Why Can't an Abstract Class with a Single Abstract Method Be a Functional Interface in Java?

Why can't an abstract class with only one abstract method be treated as a functional interface in Java?

public abstract class MyFunctionalAbstractClass {
    public abstract void myAbstractMethod();
    public void method() {  myAbstractMethod(); }
}

Java 8 introduces functional interfaces, which are interfaces that contain exactly one abstract method. While an abstract class may contain a single abstract method, it cannot be used as a functional interface due to design decisions by Java's creators.

public interface MyFunctionalInterface {
    void myAbstractMethod();  // Only one abstract method
    default void method() {
        myAbstractMethod();
    }
}

Causes

• An abstract class can have state (fields) and other methods that can complicate its use as a functional interface.
• Functional interfaces aim to keep the concept simple and focused solely on behavior, hence the single-method requirement.
• Designers wanted to maintain clarity in the lambda expressions that can utilize functional interfaces, avoiding ambiguity or misuse.

Solutions

• Instead of using an abstract class, use an interface when you only need a single abstract method and require functional characteristics.
• If you have shared code that you want across multiple implementations, consider using default methods in interfaces.

Mistake: Confusing abstract classes with functional interfaces.

Solution: Always check if you require the characteristics of an interface for functional programming, as abstract classes cannot directly serve that purpose.

Mistake: Attempting to use lambda expressions with abstract classes.

Solution: Use interfaces instead, which are specifically designed for this purpose.