What is the key difference between an abstract class and an interface? All methods in an abstract class are abstract, and all methods in an interface are concrete. There is no difference, we use these names interchangeably All methods in an abstract class are concrete, and all methods in an interface are abstract. An abstract class has a mix of abstract and concrete methods, while an interface has all abstract methods. An abstract class has all abstract methods, while an interface has a mix of abstract and concrete methods. Save