The Functon interface in Java 8 represents an operation that accepts one argument and produces a result. This functional interface contains the abstarct method apply which defines the transformation logic.

@FunctionalInterface
public interface Transformer<T, R> {
    R transform(T input);

    default <V> Transformer<V, R> combineBefore(Transformer<? super V, ? extends T> preProcessor) {
        Objects.requireNonNull(preProcessor);
        return (V v) -> transform(preProcessor.transform(v));
    }

    default <V> Transformer<T, V> combineAfter(Transformer<? super R, ? extends V> postProcessor) {
        Objects.requireNonNull(postProcessor);
        return (T t) -> postProcessor.transform(transform(t));
    }

    static <T> Transformer<T, T> noChange() {
        return t -> t;
    }
}

Basic usage of the transformation operation:

Transformer<Integer, Integer> increment = x -> x + 10;
Integer output = increment.transform(10);
System.out.println(output); // Outputs 20

Method chaining example:

Transformer<Integer, Integer> scale = a -> a * 10;
Transformer<Integer, Integer> offset = a -> a + 10;

// First offset then scale
Transformer<Integer, Integer> combined1 = scale.combineBefore(offset);
System.out.println(combined1.transform(1)); // Outputs 110

// First scale then offset
Transformer<Integer, Integer> combined2 = scale.combineAfter(offset);
System.out.println(combined2.transform(1)); // Outputs 20

Practical validation implementation:

public class ValidationHelper {
    private static final Transformer<String, String> sizeValidator = input -> {
        if (input.length() > 100) {
            throw new IllegalArgumentException("Input exceeds maximum length");
        }
        return input;
    };

    private static final Transformer<String, String> formatValidator = data -> {
        if (!data.matches("^[a-fA-F0-9]+$")) {
            throw new IllegalArgumentException("Invalid character pattern");
        }
        return data;
    };

    public static void validateInput(String value) {
        formatValidator.combineBefore(sizeValidator).transform(value);
    }
}