).
  • Nothing isn’t a functional interface because it declares no abstract method at all.

Collection

  • removeIf
  • replaceAll
  • Map.forEach
  • Entry.comparingByValue
  • Entry.comparingByKey

Null check. Optional Class

public String getCarInsuranceName(Person person) {
    if (person != null) {
        Car car = person.getCar();
        if (car != null) {
            Insurance insurance = car.getInsurance();
            if (insurance != null) {
              return insurance.getName();
            }
        }
    }
    return "Unknown";
}

java.util.Optional

Manipulating a stream of optionals

public Set<String> getCarInsuranceNames(List<Person> persons) {
   return persons.stream()

   Collect the result Strings into a Set to obtain only the distinct values.
   .map(Person::getCar)
   .map(optCar -> optCar.flatMap(Car::getInsurance))
   .map(optIns -> optIns.map(Insurance::getName))
   .flatMap(Optional::stream)
   .collect(toSet());

11. References