Built-in Functional Interfaces

Functional Interface

An interface that contains only a single abstract (unimplemented) method. It can have contain default and static methods which do have an implementation in addition to the abstract method.

Built-in Functional Interfaces

A set of functional interfaces designed for commonly occuring use cases is already built in java.

1. Function Interface: represents an abstract method, apply(), that takes a single parameter and returns a single value.

   //implementing function interface
   public class GetSquare implements Function<Integer, Integer> {
    @Override
    public Long apply(Integer number) {
        return number * number;
    }
   }
   //using the above implementation
   Function<Integer, Integer> getSquare = new GetSquare();
   Integer result = getSquare.apply(4);
   System.out.println("result = " + result);
   
   //inline implementation using lambda expression
   Function<Integer, Integer> getSquare = number -> number * number;
   Integer result = getSquare.apply(4);
   System.out.println("result = " + result);
   

2. Predicate: represents an abstract method, test(), that takes a single value as parameter, and returns true or false.

   //checks whether a value is greater than 100
   Predicate<Integer> greaterThan = v -> (v > 100);
   System.out.println(greaterThan.test(1050));
   

3. Supplier: represents an abstract method, get(), that supplies a value.

   //function returns a random value.
   Supplier<Double> randomValue = () -> Math.random();
   System.out.println(randomValue.get());
   

4. Consumer: represents an abstract method, accept(), that consumes a value without returning any value. Used for printing or writing to a file etc.

   //consumer to display a number
   Consumer<Integer> consumer = value -> System.out.println(value);
   consumer.accept(10)
   
   //consumer to display a list of numbers
   Consumer<List<Integer>> displayList = list -> list.stream().forEach(v -> System.out.print(v + " "));
   List<Integer> list = new ArrayList<Integer>();
   list.add(2);
   list.add(1);
   list.add(3);
   displayList.accept(list);