Java Interfaces

TABLE OF CONTENTS :

TABLE OF CONTENTS

Lerne mit deinen Freunden und bleibe auf dem richtigen Kurs mit deinen persönlichen Lernstatistiken

Nie wieder prokastinieren mit unseren Lernerinnerungen.

Dive into the intriguing world of Java Interfaces with this comprehensive guide. You will get insightful details about everything from the definition and principles of Java interfaces, to their various use cases in Computer Programming. The article also offers a comparative study of Java Interfaces vs Abstract classes, enriched with hands-on learning through examples. Further, it explores complex concepts like the Comparable and Functional Interfaces in Java. Whether you're a novice just starting with Java or experienced seeking advanced knowledge, you'll find all the information you need on Java Interfaces here.

Understanding Java Interfaces

Before diving into the intricacies of Java Interfaces, it's crucial to understand that this concept forms a backbone in Java programming and enables the abstraction principle.

Definition: What is an interface in Java?

An interface in Java is a blueprint of a class. It has static constants and abstract methods. The interface in Java is a mechanism to achieve abstraction and multiple inheritances.

public interface ExampleInterface {
   // abstract method
   void method1();
}

The structure of Java Interface is simplistic, and the purpose is to create a contract for classes. For instance, if a class implements an Interface, the class subscribes to all the methods within that Interface.

Java Interface Principles explained

Following are the fundamental principles to note about Java Interface:

Interfaces cannot be initiated because they preserve methods that are abstract - having no body.
In Java, a class utilising an interface must inherit all methods from the interface, even if the methods are not applicably executed within the class.
A class can implement multiple interfaces, promoting the Java principle of multiple inheritances.
As of Java 8, interfaces can hold default methods and static methods.

When java interfaces contain only single abstract methods, \(\textit{\(they're treated as functional interfaces in Java 8}\). Lambda expressions heavily utilise these interfaces.

Java Interface use cases in Computer Programming

Java interfaces find extensive usage across multiple use-cases:

Abstraction	Promotes the abstraction principle where you can hide specific details and show only essential features of an object. Java Interfaces are significant to realise abstraction.
Multiple Inheritances	Interfaces support multiple inheritances in Java wherein a class can implement multiple interfaces.
Loose Coupling	In scenario-based programming or modular programming, interfaces ensure a loose coupling between modules. It ensures that objects can interact without knowing specific details about each other.

Deep understanding of Java Interfaces paves a path towards becoming an effective communicator with the system.

Interface vs Abstract class Java: A Comparative Study

The world of Java is expansive and teeming with various elements, each lending a specific functionality and adding a unique flavour to the entire system. Two such crucial components are Java Interfaces and Abstract Classes. Both are unique entities with certain parallels and differences that are further explored in this article.

Distinguishing Java Interfaces from Abstract Classes

Before venturing into the distinction, let's establish the fundamental concepts of these two entities.

An Interface in Java is a wholly abstract class that holds static constants and abstract method. It offers a way to ensure class abstraction and multiple inheritances. In contrast, an Abstract Class in Java is a superclass that cannot be instantiated and is utilised to declare shared attributes or methods for its subclasses.

Now, let's engage in a deeper exploration of the differentiating factors between these two:

Definition and Creation	A Java Interface is created using the Interface keyword, and an Abstract Class is created using the Class keyword together with the Abstract modifier.
Implementation	The Interface is implemented using the 'implements' keyword, while an Abstract Class is extended using the 'extends' keyword.
Default Method	The Interface declares methods as abstract by default. For Abstract Classes, you must specifically declare the abstract keyword for such methods.
Multiple Inheritance	Java Interface supports multiple inheritances as a class can implement multiple interfaces. However, a class extending an Abstract Class cannot extend another class due to the limitation of multiple inheritances.

In essence, the core deviation between Java Interfaces and Abstract Classes gravitates predominantly towards the concept of implementation, extension, and inheritance ability.

Example scenarios: When to use Interface or Abstract Class in Java?

In various programming scenarios, choosing whether to use Java Interface or Abstract Class can make a great difference. Context and needs, primarily drive this decision:

If a system ought to provide multiple inheritances, go for Interfaces as a class can implement multiple interfaces.
If a system needs method implementations in a parent class, opt for Abstract Classes. Abstract Classes allow the provision of default behaviour.
If a system needs to change the API without affecting the classes that use the API, resort to Interfaces. Interfaces provide default methods that introduce non-abstract method implementations to interfaces.

