ERROR - Invalid '@FunctionalInterface' annotation; is not a functional interface

Error

Invalid '@FunctionalInterface' annotation; <BuggyBread> is not a functional interface

while using Functional Interface Annotation in Java 8.

Error Type

Compile Time

Sample Code

@FunctionalInterface
public interface BuggyBread {
      abstract void method1(); 
      abstract void method2();
}

Cause

BuggyBread is not a Functional Interface. A functional Interface should have one and only one abstract method. It may have multiple default methods. 

Resolution 

Remove abstract method method1() or method2()

@FunctionalInterface
public interface BuggyBread {
      abstract void method1(); 
}