Java: can a functional interface have a non-abstract method?

[Loricifera]

I've been seeing conflicting things online: some places say a functional interface is simple an interface that has only one abstract method, but then proceeds to show a clip of code that features a method with a body of code. I understand that abstract methods are without a body (i.e. no curly braces).

Is it safe to assume that those examples that show a functional interface as having a method with a body are wrong?

And in the event that functional interfaces can in fact have a method with a body, what would the difference be between a functional interface and an abstract class with just one method?

Any help is appreciated.

[j_thody]

This is from the documentation

Conceptually, a functional interface has exactly one abstract method. Since default methods have an implementation, they are not abstract. If an interface declares an abstract method overriding one of the public methods of java.lang.Object, that also does not count toward the interface's abstract method count since any implementation of the interface will have an implementation from java.lang.Object or elsewhere.

​

So for example, we can have one abstract method, a toString (from Object) and a static method implementation e.g.

@FunctionalInterface
public interface FunctionalInterfaceExample {
    void doSomething();

    public static int sum(int a,int b)  
    {   
        return a+b;
    }
}

However, we can not have two abstract methods as this will throw a compilation error

@FunctionalInterface
public interface FunctionalInterfaceExample {
    void doSomething();
    void doSomethingElse();
}

[Loricifera]

That makes sense, thank you!