Why do we use abstract class

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Looking to accelerate your career growth and increase your income? Learn About Certifications. When and how to use Abstract Class and Interface? Filter By Role. August 21, Author: Zsolt Nagy. What Is an Interface? What Is an Abstract Class? Enroll in Our Coding Bootcamp Program QuickStart offers coding bootcamp program partnering with the reputed universities in the United States to help you launch your career as a Developer.

Learn More! Explore More. Previous Next. Explore Our Bootcamp Programs. Learn More. Download Understanding Python Whitepaper This whitepaper has been written for people looking to learn Python Programming from scratch. First Name. Last Name. Email Address. Job Title or Role. Abstract class and abstract method are two ways through which Java assist you on coding at a certain level of abstraction. We will look at syntax level details, capability, and limitation of abstract class at the later half of article, but you must understand why an abstract class is important.

More importantly, why you, a Java programmer, must understand motive and use of an abstract class. From my years of experience, I can say that making a class at first an interface or abstract class is a good practice, it will not only help you to test your code quickly by supplying mock objects, but also help you to write code at the certain level of abstraction. In this article, we will understand why we need abstract class and abstract method and learn how to use them by writing a simple example.

Where do you use Abstract Class in Java Apart from conventional use of Abstract class to write flexible code, there are several clever uses of abstract class, which ensures certain best practices are followed.

Most of these usage take advantage of important properties of abstract class e. If you do so, compiler will flag an error. One of the clever use of abstract class I have seen is to use abstract class in conjunction with Factory method pattern to ensure that your client always use Factory method to create instance of object, instead of accidentally calling constructor. By making Product class abstract, compiler will track down any call to new Product and force them to use factory methods like makeProduct or getProduct.

One of the best example of this technique is EnumSet class in JDK, unlike other implementation of Set interface, EnumSet is abstract, which ensures that you always make use of factory methods like EnumSet. Imagine, if EnumSet was not abstract and you are using it first time, how would you have created instance of EnumSet? Factory method or Constructor? Always remember, compiler is your best friend, it finds bugs for you, So make full use of your best friend.

A best practice can be forgotten but a rule enforced by compiler will never. Why Abstract class is Important in Java Though there are lot of difference between abstract class and interface , key thing to remember is that they both provides abstraction.

Let's take an example, you need to design a program, which can produce reports for employees e. Let's assume that currently your organization only has permanent employees, which are paid monthly. You know that and you write code based upon that, after sometime your company started recruiting contract employees, which are paid at hourly rate rather than monthly salary.

Now, if you need to rewrite your program to support this, they your program is not flexible enough. On the other hand, if you just needs to write some code to plug this new type of employee into system, they your program is very much flexible and maintainable.

If you would have known about abstract class , you would have made Employee an abstract class and methods like salary abstract, because that is what varies between different types of employees.

Now introducing a new type of Employee would be cakewalk, all you need to do is to create another subclass of Employee to represent ContractEmployee , and their salary method return salary based upon a number of hours they had worked multiplied by their hourly rate. So, you get the idea right, we code at a certain level of abstraction, which allows us to accommodate new changes in our system. In order to serve fruits, we need to prepare them for eating, which involves either cutting them in slices or peel them.

We have two concrete implementations of Fruit class, Mango and Banana , I chose these two because first, they are my favorites, and second they have different methods of preparing them. You cut mangoes but you peel bananas. Color ; import java. ArrayList ; import java. Collection ; import java. As a Java programmer, you must always remember following things about abstract class and method. Think of two scenarios where you can advantage of this property, always remember a weapon can be used for both attack and defence.

One scenario I have already discussed here is to making sure that your client always use factory method to create object instead of directly calling constructor. On related note, always use Override annotation while overriding a method in Java. Use an abstract class to provide default behavior. Subclass only a base class in a hierarchy to which the class logically belongs. Use an interface When creating a standalone project which can be changed at will, use an interface in preference to an abstract class; because, it offers more design flexibility.

Use interfaces to introduce polymorphic behavior without subclassing and to model multiple inheritance—allowing a specific type to support numerous behaviors. Use an interface to design a polymorphic hierarchy for value types. Use an interface when an immutable contract is really intended. A well-designed interface defines a very specific range of functionality.

Split up interfaces that contain unrelated functionality. When you want a base class which should not be instantiated and have some common functionality for every class well as different implementation of a method in derived classes for other methods. For more information you can see Abstract Class in C with Example.

Basically Abstract class should be used for 'is-a' relationship and Interface should be used for 'has-a' relationship. If we want to retrict all the subclasses must have certain functionality, then Abstract class with implementation is recomended.

So, overriding already implemented member in the subclasses is not a good practice here. In a particular class, where there are more behaviours that will get change during runtime, then implementing from multiple interfaces are recommended.

In my knowledge the abstract class will contain a concrete method which can not be override by the derived classes. Means if you DON'T want to change one basic business rule the concrete method defined in the base abstract class by the derived classes, then go for abstract class with the concrete method. As Ricky says. When two classes share the same properties or method the common denominator is your abstract class. There are many benefits aside from this.

Each of these is a type of employee so your employee class will be the abstract base class which will contain all of the common shared properties, methods etc. Now when you create your Manager class for e. If you think about it it doesn't make sense to create an instance of an Employee class but it does to create an instance of say the Manager class or an Associate class which makes it easy to know when to have a abstract class, just ask yourself does it make sense to instantiate X type of class and if the answer is no you are probably better off having it as an abstract class, this is not a hard and fast rule but a good guide.

Another benefit of having an abstract base class is if you need to make a change you only need to make it in one place. This may help you a little more. Use Abstract class when there is a 'IS-A' relationship between classes.