Java 9 — Interfaces

Java 9 introduced private methods and private static method in interfaces. In Java 9 an interface can have seven different things:

1. constant variables (since Java 1.0)
2. abstract methods (since Java 1.0)
3. nested types (since Java 2.0)
4. default methods (since Java 8.0)
5. static methods (since Java 8.0)
6. private methods (since Java 9.0)
7. private static methods (since Java 9.0)

The private methods added in Java 9.0 improve code re-usability inside interfaces and provide choice to expose only intended method implementations to users. The private methods are only accessible within an interface and cannot be accessed or inherited from the interface to another interface or class.

Note:-

1. Every field declaration in the body of an interface is implicitly public, static, and final. It is permitted to redundantly specify any or all of these modifiers for such fields.
2. Every method declaration in an interface with body represented by a semicolon is implicitly public and abstract.It is a compile-time error if a method declaration that contains the keyword abstract also contains any one of the keywords: private, static, final, native, strictfp, or synchronized. It would be impossible for a class to implement a private abstract method, because private methods are not inherited by subclasses; therefore such a method could never be used.
3. Interfaces may contain member type declarations (nested type). A member type declaration in an interface is implicitly static and public. It is permitted to redundantly specify either or both of these modifiers.
4. The default methods are implicitly public — there's no need to specify the public modifier.

Default Methods Use Case —

a. helps minimize code duplication

b. single location to write & edit

c. can be overridden if necessary

d. can be overridden per class precision

In case of multiple inheritance , the super class method always takes preference over the interface default method

In case of multi level inheritance , the subtype interface method always takes preference over the super type interface default method

If there is a conflict, we get compile error to override the default methods

5. A static method can be invoked from other static or from default method. A static method cannot be overridden or changed in the implementation class. The static methods are implicitly public — there's no need to specify the public modifier.

6. The private methods must contain body. As private methods can only be used in the methods of the interface itself, their use is limited to being helper methods for the other methods of the interface.

Private Methods Use Case — if two default methods needed to share code, a private method in the interface would allow them to do so, but without exposing that private method to interface implementing class.

7. The private static methods are useful when you have multiple public static methods that share some common code. A private static methods can be called from instance (i.e. default or private non-static) method or static method inside the interface. A private non-static method is not allowed to be called from static or private static method within the interface

Types of methods in Java SE 9 Interfaces