Using Java 8 Functional Interfaces to Replace if...else Statements

In Java development, excessive use of if...else statements can make code hard to read. Java 8 introduces functional interfaces that allow these patterns to be expressed more concisely.

The @FunctionalInterface annotation marks an interface with a single abstract method. Common built‑in functional interfaces include Supplier (no arguments, returns a value), Consumer (accepts a value, returns void), Runnable (no arguments, no return), and Function (accepts a value and returns a result).

Example: a custom interface for throwing an exception.

/**
 * Throw exception interface
 */
@FunctionalInterface
public interface ThrowExceptionFunction {
    void throwMessage(String message);
}

A utility method that returns this interface and throws when a condition is true.

public static ThrowExceptionFunction isTrue(boolean b) {
    return errorMessage -> {
        if (b) {
            throw new RuntimeException(errorMessage);
        }
    };
}

Branch handling with two Runnable s.

@FunctionalInterface
public interface BranchHandle {
    void trueOrFalseHandle(Runnable trueHandle, Runnable falseHandle);
}

public static BranchHandle isTrueOrFalse(boolean b) {
    return (trueHandle, falseHandle) -> {
        if (b) {
            trueHandle.run();
        } else {
            falseHandle.run();
        }
    };
}

Handling optional values with Consumer and Runnable.

public interface PresentOrElseHandler<T> {
    void presentOrElseHandle(Consumer<? super T> action, Runnable emptyAction);
}

public static PresentOrElseHandler<String> isBlankOrNoBlank(String str) {
    return (consumer, runnable) -> {
        if (str == null || str.length() == 0) {
            runnable.run();
        } else {
            consumer.accept(str);
        }
    };
}

Using these interfaces simplifies conditional logic and improves code readability.