java.util.function.Function is part of java.util.function package.

In Java 8, Function is a functional interface. It accepts one argument(object of type T) and produces a result (object of type R).

@FunctionalInterface
public interface Function<T, R> {
	R apply(T t);
}

Here, T is the Type of input to the function.

R is the type of result of the function.

In addition to abstract method apply(), this interface has two default methods andThen() and compose(). Function interface has one static method – identity().

default <V> Function<V,R> andThen(Function<? super R,? extends V> after)	This method returns a composed function that first applies this function to its input, and then applies the after result to the result.
default <V> Function<V,R> compose(Function<? super V, ? extends T> before)	This method returns a composed function that first applies the before function to its input, and then applies this function to the result.
static <T> Function<T,T> identity()	This method returns a function that always returns its input argument.

Example of Function interface

We’ll use Function interface to find the length of string. This method takes String as input and returns Integer as output.

public class FunctionExample {
	public static void main(String[] args) {
		Function<String, Integer> func = s -> s.length();
		
		Integer length = func.apply("hello world");
		
		System.out.println("Length of string is: " + length);
	}
}

Output

Length of string is: 11

Chaining of Function with example of andThen()

Function chaining is nothing but syntax to invoke multiple methods calls in single line. In this example, we’ll chain Function with andThen().

In this example, we’ll change string to uppercase and then will find substring with first 5 characters.

import java.util.function.Function;

public class FunctionChainWithAndThen {
	public static void main(String[] args) {
		Function<String, String> func1 = s -> s.toUpperCase();
		
		Function<String, String> func2 = s -> s.substring(0,5);
		
		String result = func1.andThen(func2).apply("hello world");
		
		System.out.println("result is: " + result);
	}
}

Output

result is: HELLO

Example of compose() method

In the following example, first func2 will be applied. For the input integer 2, first i+2 (result is 4) will be calculated. Then square of 4 (result is 16) will be calculated.

import java.util.function.Function;

public class FunctionChainWithAndThen {
	public static void main(String[] args) {
		Function<Integer, Integer> func1 = i -> i*i;
		
		Function<Integer, Integer> func2 = i -> i+2;
		
		Integer result = func1.compose(func2).apply(2);
		
		System.out.println("result is: " + result);
	}
}

Output

result is: 16 

Example of identity() method

import java.util.function.Function;

public class IdentityExample {
	public static void main(String[] args) {
		Function<String, String> func = Function.identity(); 
		String result = func.apply("hello world");
        System.out.println(result); 
	}
}

Output

hello world

Explanation of this code is simple. For the given string “hello world”, method returns the same input argument as the result.