Java using private methods

Java 9 – Private methods in Interfaces (with examples)

As we know that Java 8 allowed us to create default and static methods in Interface. The intention was to have new methods added to the interfaces without breaking the classes that already implemented those interfaces. Java 9 has introduced another new feature, Java 9 SE onwards we can have private methods in interfaces. In this guide, we will learn why they added this feature, what is the use of it and how to work with it.

Why Java 9 allows us to have private methods in Interfaces?

Java 9 introduced private methods in interfaces to remove the redundancy by sharing the common code of multiple default methods through private methods.

To understand this we have to take an example in Java 8 (without private methods) and then we will take the same example with Java 9(using private methods).

Example in Java 8 – Multiple default methods with duplicate code (common code)
In this example we will see how default methods can have duplicate code which unnecessary increase the lines of code and makes the code less-readable. We will take the same example again using private methods to see how can private methods help us to avoid duplicate code.

interface MyInterfaceInJava8 < default void method1() < System.out.println("Starting method"); System.out.println("Doing someting"); System.out.println("This is method1"); >default void method2() < System.out.println("Starting method"); System.out.println("Doing someting"); System.out.println("This is method2"); >> public class JavaExample implements MyInterfaceInJava8 < public static void main(String args[]) < JavaExample je = new JavaExample(); je.method1(); je.method2(); >>

Output:

Example in Java 9 – Default methods sharing common code using private methods
We are taking the same example that we have seen above. This time we will introduce a private method to share the common code.

interface MyInterfaceInJava9 < default void method1() < //calling private method printLines(); System.out.println("This is method1"); >default void method2() < //calling private method printLines(); System.out.println("This is method2"); >private void printLines() < System.out.println("Starting method"); System.out.println("Doing someting"); >> public class JavaExample implements MyInterfaceInJava9 < public static void main(String args[]) < JavaExample je = new JavaExample(); je.method1(); je.method2(); >>

Starting method Doing someting This is method1 Starting method Doing someting This is method2

As you can see the output is same and the code size has been reduced.

Based on this we can say that the advantages of having private methods in interfaces are:
1. Allows default methods to share common code to avoid duplicate code (redundancy)
2. Improves readability of code.

Java 9 – Private Static methods

Till now we have leaned how to use private methods in interfaces to share common code of default methods. Java 9 also allows us to have private static methods in interfaces.

Since java 8 we can have static methods in interfaces along with default methods. We can not share the common code of static methods using the non-static private method, we must have to use the private static method to do that.

Lets take an example to understand this.

interface MyInterfaceInJava9 < static void method1() < //calling private method printLines(); System.out.println("This is method1"); >static void method2() < //calling private method printLines(); System.out.println("This is method2"); >//this must be static else we will get compilation error private static void printLines() < System.out.println("Starting method"); System.out.println("Doing someting"); >default void mymethods() < method1(); method2(); >> public class JavaExample implements MyInterfaceInJava9 < public static void main(String args[]) < JavaExample je = new JavaExample(); je.mymethods(); >>

Starting method Doing someting This is method1 Starting method Doing someting This is method2

Here is the screenshot of Eclipse Oxygen.

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About the Author

I have 15 years of experience in the IT industry, working with renowned multinational corporations. Additionally, I have dedicated over a decade to teaching, allowing me to refine my skills in delivering information in a simple and easily understandable manner.

Comments

hello, everyone I have a doubt in java 9 features in interface is that if private static method can also used in default method then what is the purpose of private instance method

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Private Methods in Java Interfaces

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1. Overview

From Java 9, private methods can be added to interfaces in Java. In this short tutorial, let’s discuss how we can define these methods and their benefits.

2. Defining Private Methods in Interfaces

Private methods can be implemented static or non-static. This means that in an interface we are able to create private methods to encapsulate code from both default and static public method signatures.

First, let’s look at how we can use private methods from default interface methods:

public interface Foo < default void bar() < System.out.print("Hello"); baz(); >private void baz() < System.out.println(" world!"); >>

bar() is able to make use of the private method baz() by calling it from it’s default method.

Next, let’s add a statically defined private method to our Foo interface:

public interface Foo < static void buzz() < System.out.print("Hello"); staticBaz(); >private static void staticBaz() < System.out.println(" static world!"); >>

Within the interface, other statically defined methods can make use of these private static methods.