But do remember, the choice between Java Interfaces and Abstract Classes is not always binary or mutually exclusive. In some scenarios, one can use both simultaneously to maximise advantages and minimize limitations. For example, consider a situation where a superclass has to define default behaviour but also needs to ensure the same method protocols for several subclasses. The superclass can be an abstract class with common methods, while an interface can ensure the method protocol.

public abstract class AbstractSuperClass {
   // Common behavior across subclasses
   public void commonBehavior() {
       // Method body 
   }
}

public interface ProtocolInterface {
   // Ensure protocol
   void ensureProtocol();
}

public class Subclass extends AbstractSuperClass implements ProtocolInterface {
   // implements the protocol
   public void ensureProtocol() {
       // Method body
   }
}

The above scenario is a fine illustration of how abstract classes and interfaces can synergise. As is true with most programming decisions, context is vital, and there are rarely absolute right or wrong choices – only options that are more appropriate given specific goals and conditions.

Hands-on Learning: Java Interface Examples

Learning by doing is always an effective approach, especially when dealing with programming principles like Java Interfaces. To ensure that you grasp the topic thoroughly, let's delve into some illustrative examples that demonstrate the practical implementation of Java Interfaces.

Beginner's guide to Java interface example

The quintessential first step in understanding Java Interfaces is to create a simple interface and a class implementing it. In this scenario, let's define an interface 'Animal' with a single method 'sound()'.

public interface Animal {
   // abstract method
   void sound();
}

Now, let's define a class 'Dog' that implements this interface:

public class Dog implements Animal {
   // implementing abstract method
   public void sound() {
       System.out.println("The dog says: bow wow");
   }
}

The class 'Dog' implements the interface 'Animal' and provides its own implementation of the method 'sound()'. To use this, let's create a simple test class:

public class Test {
   public static void main(String args[]) {
       Dog dog = new Dog();
       dog.sound();
   }
}

When you run this 'Test' class, you'll see an output of "The dog says: bow wow". This is an elementary example of how you'd implement an interface in Java.

Advanced Java interface example for experienced learners

Now, let's explore a more complex example that illustrates how multiple interfaces and methods function together in a Java program. Imagine you're building a game, and your game has both flying entities and ground entities. Firstly, let's define an interface 'Flyable' for entities capable of flying:

public interface Flyable {
   void fly();
}

Then, let's define another interface 'Walkable' for entities that can walk:

public interface Walkable {
   void walk();
}

Now, let's define an 'Eagle' class that implements 'Flyable' and 'Bird' class that implements both the interfaces 'Flyable' and 'Walkable'.

public class Eagle implements Flyable {
   public void fly() {
       System.out.println("Eagle is flying");
   }
}

public class Bird implements Flyable, Walkable {
   public void fly() {
       System.out.println("Bird is flying");
   }

   public void walk() {
       System.out.println("Bird is walking");
   }
}

In our game, 'Eagle' can only fly, whereas 'Bird' can both fly and walk. These classes provide implementation for all the methods of the interfaces they implement. Finally, let's create a game class to test these:

public class Game {
   public static void main(String args[]) {
       Eagle eagle = new Eagle();
       eagle.fly();

       Bird bird = new Bird();
       bird.fly();
       bird.walk();
   }
}

When we run this 'Game' class, we obtain "Eagle is flying", "Bird is flying", and "Bird is walking" as outputs. Here, we observe two prime principles. Firstly, a class can implement any number of interfaces. Secondly, each class must provide its own implementation of the methods of the interfaces it implements. These examples give a solid understanding of how interfaces operate within Java and how they promote multiple inheritances and abstraction.

Digging Deeper: Comparable Interface in Java

Moving on, there's another aspect of Java interfaces that's worth discussing, and that's the Comparable Interface. This particular interface is employed to order the objects of the user-defined class.

Conceptual Understanding: What is Comparable Interface in Java?

In essence, interfaces in Java are mechanisms for creating custom types and achieving fully abstract classes that possess static constants and abstract methods. One such built-in interface within the Java Collection Framework is the Comparable Interface. This Interface is housed in java.lang package and contains a solitary method, namely, 'compareTo()'. This method offers natural ordering, characteristically defined by the natural order of the object.

The Comparable Interface is used to order the objects of a user-defined class. Typically, objects of built-in classes like String and Date implement java.lang.Comparable by default. For user-defined classes to be sorted according to a natural order, they should implement this interface.

