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Java Default Method Interface: Java Explained

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The Java language has evolved a lot since its debut in 1995. With each addition and revision, new features such as generics and lambda expressions have been added to make programming easier and better. The latest addition to the Java sphere is the default method interface, and it promises to make writing code simpler and easier. In this article, we explore what a default method interface is, what its benefits are, how to implement one in Java, plus advantages, disadvantages and use cases for them.

What Is a Default Method Interface?

A default method interface in Java is an interface with a method signature that has been declared and implemented with the keyword “default”. The purpose of this type of interface is to allow developers to add default behaviour to an existing interface without breaking existing code that implements it. This feature allows for much greater flexibility in writing code, as it makes it possible for developers to supply implementations for existing interfaces without having to rewrite or change existing code.

Default methods are also useful for adding new methods to existing interfaces without breaking existing code. This is especially useful when working with legacy code, as it allows developers to add new functionality without having to rewrite the entire codebase. Additionally, default methods can be used to add functionality to existing interfaces that are not part of the Java language, such as third-party libraries.

What Are Its Benefits?

The benefits of the default method interface are numerous. The first benefit is that it allows developers to keep existing code that uses an interface unaltered while providing them with options for including additional methods or implementations. This eases maintenance and helps prevent code bloat, since new default implementations can be added without changing existing code. In addition, because existing implementations of interfaces are not affected by the addition of default methods, developers do not have to worry about compatibility issues.

Another benefit of the default method interface is that it allows for much greater flexibility when writing code. Since a single interface can contain multiple implementations, developers can easily create complex functionality without having to write long blocks of code. Additionally, this flexibility helps developers create code that is far more testable and maintainable, since they can easily add or remove default implementations.

The default method interface also allows developers to create code that is more extensible. By adding default implementations to an interface, developers can easily add new features to existing code without having to rewrite the entire codebase. This makes it easier to keep code up to date with the latest technologies and trends, while also making it easier to maintain and debug existing code.

How to Implement a Default Method Interface in Java

Implementing a default method interface in Java is easy. The first step is to declare your interface. To do so, use the “interface” keyword and the name of your interface. Then, write the signature of your method, making sure to include the keyword “default” before it. Finally, provide an implementation for the method. Here is an example of a simple default method interface:

interface MyInterface {  default void myMethod() {    System.out.println("Hello World!");  }}

To implement this interface in your code, simply use the implements keyword when creating your class:

class MyClass implements MyInterface {  // ...  }

Once you have declared your class as implementing the interface, you can call the default method directly:

MyClass myClass = new MyClass();myClass.myMethod(); // prints "Hello World!"

It is important to note that the default method interface is only available in Java 8 and later. If you are using an earlier version of Java, you will need to use a different approach to implement your interface.

Advantages of Using Default Method Interfaces

The advantages of using a default method interface are vast. For one thing, they make it easier to manage legacy code since developers can easily add default implementations without worrying about breaking existing code that uses the interface. In addition, default methods promote testability and maintainability by making it easy to add new functionality without requiring major code changes.

Default methods also provide an advantage for developers who need to quickly develop code that conforms to an existing interface. By using the default keyword, developers can quickly code up an implementation that satisfies the requirements of an existing interface while allowing them to focus their efforts on other parts of their project.

Furthermore, default methods can help reduce the amount of code duplication that can occur when implementing multiple interfaces. By using the default keyword, developers can create a single implementation that satisfies the requirements of multiple interfaces, thus reducing the amount of code that needs to be written and maintained.

Disadvantages of Default Method Interfaces

While there are many advantages to using a default method interface in Java, there are also some drawbacks. For example, because the default keyword does not actually create a class, if two interfaces that use the same method name have different defaults, the compiler will not be able to resolve the ambiguity and will throw an error. Additionally, using default methods makes it harder for developers to remember which methods come from which interfaces.

Another disadvantage of default method interfaces is that they can lead to code bloat. Since the default methods are included in the interface, they must be included in the compiled code, even if they are not used. This can lead to larger compiled code sizes, which can be an issue for applications that need to be as small as possible.

Common Use Cases for Default Method Interfaces

Default method interfaces are especially useful for extending existing interfaces with additional behaviour without bothering with changing current implementations. Many Java frameworks such as Spring and Hibernate use them for this purpose. Additionally, they are useful when an application needs to allow developers to implement an abstract class with completely different behaviour than what was originally expected.

Default method interfaces are also useful for providing backward compatibility when an interface needs to be updated. By using default methods, developers can add new methods to an existing interface without breaking existing implementations. This allows developers to add new features to an interface without having to worry about breaking existing code.

Practical Examples of Default Method Interfaces in Java

The most common use case for a default method interface is adding additional methods to an existing interface. For instance, consider the Comparator interface in Java. It already has two methods defined – compare() and equals() – however they can be extended by using a default method:

interface Comparator {  int compare(Object o1, Object o2);  boolean equals(Object obj);  // additional behaviour provided by default method:  default void sort(List list) {     list.sort(this);   }  }  

In this example, we added an additional method – sort() – without requiring any changes to any existing implementations of the Comparator interface. As you can see, this is a powerful and useful way of extending an existing interface without affecting any existing code.

Conclusion

The default method interface is a powerful feature of Java that allows developers to easily extend existing interfaces with additional behaviour without modifying existing code. This makes it a great choice for developers who need extra flexibility when writing code. However, it is not without its disadvantages – such as potential conflicts between methods with the same name across different interfaces – so it should be used with care.

Anand Das

Anand Das

Anand is Co-founder and CTO of Bito. He leads technical strategy and engineering, and is our biggest user! Formerly, Anand was CTO of Eyeota, a data company acquired by Dun & Bradstreet. He is co-founder of PubMatic, where he led the building of an ad exchange system that handles over 1 Trillion bids per day.

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