Java Function Interface

It is a functional interface. It is used to refer method by specifying type of parameter. It returns a result back to the referred function.

Java Function Interface Methods

Method	Description
default <V> Function<T,V> andThen(Function<? super R,? extends V> after)	It returns a composed function that first applies this function to its input, and then applies the after function to the result. If evaluation of either function throws an exception, it is relayed to the caller of the composed function.
static <T> Function<T,T> identity()	It returns a function that always returns its input argument.
R apply(T t)	It applies this function to the given argument.
default <V> Function<V,R> compose(Function<? super V,? extends T> before)	It Returns a composed function that first applies the before function to its input, and then applies this function to the result. If evaluation of either function throws an exception, it is relayed to the caller of the composed function.

Java Function Interface Example 1

// importing Function interface
import java.util.function.Function;
public class FunctionInterfaceExample {
	static String show(String message){
		return "Hello "+message;
	}
	public static void main(String[] args) {
		// Function interface referring to a method
		Function<String, String> fun = FunctionInterfaceExample::show;
		// Calling Function interface method
		System.out.println(fun.apply("Peter"));
	}
}

Output:

Hello Peter

Java Function Interface Example 2

// importing Function interface
import java.util.function.Function;
import java.util.List;
import java.util.ArrayList;
public class FunctionInterfaceExample {
	static Integer addList(List<Integer> list){
		return list.stream()
				   .mapToInt(Integer::intValue)
				   .sum();
	}
	public static void main(String[] args) {
		// Creating a list and adding values
		List<Integer> list = new ArrayList<Integer>();
		list.add(10);
		list.add(20);
		list.add(30);
		list.add(40);
		// Referring addList() method
		Function<List<Integer>, Integer> fun = FunctionInterfaceExample::addList;
		// Calling Function interface method
		int result = fun.apply(list);
		System.out.println("Sum of list values: "+result);
	}
}

Output: