java-8-playground

What is a Functional interface?

1. A functional interface allows only 1 abstract method
2. More than 1 default methods
3. More than 1 static methods
4. Let us override java.lang.Object ckass methods

What functional interface can't contain?

1. Concrete method

Purpose?

1. To enable functional programming in java.
2. To enable lambda expression
3. To use method referencing

What is functional programming?

In functional programming we can send another function as method parameter.

How to enable a functional interface?

By using @FunctionalInterface annotation

How many ways can we implement an Interface in Java 8?

1. By writing separate Implementation class
2. By annonymous inner class
3. By lambda expression
4. By Method references

Something about Method referencing

1. Operator ::
2. Signature must be same (name is not mandatory)
3. Implementation of method must be static

There are few functional interfaces namely Consumer, Supplier, Predicate provided by Java 8.

Read more

Consumer

• A Consumer is a functional interface that accepts a single input and returns no output.
• Consumer interface has two methods:

void accept(T t);
default Consumer<T> andThen(Consumer<? super T> after);

• The accept method is the Single Abstract Method (SAM) which accepts a single argument of type T.
• andThen is a default method used for composition or chaining.
• andThen takes a consumer as input and returns a consumer.

accept method example

	public static void printUsingAcceptMethod(){
		// Create a function that prints what ever is passed through accept method of consumer interface.
	    Consumer<String> printConsumer = t -> System.out.println(t);
	    Stream<String> cities = Stream.of("Sydney", "Dhaka", "New York", "London");
	    // foreach method of stream interface takes a consumer through parameter.
	    cities.forEach(printConsumer);
	}

andThen method example.

public void printUsingAndTherMethod(){
    List<String> cities = Arrays.asList("Sydney", "Dhaka", "New York", "London");

// This consumer converts an string list into upper case.
    Consumer<List<String>> upperCaseConsumer = list -> {
        for(int i=0; i< list.size(); i++){
            list.set(i, list.get(i).toUpperCase());
        }
    };
    
// This Consumer print list of string 
    Consumer<List<String>> printConsumer = list -> list.stream().forEach(System.out::println);

// Chaining consumers using andThen
    upperCaseConsumer.andThen(printConsumer).accept(cities);
}

Predicate

In mathematics, a predicate is commonly understood to be a boolean-valued function P: X? {true, false}, called the predicate on X. Informally, a strong. It can be thought of as an operator or function that returns a value that is either true or false.

In Java 8, Predicate is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference. So, where you think, we can use these true/false returning functions in day to day programming? I will say you can use predicates anywhere where you need to evaluate a condition on group/collection of similar objects such that evaluation can result either in true or false.

public class Main {

	public static void main(String[] args) {
	List<Integer> values=Arrays.asList(1,2,3,4,5,6,7,8,9,10);
	Main main=new Main();
	List<Integer> tempList=main.mapResult(values, (val)->val%2==0);
	
	tempList.forEach(temp->{
		System.out.println(temp);
	});
	
	}
	
	public List<Integer>  mapResult(List<Integer> values,Predicate<Integer> condition){
		List<Integer>  tempList=new ArrayList<>();
		values.forEach(val->{
			if(condition.test(val)) {
				tempList.add(val);
			}
		});
		return tempList;
	}

}