Do note that this natural ordering is possible only when there is a meaningful comparison between the elements of the class. The method to compare in the Comparable Interface resembles:

int compareTo(T obj)

This method is utilised to compare the current object with the specified object and returns a

Negative integer if the current object is less than the specified object
Zero if the current object equals the specified object
Positive integer if the current object is greater than the specified object

Also noteworthy is the fact that natural ordering can be used with other classes, such as TreeSet and TreeMap, to maintain the order of their elements automatically.

Practical Use: Examples of Comparable Interface in Java

To fully understand the implementation of the Comparable Interface in Java, let's delve into some practical examples. Suppose you're creating an application where you need to sort a list of students according to their age. For this, you need your Student class to implement the Comparable Interface:

public class Student implements Comparable {
   private String name;
   private int age;

   public Student(String name, int age) {
       this.name = name;
       this.age = age;
   }

   public int compareTo(Student s) {
       return this.age - s.age;
   }
}

In this class, the 'compareTo()' method compares Student objects based on their age. To test this, let's create a List of Student objects and sort the List:

import java.util.*;

public class Test {
   public static void main(String args[]) {
       List students = new ArrayList<>();
       students.add(new Student("John", 20));
       students.add(new Student("Alice", 18));
       students.add(new Student("Bob", 19));

       Collections.sort(students);

       for(Student s: students){
           System.out.println(s.getName());
       }
   }
}

When you run the above code, it will print the names of the students in the order of their ages. This means our Student class correctly implemented the Comparable interface, and instances can be sorted based on their natural ordering. Remember, the Comparable interface in Java can play a significant role while dealing with sorted collections. Creating a custom sorting logic for objects of a user-defined class becomes a lot simpler with the compare() method. With practical exposure and understanding, you can leverage comparable and comparator interfaces effectively in Java.

Introduction to Functional Interface in Java

On the journey to mastering Java, encountering the Functional Interface is inevitable. It's not just an underlying component; it's one that comes packaged with enormous benefits. This section uncovers what a Functional Interface is, its unique aspects, and how it fits into the world of Java Programming.

An Overview of Functional Interface in Java

In Functional Programming, an interface with a single abstract method is known as a Functional Interface. Introduced in Java 8, these interfaces are intended to be implemented by a Lambda expression, an anonymous function that allows you to pass methods just like variables.

It's noteworthy that while a Functional Interface can possess any number of default and static methods, it can only have one abstract method. The Java API touts several functional interfaces, such as the Consumer, Predicate, and Function interfaces, all housed in the java.util.function package.

Java 8 also introduced the @FunctionalInterface annotation, an informative annotation that ensures the interface adheres to the strict definition of a Functional Interface. If an interface annotated with @FunctionalInterface doesn't meet the criteria (i.e. more than one abstract method is present), the Compiler raises an error.

@FunctionalInterface
public interface GreetingService {
    void sayMessage(String message);
}

In the above code example, GreetingService, adorned with the @FunctionalInterface annotation, qualifies as a functional interface as it has only one abstract method, sayMessage().

Using the @FunctionalInterface annotation is optional but it's a best practice when creating functional interfaces. This annotation helps in maintaining the contract of having a single abstract method in the interface, stopping developers from accidentally adding a second one and breaking the interface’s “functional”-ness.

Use cases of Functional Interface in Java Programming

Functional Interfaces bring several consequential perks into the horizon of Java programming. Aside from their use with Lambda Expressions to simplify and streamline the code, they are used in method references and constructor references. In particular, their potential shines through in the following use cases:

Streams: The Stream API takes extensive advantage of lambda functions and functional interfaces. Things like mapping, filtering, and reducing operations all work on functions passed as lambda expressions implementing functional interfaces.
Event Listeners: Event listener interfaces like ActionListener for click event handling in GUI applications are classic examples of functional interfaces. With lambda expressions, the code becomes much more readable.
Implementing Runnable: The Runnable interface, which is frequently used in multithreaded programming, is a functional interface as it contains only one method, run(). We can use lambda expressions in place of anonymous Runnable classes to write more succinct code.

An exemplar of a Functional Interface easing event handling would be:

button.addActionListener(e -> System.out.println("Button clicked"));

This code uses a lambda expression to provide a concise, functionally equivalent alternative to an anonymous class. The value ‘e’ represents the event being handled - in this case, the act of a button being clicked. Functional Interfaces have further amplified the expressive power of Java, allowing for cleaner, less verbose code that is easier to read and understand. Through examples, one can grasp the power they wield in simplifying and refining the code, thereby enhancing the overall Java programming experience. Indeed, Functional Interfaces are a transformative and vital inclusion in the toolset of every Java programmer.

