1. Introduction
Welcome to this blog on Java Object-Oriented Programming: Classes, Objects and Inheritance. In this blog, you will learn the basic concepts and principles of object-oriented programming (OOP) in Java, one of the most popular and widely used programming languages in the world.
OOP is a programming paradigm that organizes data and behavior into reusable units called objects. Objects have attributes (data) and methods (behavior) that define their state and functionality. Objects can also interact with each other through messages, creating complex and dynamic systems.
Java is an object-oriented language that supports the creation and manipulation of objects. Java also provides features such as inheritance, polymorphism, and abstraction, which allow you to design and implement flexible and modular code. These features make Java a powerful and versatile language for developing applications ranging from desktop to web to mobile.
By the end of this blog, you will be able to:
- Explain what is object-oriented programming and why it is useful.
- Create and use classes and objects in Java.
- Define and invoke constructors and methods in Java.
- Apply access modifiers and encapsulation to control the visibility and access of data and methods.
- Use inheritance and polymorphism to extend and modify the behavior of classes and objects.
- Understand the difference between abstract classes and interfaces and how to use them in Java.
- Perform dynamic binding and casting of objects in Java.
Are you ready to dive into the world of Java OOP? Let’s get started!
2. What is Object-Oriented Programming?
Object-oriented programming (OOP) is a programming paradigm that organizes data and behavior into reusable units called objects. Objects have attributes (data) and methods (behavior) that define their state and functionality. Objects can also interact with each other through messages, creating complex and dynamic systems.
OOP is based on four main principles: abstraction, encapsulation, inheritance, and polymorphism. These principles help you to design and implement flexible and modular code that can be easily modified and extended.
- Abstraction is the process of hiding the implementation details and exposing only the essential features of an object. Abstraction helps you to reduce complexity and focus on the functionality of an object.
- Encapsulation is the mechanism of wrapping the data and methods of an object together and restricting the access to them from outside. Encapsulation helps you to protect the integrity and consistency of an object and prevent unwanted interference.
- Inheritance is the mechanism of creating new classes from existing ones by inheriting their attributes and methods. Inheritance helps you to reuse and extend the code and create a hierarchical relationship between classes.
- Polymorphism is the ability of an object to take different forms and behave differently depending on the context. Polymorphism helps you to create generic and flexible code that can handle different types of objects.
OOP is one of the most widely used programming paradigms in the world, as it offers many benefits such as code reuse, modularity, maintainability, and readability. OOP also supports the development of complex and large-scale applications that can be easily tested and debugged.
How do you apply OOP principles in Java? Let’s find out in the next section.
3. Java Classes and Objects
In this section, you will learn how to create and use classes and objects in Java. Classes and objects are the fundamental building blocks of Java OOP. A class is a blueprint that defines the attributes and methods of a type of object. An object is an instance of a class that has a specific state and behavior.
To create a class in Java, you use the class keyword followed by the name of the class. The name of the class should start with a capital letter and follow the camel case convention. For example, to create a class named Person, you write:
class Person {
// attributes and methods of the class
}
To create an object of a class, you use the new keyword followed by the name of the class and a pair of parentheses. You can assign the object to a variable of the same type as the class. For example, to create an object of the Person class and assign it to a variable named p, you write:
Person p = new Person();
To access the attributes and methods of an object, you use the dot operator (.) followed by the name of the attribute or method. For example, to access the name attribute and the greet method of the p object, you write:
p.name = "Alice"; p.greet();
How do you define the attributes and methods of a class? Let’s see in the next subsection.
3.1. Creating and Using Classes
In this subsection, you will learn how to create and use classes in Java. A class is a blueprint that defines the attributes and methods of a type of object. You can create multiple objects of the same class, each with its own state and behavior.
To create a class in Java, you use the class keyword followed by the name of the class. The name of the class should start with a capital letter and follow the camel case convention. For example, to create a class named Person, you write:
class Person {
// attributes and methods of the class
}
The attributes and methods of the class are defined inside the curly braces. Attributes are variables that store the data of the object, such as name, age, or height. Methods are functions that perform the actions of the object, such as greet, walk, or sleep.
