Chapter 79: Function in Java

Function<T,R> models one input → one output: R apply(T t). It is the shape behind Stream.map, many Optional.map calls, and data conversion utilities.

Predicate (78) returned boolean; Function returns whatever type R you need.

1. Topic title

Function: a typed transformer you can chain like plumbing fittings

2. What it means

andThen runs this function first, then the next function on the result: f.andThen(g) means g(f(x)).

compose runs the argument function first, then this: f.compose(g) means f(g(x)).

Function.identity() returns a function that hands the value through unchanged—useful when an API demands a Function but sometimes you want a no-op step.

3. Why it is used

Parsing layers (String → int → Money), DTO mapping, pipeline steps in streams—anywhere transform is clearer than five nested helper calls.

4. Mental sketch

Function is a machine in a factory line: crates enter (T), crates leave (R). andThen bolts the next machine after; compose slides a pre-processor before your machine.

5. Java code example

import java.util.function.Function;
 
public class FunctionDemo {
    public static void main(String[] args) {
        Function<String, String> trim = String::trim;
        Function<String, Integer> len = String::length;
 
        Function<String, Integer> pipeline = trim.andThen(len);
        System.out.println(pipeline.apply("  hello  "));
 
        Function<Integer, Integer> timesTwo = x -> x * 2;
        Function<Integer, Integer> plusOne = x -> x + 1;
        Function<Integer, Integer> math = timesTwo.compose(plusOne);
        System.out.println(math.apply(3));
    }
}

timesTwo.compose(plusOne) applies plusOne first → 3+1=4, then timesTwo → 8.

6. Explanation of code

String::trim is a method reference—Chapter 88 dives deeper; here read it as s -> s.trim() shorthand.

7. Common mistakes

Confusing compose vs andThen—draw f(g(x)) vs g(f(x)) on paper once; future-you will thank present-you.

Returning null from apply—downstream map steps may NullPointerException; prefer Optional when absence is normal (Chapter 87).

8. Best practices

Prefer IntFunction, ToIntFunction, etc., when primitives dominate to reduce boxing.

Keep apply pure when used in parallel streams—side effects race.

9. Small practice task

Build Function<String, String> that trims, toLowerCases, then replace(' ', '_') using andThen. Print the result for " Hello Big World ".

Beginner tip

When stuck naming R, think “what leaves the function?”—that type is R, even if it matches T sometimes.