# Composing Functions

Paul Tomoiu

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Examples,

Java 8,

Functions

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Published 2022-12-03

This tutorial explains to you how we can compose functions in Java.

The "functions" are implementations of the Functions<T,R> functional interfaces.

In my example, I start from a simple Spring Boot application created using Spring initializr.

Let's have the following functions:

f:Z -> Z, f(x) = x + 1
g:Z -> Z, g(x) = 2 * x

Info

Z stands for integers, including all negative and positive integers.
f:Z -> Z means that the function takes as parameter an integer and return an integer.

In the following example we will create another 2 functions:

h1:Z -> Z, h1(x) = g(f(x)) = (g o f)(x) --> f.andThen(g);
h2:Z -> Z, h2(x) = f(g(x)) = (f o g)(x) --> f.compose(g)

Here is the Java code for testing the situation above:

package com.exampe.java;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

import java.util.function.Function;

@SpringBootApplication
public class DemoApplication {

  public static void main(String[] args) {

    SpringApplication.run(DemoApplication.class, args);

    System.out.println("--------------------------------------------");

    Function<Integer, Integer> f = x -> x + 1;
    Function<Integer, Integer> g = x -> 2 * x;

    // f(x) is calculated and will be the input parameter for g(x) || h1(x) = g(f(x))
    Function<Integer, Integer> h1 = f.andThen(g);
    Integer result1 = h1.apply(2);
    System.out.println("Result1 = "+result1);

    // g(x) is calculated and will be the input parameter for f(x) || h2(x) = f(g(x))
    Function<Integer, Integer> h2 = f.compose(g);
    Integer result2 = h2.apply(2);
    System.out.println("Result2 = "+result2);

    System.out.println("--------------------------------------------");
    System.out.println("End of the execution");
  }
}

When we run the application we can see:

--------------------------------------------
Result1 = 6
Result2 = 5
--------------------------------------------
End of the execution

Please take a look at the comments in the code.

Info

We can create transformation pipelines using functions and andThen or compose methods.