Function.andThen() vs Function.compose()

Direct Answer

f.andThen(g) applies f first and passes its result to g, so it represents g(f(x)). f.compose(g) applies g first and passes that result to f, so it represents f(g(x)). Choose the direction that makes the transformation and compatible types clearest.

Code Example

import java.util.function.Function;

public class AndThenVsCompose {
    public static void main(String[] args) {
        Function<Integer, Integer> times2 = x -> x * 2;
        Function<Integer, Integer> plus3  = x -> x + 3;

        // andThen: times2 first, then plus3 -> (5*2)+3
        System.out.println(times2.andThen(plus3).apply(5)); // 13

        // compose: plus3 first, then times2 -> (5+3)*2
        System.out.println(times2.compose(plus3).apply(5)); // 16
    }
}

Common Mistakes & Pitfalls

• Reversing compose: f.compose(g) runs g first and then f.
• Ignoring type flow: with f.andThen(g), the output of f must be compatible with the input accepted by g.
• Assuming order cannot change the result: parsing, rounding, validation, and side effects may produce different results when reversed.
• Choosing by novelty: use the direction that makes the intended data flow easiest to verify.