Java 9 Interface Private Methods

In Java 9, we can create private methods inside an interfaces. Interface allows us to declare private methods that help to share common code between non-abstract methods. It helps to make the interfaces cleaner and more organized.

Before Java 9, creating private methods inside an interface cause a compile time error. 

Private Methods in Interface

Before Java 9, the private methods in interfaces could only have:

• Abstract Methods (methods without a body)
• Static Methods (methods with a body and the static keyword)
• Default Methods (methods with a body and default keyword)

If we need to reuse code inside default or static methods then we need to repeat it. To solve the problem, Java 9 helps by allowing private methods inside interfaces.

A private method in an interface cannot be called outside the interface and can only be used inside default or static methods of the same interface. It avoids duplicating the code and makes the interface cleaner.

Why use Private Methods?

To solve the problem faced by developers while using default and static methods in interfaces, the private methods were introduced in Java 9. Those methods improve the quality of code and design.

Code Reusability: The methods such as Default and static, often share common steps such as validation or checks. We can write the logic once in the private methods and reuse it inside the interface which avoids repeated code and reduces errors.

Cleaner and Readable Code: The interface becomes messy by repeating the code. The private methods keep the default and static methods short and clear which makes the interface easy to read and understand.

Better Encapsulation: Private methods are hidden from the classes that implement the interface. They are used only inside the interface which keeps the internal logic private and the design clean.

Easy Maintenance: Only updating the private method is enough when changes are needed. All the methods using it automatically get the update, save time and prevent bugs.

Output:

Message is valid
Message sent: Hello Java
Message is valid
Message received: Welcome to Java 9

Explanation

We have shown private methods in a Java 9 interface in the above program. There are two default methods in the MessageService interface named sendMessage() and receiveMessage(). Both used the private method named validateMessage() to validate if the message is empty or null.

We can use the private class with the interface only and it cannot be accessed by the implementing class. The interface is then implemented by the MessageApp and uses the default methods without overriding them. Overall, the program helps to reduce code duplication, keeps the interface organized and becomes easy to maintain.

Private Static Method in Interface from Java 9

Java 9 interfaces allow private static methods. Inside the interface it can be called by other static methods and cannot be accessed by classes that implement the interface. Also, it helps to avoid the repetition of code in static methods and keeps the interface clean.

Output:

Performing calculation for: 5
Square of 5: 25
Performing calculation for: 3
Cube of 3: 27

Explanation

The program shows private static methods. Here, the MathOperations class has two static methods, square() and cube(). These two methods use the private static method calculate() for common calculations.

We cannot access the private method outside the interface and it helps to avoid repeating code. With the help of the interface, the static methods are called and print the square of 5 and the cube of 3. It keeps the interface organized and makes the code clean.

Advantages

• Reduces Error: There is less chance of inconsistency caused by copying code incorrectly.
• Easy to Maintain: We need to update only the private method if we need to change some logic. It saves time and makes fewer mistakes.
• Improves Code Readability: The private methods help to break complex logic into smaller parts so the code becomes easy to understand.
• Organized Design: It is a good coding practices to use private methods, especially in large applications.

Disadvantages

• Limited Usage Scope: It limits flexibility because it cannot be accessed or reused by implementing classes and we can only use private methods inside the same interface.
• Difficult to Debug: While debugging tracing issues can be tricky sometimes but as private methods are hidden so they cannot be accessed directly.
• Increase Interface Complexity: If we add many private methods then it becomes harder to understand by the beginners.
• Not Available in Older Java Versions: From Java 9 onwards, we can work with private methods in interfaces. We cannot use the feature in Java 8.

Conclusion

Private methods in Java 9 interfaces make the coding simple, clean and easy to understand. Keeping the common logic in one place, it helps to avoid writing the same code again and again. It becomes easy to read and update the interface. They keep the internal work hidden and safe because we cannot use these methods outside the interface. The developers can write the code better and more organized if they use it in the right way.