To define an attribute, you specify its data type and name. For example, to define an attribute named name of type String, you write:
String name;
To define a method, you specify its return type, name, and parameters. For example, to define a method named greet that returns nothing (void) and takes no parameters, you write:
void greet() {
// code of the method
}
The code of the method is written inside the curly braces. To access the attributes of the object within the method, you use the this keyword followed by the dot operator and the name of the attribute. For example, to print the name of the object in the greet method, you write:
void greet() {
System.out.println("Hello, my name is " + this.name);
}
Now that you know how to create a class, let’s see how to use it in the next subsection.
3.2. Constructors and Methods
In this subsection, you will learn how to define and use constructors and methods in Java. Constructors and methods are special types of functions that belong to a class and operate on its objects. Constructors are used to initialize the state of an object, while methods are used to perform the behavior of an object.
To define a constructor, you use the same name as the class and specify the parameters. For example, to define a constructor for the Person class that takes a String parameter named name, you write:
Person(String name) {
// code of the constructor
}
The code of the constructor is written inside the curly braces. To assign the value of the parameter to the attribute of the object, you use the this keyword followed by the dot operator and the name of the attribute. For example, to assign the value of the name parameter to the name attribute of the object, you write:
Person(String name) {
this.name = name;
}
To use a constructor, you invoke it with the new keyword followed by the name of the class and the arguments. For example, to create an object of the Person class and pass the argument “Alice” to the constructor, you write:
Person p = new Person("Alice");
To define a method, you use the same syntax as a regular function, except that you do not need to use the static keyword. For example, to define a method named greet that returns nothing (void) and takes no parameters, you write:
void greet() {
// code of the method
}
The code of the method is written inside the curly braces. To access the attributes and methods of the object within the method, you use the this keyword followed by the dot operator and the name of the attribute or method. For example, to print the name of the object in the greet method, you write:
void greet() {
System.out.println("Hello, my name is " + this.name);
}
To use a method, you invoke it with the dot operator followed by the name of the method and the arguments. For example, to call the greet method on the p object, you write:
p.greet();
Now that you know how to define and use constructors and methods, let’s see how to control the access and visibility of them in the next subsection.
3.3. Access Modifiers and Encapsulation
In this subsection, you will learn how to control the access and visibility of the attributes and methods of a class using access modifiers and encapsulation. Access modifiers are keywords that specify the level of access that other classes and objects have to the attributes and methods of a class. Encapsulation is the principle of hiding the internal details of a class and exposing only the necessary interface to the outside world.
Java provides four access modifiers: public, private, protected, and default. The table below summarizes the meaning and scope of each modifier:
| Modifier | Meaning | Scope |
|---|---|---|
| public | The attribute or method is accessible by any class or object. | Anywhere |
| private | The attribute or method is accessible only by the class itself. | Within the same class |
| protected | The attribute or method is accessible by the class itself and its subclasses. | Within the same package or subclass |
| default | The attribute or method is accessible by the class itself and other classes in the same package. | Within the same package |
To apply an access modifier to an attribute or method, you write the modifier keyword before the data type or return type. For example, to make the name attribute and the greet method of the Person class public, you write:
public String name;
public void greet() {
// code of the method
}
By default, if you do not specify any modifier, the attribute or method is assigned the default modifier. For example, the following code is equivalent to the previous one:
String name;
void greet() {
// code of the method
}
The access modifiers help you to implement the principle of encapsulation, which is to hide the internal details of a class and expose only the necessary interface to the outside world. Encapsulation helps you to protect the integrity and consistency of the data and methods of a class and prevent unwanted interference or misuse.
A common practice of encapsulation is to make the attributes of a class private and provide public methods to access and modify them. These methods are called getters and setters. For example, to make the name attribute of the Person class private and provide a getter and a setter for it, you write:
private String name;
public String getName() {
return this.name;
}
public void setName(String name) {
this.name = name;
}
Now that you know how to control the access and visibility of the attributes and methods of a class, let’s see how to extend and modify the behavior of a class using inheritance and polymorphism in the next section.
