Abstract class and Interface are two closely related Java constructs. Although both can’t be instantiated, meaning you cannot create an object from them, Each plays a significant role in creating abstractions that provide classes or abstract methods. The abstract methods are crucial in organizing and defining the functions that an object can assume. Therefore, it will be helpful to have a clear understanding of what Abstract Class and Interface can do and how they help implement objects correctly.

Abstract Class vs Interface: 12 Must-Know Facts 

  • Both abstract class and Interface can’t be instantiated.
  • Abstract Class can include abstract and non-abstract methods, while Interface can only have abstract methods.
  • When you declare a method as abstract, other non-abstract methods can access it.
  • An interface can be declared a public constant.
  • An abstract class can extend another Java class and implement multiple Java interfaces, while interfaces can extend another Java interface.
  • You don’t need an abstract method in your Interface because every method is said to be abstract.
  • Both Abstract Class and Interface can contain methods and variables.
  • Both Abstract class and interface methods have no code.
  • All abstract methods must be implemented when using the Interface.
  • A regular class can only inherit from a single abstract class but can inherit from multiple interfaces.
  • The abstract class uses the keyword “abstract,” while Interface uses the keyword “interface.”
  • An abstract is extended using the keyword “extends,” while interfaces are implemented using the keyword “implements.”

Abstract Class vs Interface: Side-by-Side Comparison

Abstract ClassInterface
What it isIt is a class that is declared an abstract It is an abstract type that specifies functionality.
Primary useUsed in implementing functionality across a set of classUsed in defining functionality that a class must implement
Keywords usedAbstract, extendedInterface, implements
ExtensionCan extend a regular classCan extend another Java interface
ImplementationCan implement an interfaceCannot implement an abstract class
Methods availableAbstract and non-abstractAbstract
Inheritance supportDoesn’t support inheritanceSupport multiple inheritances
Available variablesFinal, non-final, static, and non-staticStatic and final define by default
SpeedFastSlow

What is Abstract Class?

In short, an abstract class is simply a class that is declared as an abstract. To declare a regular class as an abstract, you must use the keyword “abstract.” you can use an abstract class to create a default functionality for the subclasses. The subclasses are essential because you can use them to modify or add instance variables and methods to the class. However, abstract classes don’t have a code, which means they cant create an object. So, why is it important?

If you program a regular class, you can create an object. However, changing the regular class into an abstract class will result in an error because abstracts cannot instantiate an object. But take an instance where you have a class you don’t want to develop as an object, but you need to create objects on its subclasses. Here is where the Abstract class is helpful. 

How does Abstract Class Work? 

Once you add the keyword abstract to the regular class, you have created an abstract class meaning it can’t be instantiated, but you can have subclasses that can create objects. Also, abstract classes enable you to share common fields with every subclass you are programming.

For instance, we have a subclass “dog” and “cat,” which is extended from “animals.” so we want to program our “dog” and “cat” to be able to make noise but produce different sounds. 

Because we only need the subclasses “dog” and “cat” to make noise but not our regular class “animals,” we change the normal class into an abstract class. This means we still have “animals” as the class but cannot be instantiated.

Therefore, we can use the abstract class “animals” to define and organize what we want our subclasses to have. In our case, we add a construct that reads: public abstract void make noise(); which means our subclasses “dog” and “cat” must implement the program to make noise. Once you develop an abstract class, all the child classes that you create will have to implement the method “make noise.”

Sample code for Abstract Class in Java

Abstract Class Syntax

abstract class name{
    // code
}

Abstract class example:

abstract class Shape {
    int b = 20;
    abstract public void calculateArea();
}

public class Rectangle extends Shape {
    public static void main(String args[]) {
        Rectangle obj = new Rectangle();
        obj.b = 200;
        obj.calculateArea();
    }
    public void calculateArea() {
        System.out.println("Area is " + (b * b));
    }
}

What is Interface?

An interface is similar to an Abstract class because it can’t be instantiated. However, some differences help define the Interface. First, when creating an interface, you will use the keyword “interface.” you don’t need an abstract method in your Interface because all methods are said to be abstracts that are not implemented. 

Any field you declare in the Interface should be static and final. It means you must instantiate each field with a value. Because interfaces are static, the values will be presented in every object in that class. Additionally, implementing a field in the Interface will result in an error that states; abstract methods do not specify a body. This is because all interfaces are assumed to be unimplemented abstract methods. 

