Java 9 – Private methods in interface

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In this topic, we will discuss the private and private static methods inside an interface.

interface in Java is a concept which is used to achieve abstraction. In early versions of Java, an interface can contain only abstract methods and constants until Java 8 which allows to add default and static methods as well.

In Java 9, interface allows creating private and private static methods. It means now an interface can have the following:

  1. Abstract methods
  2. Constant variables
  3. Default methods (added in Java 8)
  4. Static methods (added in Java 8)
  5. Private methods (added in Java 9)
  6. Private static methods (added in Java 9)

Now, let’s understand what is private method and why it is important.

Interface Private Method

A private method declared inside an interface is similar to declared inside a class. It is declared using private access modifier to keep it limited to the interface. These methods can’t be accessible outside the interface and don’t inherit to the interface or implementing class.

The primary purpose behind to add private method was to share common code between non-abstract methods within the interface. Let’s see an example.

Output:

Let’s have a Coffee

Interface Private Static Methods

Like private instance methods, private methods can be static as well. A private static method can not be accessible from outside the interface and can only be accessible inside the default or static methods.

Output:

Let’s have a Coffee
Let’s have a Tea

If we want to share code between instance methods, private instance methods and private static methods both can be used. But If we want to share code between static methods, use private static methods only. See, what happens, if we try to access a private method inside a static method.

Output:

Error : Cannot make a static reference to the non-static method

Key-points to remember

  1. Interface private methods must have implementation body; they can’t be declared as abstract methods.
  2. A static method can call only private static method, but a default method can call both the static and non-static(instance) private methods.
  3. The interface private methods support sharing common code between non-abstract methods(default, static and private).

Conclusion

This topic is all about the private methods in the interface. This feature makes the interface more powerful and advance. Now, interface supports both private and private static methods within an interface.

That’s all about Java 9 Private methods in interface.

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