Is it possible to create methods for accepting and returning values in functional interfaces?

In a functional interface in Java, there must be exactly one abstract method (SAM — Single Abstract Method). This method defines the signature for accepting and returning values.

You can create additional methods, but:

• They must be either default (with implementation),
• Or static,
• Or methods from java.lang.Object (e.g., equals, hashCode, toString).

There should be no abstract methods other than the one.

Example:

@FunctionalInterface
public interface MyFunction {
    int apply(int x); // main abstract method

    default int doubleApply(int x) { // additional method with implementation
        return apply(x) * 2;
    }

    static int staticMethod() { // static method
        return 42;
    }
}

Thus, the method for accepting and returning values is the only abstract method of the interface. Other methods with implementation can be created, but they do not affect the interface's functionality.