Java Function compose()

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In this guide, you will learn about the Function compose() method in Java programming and how to use it with an example.

1. Function compose() Method Overview

Definition:

The Function compose() method allows for the composition of functions, i.e., it returns a composed function that first applies the provided function to its input, and then applies the current function to the result.

Syntax:

default <V> Function<V, R> compose(Function<? super V, ? extends T> before)

Parameters:

- before: The function to apply before the current function.

Key Points:

- The composed function's input type is a type of the before function's input, and its output type is a type of the current function's output.

- If the evaluation of either function throws an exception, it is relayed to the caller of the composed function.

- It's useful for creating a sequence of transformations on data.

2. Function compose() Method Example

import java.util.function.Function;

public class FunctionComposeExample {
    public static void main(String[] args) {
        // Define a Function to double an integer
        Function<Integer, Integer> doubleNumber = num -> num * 2;

        // Define a Function to increment an integer by 1
        Function<Integer, Integer> incrementByOne = num -> num + 1;

        // Compose the functions: double the number and then increment
        Function<Integer, Integer> doubleThenIncrement = doubleNumber.compose(incrementByOne);

        // Apply the composed function
        int result = doubleThenIncrement.apply(5);

        System.out.println("Result after applying composed function: " + result);
    }
}

Output:

Result after applying composed function: 12

Explanation:

In the example provided, we first define two Functions: one to double an integer and another to increment an integer by 1. We then compose these functions using the compose() method. 

The order matters: the increment function is applied first, and then the result is doubled. Thus, for an input of 5, the function first increments it to 6 and then doubles it to 12.