Function<T,R>

Learn transformation functions with Function<T,R>.

The Function<T,R> interface

Function<T, R> represents a function that takes a T value and returns an R value. Its single abstract method is apply. This interface is the standard way to write transforming code concisely.

Example: Function<String, Integer> len = String::length;. Then len.apply("Alex") returns 4. You can also write the function as a lambda: s -> s.length(). Function is also often passed to map in streams.

compose and andThen

To apply two functions in sequence you use compose and andThen. Both return a new Function; the difference is the application order.

Example: f.andThen(g) applies f first, then g. f.compose(g) applies g first, then f. This is similar to function composition in mathematics; it is the clean way to pipe data through one stage into the next.

Exercise Find string length with Function

Define a Function<String, Integer> that uses the String::length method reference. Then print the result of apply("Hello") with System.out.println.

Show example solution

import java.util.function.Function;

public class FunctionExample {
    public static void main(String[] args) {
        Function<String, Integer> len = String::length;
        System.out.println(len.apply("Hello"));
    }
}

Test yourself

What is the single abstract method of Function<T,R>?
- apply
- get
- test
- accept
Show answer

apply — apply is Function's call method.

Complete the Function call

Function<String, Integer> len = String::length;
int n = len.____("Ayşe");

Show answer

Function<String, Integer> len = String::length;
int n = len.apply("Ayşe");

Match the Function concepts

Function<T,R>

Takes T returns R
apply

Call method
andThen

Calls afterwards
compose

Calls before

Next lesson: Predicate<T>

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