Java Interfaces - Key takeaways

Java Interfaces are used for abstraction, supporting multiple inheritance, and ensuring loose coupling in modular programming.
An Interface in Java is a fully abstract class containing static constants and abstract methods. It differs from Abstract Classes which are superclasses that cannot be instantiated and are used for declaring shared attributes or methods.
A Java Interface is created using the Interface keyword and implemented using the 'implements' keyword. An Abstract Class is created using the Class keyword and the Abstract modifier, and is extended using the 'extends' keyword.
Java Interface and Abstract Class are not mutually exclusive; they can be used simultaneously in certain scenarios to maximize advantages and minimize limitations.
The Comparable Interface in Java is a built-in interface used to order the objects of a user-defined class based on a natural ordering of the object.
The Functional Interface in Java, introduced in Java 8, is an interface with a single abstract method. It can be implemented by a Lambda expression, an anonymous function allowing the passing of methods like variables.

Frequently Asked Questions about Java Interfaces

Java Interfaces provide a blueprint for classes, ensuring a particular set of methods are implemented. They promote loose coupling, enhance flexibility, support multiple inheritance, and facilitate polymorphism - allowing objects of different classes to be treated as objects of a super class.

The fundamental purpose of Java Interfaces in software development is to define a contract for classes to follow. It ensures a standard structure, promotes reusability, and enables polymorphism, which facilitates loose coupling and high cohesion in programming principles.

In Java, a class can implement multiple interfaces, allowing it to inherit the abstract methods of these interfaces. This gives a certain degree of multiple inheritance, as the class can have different behaviours defined by multiple interfaces.

Yes, you can override default methods in Java interfaces. However, you cannot override static methods or methods from the Object class. It's also necessary to match the method's signatures exactly.

Yes, we can define variables within Java Interfaces. They are implicitly public, static and final so they need to be initialised at the time of declaration. They don't have a default value.

Final Java Interfaces Quiz

Java Interfaces Quiz - Teste dein Wissen

Question

What is an interface in Java?

Show answer

Answer

An interface in Java is a blueprint of a class, including static constants and abstract methods, used to achieve abstraction and multiple inheritances.

Show question

Question

What are the fundamental principles of Java Interface?

Show answer

Answer

Interfaces cannot be initiated due to abstract methods, a class using an interface must inherit all methods, a class can implement multiple interfaces enabling multiple inheritances, and since Java 8, interfaces can contain default and static methods.

Show question

Question

What are some common use cases of Java Interfaces in computer programming?

Show answer

Answer

Java interfaces are commonly used for abstraction (hiding specific details and showing only the essential features of an object), multiple inheritances (allowing a class to implement multiple interfaces), and loose coupling in scenario-based or modular programming.

Show question

Question

Which keyword is used to create an Interface in Java?

Show answer

Answer

An Interface in Java is created using the Interface keyword.

Show question

Question

What is the main difference in terms of inheritance between Java Interface and Abstract Class?

Show answer

Answer

Java Interface supports multiple inheritances, meaning a class can implement multiple interfaces. However, a class extending an Abstract Class cannot extend another class due to the limitation of multiple inheritances.

Show question

Question

In what case would you use both an Abstract Class and an Interface in Java?

Show answer

Answer

For a situation where a superclass needs to define default behaviour but also needs to ensure the same method protocols for several subclasses, both Abstract Class and Interface can be utilised. The superclass can be an abstract class with common methods, while an interface can ensure the method protocol.

Show question

Question

What is the purpose of Java Interfaces?

Show answer

Answer

Java Interfaces allow for multiple inheritance and abstraction in Java programming, prompting the implementing classes to provide their own implementation of the methods of the interfaces.

Show question

Question

How does a class implement an interface in Java?

Show answer

Answer

A class implements an interface by using the keyword 'implements' followed by the interface name. It then provides its own implementation for every method defined in the interface(s).

Show question

Question

Are Java interfaces limited to single method or single inheritance?

Show answer

Answer

No, Java interfaces can define any number of methods, and a class can implement any number of interfaces, thereby enabling multiple inheritances.

Show question

Question

What is the Comparable Interface in Java used for?

Show answer

Answer

The Comparable Interface in Java is used to order the objects of a user-defined class according to a natural order, defined by the natural order of the object.

Show question

Question

What does the compareTo() method, within the Comparable interface, return?

