Inheritance and Polymorphism

Dive deeper into object-oriented Java with inheritance and polymorphism.

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Key Concepts

Review core concepts you need to learn to master this subject

Inheritance in Java

Main() method in Java

super() in Java

Protected and Final keywords in Java

Polymorphism in Java

Method Overriding in Java

Child Classes in Arrays and ArrayLists

Inheritance in Java

// Parent Class
class Animal {
  // Animal class members
}

// Child Class
class Dog extends Animal {
  // Dog inherits traits from Animal 
  
  // additional Dog class members
}

Inheritance is an important feature of object-oriented programming in Java. It allows for one class (child class) to inherit the fields and methods of another class (parent class). For instance, we might want a child class Dog to inherent traits from a more general parent class Animal.

When defining a child class in Java, we use the keyword extends to inherit from a parent class.

Inheritance and Polymorphism

Lesson 1 of 1

1
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 s…
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2
Inheritance in Practice
So how do we define a child class so that it inherits from a parent class? We use the keyword extends like this: class Shape { // Shape class members } class Triangle extends Shape { // add…
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3
Inheriting the Constructor
Hang on, you might be thinking, if the child class inherits its parent’s fields and methods, does it also inherit the constructor? Let’s take a look at how the super() constructor works! Let’s say…
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4
Parent Class Aspect Modifiers
You may recall that Java class members use private and public access modifiers to determine whether they can be accessed from outside the class. So does a child class inherit its parent’s private m…
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5
Introducing Polymorphism
In Java, if Orange is a Fruit through inheritance, you can then use Orange in the same contexts as Fruit like this: String makeJuice(Fruit fruit) { return “Apple juice and “ + fruit.squeeze(); …
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6
Method Overriding
One common use of polymorphism with Java classes is something we mentioned earlier — overriding parent class methods in a child class. Like the + operator, we can give a single method sligh…
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7
Using a Child Class as its Parent Class
An important facet of polymorphism is the ability to use a child class object where an object of its parent class is expected. One way to do this explicitly is to instantiate a child class object…
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8
Child Classes in Arrays and ArrayLists
Usually, when we create an array or an ArrayList, the list items all need to be the same type. But polymorphism puts a new spin on what is considered the same type… In fact, we can put instances…
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9
Child Classes in Method Parameters
When we call a method that contains parameters, the arguments we place in our method call must match the parameter type. Similar to the previous exercise, polymorphism gives us a little more flexib…
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10
Review of Inheritance and Polymorphism
Excellent work! You’ve learned quite a bundle about inheritance and polymorphism in Java: - A Java class can inherit fields and methods from another class. - Each Java class requires its own file, …
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