Java Inheritance


Java inheritance refers to the ability of a Java Class to inherit the properties from some other Class. Think of it like a child inheriting properties from its parents, the concept is very similar to that. In Java lingo, it is also called extend-ing a class.

Some simple things to remember:

  • The Class that extends or inherits is called a subclass
  • The Class that is being extended or inherited is called a superclass

Why use inheritance in Java

  • For Method Overriding (so runtime polymorphism can be achieved).
  • For Code Reusability.

Terms used in Inheritance

  • Class – A class is a group of objects which have common properties. It is a template or blueprint from which objects are created.
  • Sub Class/Child Class – Subclass is a class that inherits the other class. It is also called a derived class, extended class, or child class.
  • Super Class/Parent Class – Superclass is the class from where a subclass inherits the features. It is also called a base class or a parent class.
  • Reusability – As the name specifies, reusability is a mechanism that facilitates you to reuse the fields and methods of the existing class when you create a new class. You can use the same fields and methods already defined in the previous class.

Thus, inheritance gives Java the cool capability of re-using code, or sharing code between classes!

Syntax

class Subclass-name extends Superclass-name { //methods and fields }

The extends keyword indicates that you are making a new class that derives from an existing class. The meaning of “extends” is to increase the functionality.

In the terminology of Java, a class which is inherited is called a parent or superclass, and the new class is called child or subclass.

Let’s describe it with the classic example of a Vehicle class and a Car class :

public class Vehicle { public void start() { // starting the engine } public void stop() { // stopping the engine } } public class Car extends Vehicle { int numberOfSeats = 4; public int getNumberOfSeats() { return numberOfSeats; } }

Here, we can see the Car class inheriting the properties of the Vehicle class. So, we don’t have to write the same code for the methods start() and stop() for Car as well, as those properties are available from its parent or superclass. Therefore, objects created from the Car class will also have those properties!

Car tesla = new Car(); tesla.start(); tesla.stop();

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But, does the parent class have the methods of the child? No, it doesn’t.

Therefore, whenever you need to share some common piece of code between multiple classes, it is always good to have a parent class, and then extend that class whenever needed! Reduces the number of lines of code, makes code modular, and simplifies testing.

What can be inherited?

  • All protected and public members (fields, methods, and nested classes) from parent, regardless what package the sub-class is in. If a sub-class is in the same package as its parent, it will also inherit package-private members from parent.

What cannot be inherited?

  • private fields and methods
  • Constructors. Although, the subclass constructor has to call the superclass constructor if its defined (More on that later!)
  • Multiple classes. Java supports only single inheritance, that is, you can only inherit one class at a time.
  • Fields. Individual fields of a class cannot be overriden by the subclass.

Type Casting & Reference

In Java, it is possible to reference a subclass as an instance of its superclass. It is called Polymorphism in Object Oriented Programming (OOP), the ability for an object to take on many forms. For example, the Car class object can be referenced as a Vehicle class instance like this :

Vehicle car = new Car();

Although, the opposite is not possible :

Car car = new Vehicle(); // ERROR

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Since you can reference a Java subclass as a superclass instance, you can easily cast an instance of a subclass object to a superclass instance. It is possible to cast a superclass object into a subclass type, but only if the object is really an instance of the subclass. So keep this in mind :

Car car = new Car(); Vehicle vehicle = car; // upcasting Car car2 = (Car)vechile; //downcasting Bike bike = new Bike(); // say Bike is also a subclass of Vehicle Vehicle v = bike; // upcasting, no problem here. Car car3 = (Car)bike; // Compilation Error : as bike is NOT a instance of Car

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Now you know how to share code through a parent-child relationship. But, what if, you do not like the implementation of a particular method in the child class and want to write a new one for it? What do you do then?

Override it!

Java lets you override or redefine the methods defined in the superclass. For example, your Car class has a different implementation of start() than the parent Vehicle, so you do this :

public class Vehicle { public void start() { System.out.println("Vehicle start code"); } } public class Car extends Vehicle { public void start() { System.out.println("Car start code"); } } Car car = new Car(); car.start(); // "Car start code"

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So, it’s pretty simple to override methods in the subclass. Although, there is a catch. Only that superclass method with the exact same method signature as the subclass method will be overriden. That means the subclass method definition must have the exact same name, same number and type of parameters, and in the exact same sequence. Thus, public void start(String key) would not override public void start().

Notes

  • You cannot override private methods of the superclass. (Quite obvious, isn’t it?)
  • What if the method of superclass which you are overriding in the subclass suddenly gets obliterated or methods changed? It would fail in runtime! So Java provides you a nifty annotation @Override which you can place over the subclass method, which will warn the compiler of those incidents!

Annotations in Java is a good coding practice, but they are not a necessity. The compiler is smart enough to figure out overriding on its own though. Unlike other OOP languages, Annotations in Java it doesn’t necessarily modify the method or add extra functionality.

How to call super class methods?

Funny you ask about it! Just use the keyword super :

public class Vehicle() { public void start() { System.out.println("Vehicle start code"); } } public class Car extends Vehicle { public void run() { super.start(); } } Car car = new Car(); car.run(); // "Vehicle start code"

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Note: Although you can call the parent method by using a super call, you cannot go up the inheritance hierarchy with chained super calls.

How to know the type of a class?

Using the instanceof keyword. Having lots of classes and subclasses it would be a little confusing to know which class is a subclass of which one in runtime. So, we can use instanceof to determine whether an object is an instance of a class, an instance of a subclass, or an instance of an interface.

Car car = new Car(); boolean flag = car instanceof Vehicle; // true in this case!

Constructors & Inheritance

As mentioned earlier, constructors cannot be directly inherited by a subclass. Although, a subclass is required to call its parent’s constructor as the first operation in its own constructor. How? You guessed it, using super :

public class Vehicle { public Vehicle() { // constructor } public void start() { System.out.println("Vehicle start code"); } } public class Car extends Vehicle { public Car() { super(); } public void run() { super.start(); } }

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Remember, if the superclass does not have any constructors defined, you don’t have to call it explicitely in the subclass. Java handles that internally for you! Invocation to super constructor is done in the case when the super class is to be called with any other constructor other than the default constructor.

If no other constructors are defined, then Java invokes the default super class constructor (even if not defined explicitly).

Congrats, now you know all about Inheritance! Read more about advanced ways to inherit things in Abstract Classes and Interfaces!

Here are some useful links to learn more about Inheritance in Java (great for those starting out with Java):

  • https://www.codejava.net/java-core/the-java-language/12-rules-and-examples-about-inheritance-in-java
  • https://medium.com/java-for-absolute-dummies/inheritance-in-java-programming-39176e0016f3

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