Finally, let’s call the defined default and static methods from a concrete class:

public class CustomFoo implements Foo < public static void main(String. args) < Foo customFoo = new CustomFoo(); customFoo.bar(); Foo.buzz(); >>

The output is the string “Hello world!” from the call to the bar() method and “Hello static world!” from the call to the buzz() method.

3. Benefits of Private Methods in Interfaces

Let’s talk about the benefits of private methods now that we have defined them.

Touched on in the previous section, interfaces are able to use private methods to hide details on implementation from classes that implement the interface. As a result, one of the main benefits of having these in interfaces is encapsulation.

Another benefit is (as with private methods in general) that there is less duplication and more re-usable code added to interfaces for methods with similar functionality.

4. Conclusion

In this tutorial, we have covered how to define private methods within an interface and how we can use them from both static and non-static contexts. The code we used in this article can be found over on GitHub.

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Private and final methods in Java Programming

In Java private methods are the methods having private access modifier and are restricted to be access in the defining class only and are not visible in their child class due to which are not eligible for overridden. However, we can define a method with the same name in the child class and could access in parent class.

Like private methods final methods in Java are the methods having final non-access modifier instead of private and are again restricted to be accessed in the defining class only and are not visible in their child class due to which are not eligible for overridden. The only difference between private and final methods is that in case of final methods we even can’t define a method with the same name in child class while in case of private methods we could define.

In Java as both private and final methods do not allow the overridden functionality so no use of using both modifiers together with same method.

Example

public class PrivateFinalMethods < private void print() < System.out.println("in parent print"); >public static void main(String[] args) < PrivateFinalMethods obj = new PrivateFinalMethodsChild(); obj.print(); PrivateFinalMethodsChild obj1 = new PrivateFinalMethodsChild(); obj1.print(); >> class PrivateFinalMethodsChild extends PrivateFinalMethods < public void print()< System.out.println("in child print method"); >>

Output

in parent print in child print method

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Private Methods in Java

Private methods in Java have a private access modifier which means they have limited access to the defining class and are not accessible in the child class in inheritance; that is why they are not eligible for overriding.

However, a method can be created in the child class and could be accessed in the parent class. This tutorial demonstrates how to create and use private methods in Java.

Private Methods in Java

As mentioned above, private methods are only accessible in the defining class; we consider the points below for the private methods.

1. Private methods are Class X are only accessible in Class X.
2. Package-private members or methods of package X are only accessible within all the classes of the X package.

Let’s try to create and use private methods in Java. See example:

package delftstack;  public class Private_Methods   private void print()   System.out.println("The Private Method can only be printed in the defining Class");  >  public static void main(String[] args)   Private_Methods Demo = new Private_Methods();  Demo.print();  Private_Methods_Child Demo1 = new Private_Methods_Child();  Demo1.print();  > > class Private_Methods_Child extends Private_Methods   public void print()  System.out.println("The Public Method can be printed anywhere");  > > 

The code above creates a private method and calls it in the same class and also a public method to call it in the parent class; the output will be:

The Private Method can only be printed in the defining Class The Public Method can be printed anywhere 

If we change the public method to private in the child class, it will throw an exception. See example:

package delftstack;  public class Private_Methods   private void print()   System.out.println("The Private Method can only be printed in the defining Class");  >  public static void main(String[] args)   Private_Methods Demo = new Private_Methods();  Demo.print();  Private_Methods_Child Demo1 = new Private_Methods_Child();  Demo1.print();  > > class Private_Methods_Child extends Private_Methods   private void print()  System.out.println("The Public Method can be printed anywhere");  > > 

We cannot access the print method from the child class. See output:

Exception in thread "main" java.lang.Error: Unresolved compilation problem:  The method print() from the type Private_Methods_Child is not visible   at delftstack.Private_Methods.main(Private_Methods.java:11) 

Sheeraz is a Doctorate fellow in Computer Science at Northwestern Polytechnical University, Xian, China. He has 7 years of Software Development experience in AI, Web, Database, and Desktop technologies. He writes tutorials in Java, PHP, Python, GoLang, R, etc., to help beginners learn the field of Computer Science.

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