How does the Interface work?

You will use the keyword “implement” on the regular class followed by the interface name to create an interface. Once you have completed your Interface, you need to organize what you want your object to have. Additionally, an interface is public, static, and final by default.

Therefore, you need to instantiate your fields with values. This means any class implementing the Interface will have all the properties you indicate. For instance, if you implement a “dogInterface” you may create a field-defining how you want it to bark or poop. Any class that will implement the “dogInterface” will develop an object with all the characters defined in the Interface’s field. 

Sample code for Interface

Interface Syntax

interface name{
//methods
}

Java Interface Example:

interface Pet {
    public void test();
}
class Dog implements Pet {
    public void test() {
        System.out.println("Interface Method Implemented");
    }
    public static void main(String args[]) {
        Pet p = new Dog();
        p.test();
    }
}

The Differences Between Abstract Class and Interface

Here is a quick breakdown of the differences between Abstract Class and Interface:

Abstract ClassInterface
Both abstract and non-abstract methods can be used in abstract classOnly an abstract method can be used in interface
Abstract class cannot support multiple inheritancesInterface can support multiple inheritances
The variables used in Abstract class is final, non-final, static, and non-staticThe variables used in Interface is static and final 
Its possible for abstract class to provide an implementation of interfaceIt’s impossible for an interface to provide for the implementation of abstract class
A regular class is declared abstract class using the abstract keywordA regular class is declared interface using the interface keyword
Abstract class can be used to extended a regular class Interface can only extended another java interface
Abstract class can have multiple member class such as private and protected Interface members are public by default

Abstract Class vs Interface Summary

Keywords are crucial in defining the regular class as either Abstract class or Interface. The keyword abstract is used to create an abstract class, and Interface is used in developing an interface. Abstract class and Interface share some similarities, such as they can both have methods and variables. But, abstract methods support both abstract and non-abstract methods, while you can only use abstract methods in the Interface. Therefore, abstract classes allow body implementations through non-abstract methods, unlike interfaces where they are assumed to be abstract. 

Also, as much as abstract classes and interfaces have variables, interfaces can only have static and final variables defined by default. In contrast, abstract classes have final, non-final, static, and non-static variables. Interface variables are static and final because java interfaces cannot be instantiated independently, which means the variables must be assigned in a static context.

Regular classes can implement multiple interfaces and only one abstract. It means you can only extend one class, but you can implement many interfaces. Also, a user-defined class can be extended and implemented simultaneously. 

Abstract Class vs Interface: Which One Is Better? Which One Should You Use?

Abstract class and Interface play different roles in abstraction; therefore, what you are programming will guide your selection. Let’s see how best you can select each of them:

Use an abstract class if you want to achieve a common functionality across a set of classes. All the subclasses under your abstract class implement the functionality you have created. For example, start an abstract class animal and give a functionality to make noise. For each object you develop in the abstract class, animals will have to implement the functionality to make noise.

Use an interface if you want to define your functionality across a set of classes. It’s vital to note that interfaces can’t implement the functionality. Also, an interface is static and final by default; thus, it should be instantiated with values. For example, if you create an AnimalInterface and define the functionality walk, any object you make on other types of classes will automatically be able to walk. 

What of the speed, does it matter? The speed of either Abstract Class or interface should not determine when choosing one to use. However, you should note that an interface is slower than an abstract class because Interface involves a search before providing an actual method in java.

Abstract Class Vs Interface: Full Comparison FAQs (Frequently Asked Questions) 

Which is better to use: abstract class or Interface?

We can’t rate which is best between abstract class and interface in terms of use because they both play important but distinguished roles in abstraction. For example, an abstract class will help create functionality that subclasses can implement, but you will need to use the Interface to define the functionality.

What is the difference between an abstract class and an Interface?

There are many differences between abstract class and Interface, such as:

  • An abstract class can use abstract and non-abstract methods, whereas Interface can only use abstract methods.
  • When creating an Abstract class, you use the keyword abstract, but you use the keyword interface when creating an interface.
  • An abstract class has final, non-final, static and non-static variables, while an interface has static and final variables.
  • Abstract classes are extended using the keyword extends, while an interface is implemented using the keyword implement.
  • Abstract classes do not support multiple inheritances, while an interface supports various inheritances.

Why is abstract class faster than Interface?

An abstract class is faster because an interface takes time to search for the actual method in java.

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