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Inheritance and Polymorphism
Introducing Inheritance

When you hear the word “inheritance”, code may not be the first thing that springs to mind; you’re probably more likely to think of inheriting genetic traits, like eye color from your mother or a smile from your grandfather. But inheritance is also an important feature of object-oriented programming in Java.

Suppose we are building a Shape class in Java. We might give it some points in 2D, a method for calculating the area, and another method for displaying the shape. But what happens if we want a class for a triangle that has some triangle-specific methods? Do we need to redefine all of the same methods that we created for Shape?

No! (Phew.) Lucky for us, a Java class can also inherit traits from another class. Because a Triangle is a Shape, we can define Triangle so that it inherits fields and methods directly from Shape. A reference of type Shape can refer to an object of Shape or an object of Triangle. The object-oriented principle of inheritance saves us the headache of redefining the same class members all over again.

Our Triangle class will inherit all the traits of Shape, but Triangle can also contain its own unique methods and variables. For example, we could have an instance variable called hypotenuse and a method called findHypotenuse() that can only be accessed by Triangle class references. Objects of Triangle can call any method contained in Triangle or Shape. This gives us a bunch of possibilities!

There are several terms you’ll encounter frequently:

  • Parent class, superclass, and base class refer to the class that another class inherits from (like Shape).
  • Child class, subclass, and derived class refer to a class that inherits from another class (like Triangle).


Take a look at the image to see how inheritance works in object-oriented programming.

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