Show answer

Answer

The compareTo() method returns a negative integer if the current object is less than the specified object, zero if they are equal, and a positive integer if the current object is greater than the specified object.

Show question

Question

How can the Comparable interface be implemented in a custom class in Java?

Show answer

Answer

To implement the Comparable interface in a custom class in Java, one needs to define the compareTo(T obj) method in such a class. For instance, in a Student class, one could order students by age by subtracting one student's age from the other's.

Show question

Question

What are Functional Interfaces in Java?

Show answer

Answer

In Java, a Functional Interface is an interface with a single abstract method. Although it can have any number of default and static methods, it can only have one abstract method. Functional Interfaces are typically implemented by a Lambda expression and were introduced in Java 8.

Show question

Question

Which annotation is used with Functional Interfaces in Java and what is its purpose?

Show answer

Answer

The '@FunctionalInterface' annotation is used in Java to ensure that the interface adheres to the strict definition of a Functional Interface. If an interface with this annotation doesn't meet the criteria (has more than one abstract method), the compiler raises an error.

Show question

Question

What are some use cases of Functional Interfaces in Java programming?

Show answer

Answer

Functional Interfaces in Java are used in conjunction with Lambda expressions, method references, and constructor references. Specifically, they are used extensively in Stream API for mapping, filtering, and reducing operations. They aid in improving the readability of event listener interfaces in GUI applications and can be used to implement the Runnable interface in multi-threaded programming.

Show question

Flashcards in Java Interfaces15

Start learning

What is an interface in Java?

An interface in Java is a blueprint of a class, including static constants and abstract methods, used to achieve abstraction and multiple inheritances.

What are the fundamental principles of Java Interface?

What are some common use cases of Java Interfaces in computer programming?

Which keyword is used to create an Interface in Java?

An Interface in Java is created using the Interface keyword.

What is the main difference in terms of inheritance between Java Interface and Abstract Class?

In what case would you use both an Abstract Class and an Interface in Java?

Learn with 15 Java Interfaces flashcards in the free StudySmarter app

Already have an account? Log in

More about Java Interfaces

Save Article

How would you like to learn this content?

Creating flashcards

Studying with content from your peer

Taking a short quiz

How would you like to learn this content?

Creating flashcards

Studying with content from your peer

Taking a short quiz

Free computer-science cheat sheet!

Everything you need to know on . A perfect summary so you can easily remember everything.

Email Address*

Select your language

Java Interfaces

Understanding Java Interfaces

Definition: What is an interface in Java?

Java Interface Principles explained

Java Interface use cases in Computer Programming

Interface vs Abstract class Java: A Comparative Study

Distinguishing Java Interfaces from Abstract Classes

Example scenarios: When to use Interface or Abstract Class in Java?

Hands-on Learning: Java Interface Examples

Beginner's guide to Java interface example

Advanced Java interface example for experienced learners

Digging Deeper: Comparable Interface in Java

Conceptual Understanding: What is Comparable Interface in Java?

Practical Use: Examples of Comparable Interface in Java

Introduction to Functional Interface in Java

An Overview of Functional Interface in Java

Use cases of Functional Interface in Java Programming

Java Interfaces - Key takeaways

Frequently Asked Questions about Java Interfaces

What are the main benefits of using Java Interfaces in object-oriented programming?

What is the fundamental purpose of Java Interfaces in software development?

How can multiple inheritance be achieved through Java Interfaces?

Can you override methods in Java Interfaces?

Can we define variables within Java Interfaces and what are their default values?

Final Java Interfaces Quiz

Java Interfaces Quiz - Teste dein Wissen

Test your knowledge with multiple choice flashcards

Join the StudySmarter App and learn efficiently with millions of flashcards and more!

Learn with 15 Java Interfaces flashcards in the free StudySmarter app

Learn with 15 Java Interfaces flashcards in the free StudySmarter app

Join over 22 million students in learning with our StudySmarter App

More explanations about Computer Programming

Discover the right content for your subjects

Biology

Business Studies

Chemistry

Combined Science

Economics

Engineering

English

English Literature

Environmental Science

Geography

History

Human Geography

Law

Macroeconomics

Marketing

Math

Microeconomics

Physics

Politics

Psychology

Sociology

This is still free to read, it's not a paywall.

You need to register to keep reading

This is still free to read, it's not a paywall.

You need to register to keep reading

Start learning with StudySmarter, the only learning app you need.

Create a free account to save this explanation.

StudySmarter bietet alles, was du für deinen Lernerfolg brauchst - in einer App!