4. Java Inheritance and Polymorphism
In this section, you will learn how to extend and modify the behavior of classes and objects using inheritance and polymorphism in Java. Inheritance and polymorphism are two powerful features of OOP that allow you to reuse and adapt the code and create a hierarchical relationship between classes.
Inheritance is the mechanism of creating new classes from existing ones by inheriting their attributes and methods. The new class is called the subclass or the child class, and the existing class is called the superclass or the parent class. The subclass can access and use the attributes and methods of the superclass, as well as define its own attributes and methods.
To create a subclass from a superclass in Java, you use the extends keyword followed by the name of the superclass. For example, to create a subclass named Student from the superclass named Person, you write:
class Student extends Person {
// attributes and methods of the subclass
}
The subclass inherits all the attributes and methods of the superclass, unless they are declared as private. To access the attributes and methods of the superclass within the subclass, you use the super keyword followed by the dot operator and the name of the attribute or method. For example, to call the constructor of the superclass in the subclass, you write:
Student(String name) {
super(name);
// code of the subclass constructor
}
Polymorphism is the ability of an object to take different forms and behave differently depending on the context. Polymorphism allows you to create generic and flexible code that can handle different types of objects. There are two types of polymorphism in Java: compile-time polymorphism and run-time polymorphism.
Compile-time polymorphism is also known as method overloading. It occurs when you define multiple methods with the same name but different parameters in the same class. The compiler decides which method to invoke based on the number and type of the arguments. For example, to overload the greet method in the Person class with two versions, one that takes no parameters and one that takes a String parameter, you write:
void greet() {
System.out.println("Hello, my name is " + this.name);
}
void greet(String name) {
System.out.println("Hello, " + name + ". My name is " + this.name);
}
Run-time polymorphism is also known as method overriding. It occurs when you define a method with the same name and parameters in both the superclass and the subclass. The compiler decides which method to invoke based on the type of the object at run time. For example, to override the greet method in the Student subclass, you write:
void greet() {
System.out.println("Hi, my name is " + this.name + " and I am a student.");
}
Now that you know how to use inheritance and polymorphism in Java, let’s see how to use abstract classes and interfaces in the next subsection.
4.1. Extending Classes and Overriding Methods
In this subsection, you will learn how to extend and override the behavior of classes and objects using inheritance and polymorphism in Java. Inheritance and polymorphism are two powerful features of OOP that allow you to reuse and adapt the code and create a hierarchical relationship between classes.
Inheritance is the mechanism of creating new classes from existing ones by inheriting their attributes and methods. The new class is called the subclass or the child class, and the existing class is called the superclass or the parent class. The subclass can access and use the attributes and methods of the superclass, as well as define its own attributes and methods.
To create a subclass from a superclass in Java, you use the extends keyword followed by the name of the superclass. For example, to create a subclass named Student from the superclass named Person, you write:
class Student extends Person {
// attributes and methods of the subclass
}
The subclass inherits all the attributes and methods of the superclass, unless they are declared as private. To access the attributes and methods of the superclass within the subclass, you use the super keyword followed by the dot operator and the name of the attribute or method. For example, to call the constructor of the superclass in the subclass, you write:
Student(String name) {
super(name);
// code of the subclass constructor
}
Polymorphism is the ability of an object to take different forms and behave differently depending on the context. Polymorphism allows you to create generic and flexible code that can handle different types of objects. There are two types of polymorphism in Java: compile-time polymorphism and run-time polymorphism.
Compile-time polymorphism is also known as method overloading. It occurs when you define multiple methods with the same name but different parameters in the same class. The compiler decides which method to invoke based on the number and type of the arguments. For example, to overload the greet method in the Person class with two versions, one that takes no parameters and one that takes a String parameter, you write:
void greet() {
System.out.println("Hello, my name is " + this.name);
}
void greet(String name) {
System.out.println("Hello, " + name + ". My name is " + this.name);
}
Run-time polymorphism is also known as method overriding. It occurs when you define a method with the same name and parameters in both the superclass and the subclass. The compiler decides which method to invoke based on the type of the object at run time. For example, to override the greet method in the Student subclass, you write:
void greet() {
System.out.println("Hi, my name is " + this.name + " and I am a student.");
}
Now that you know how to use inheritance and polymorphism in Java, let’s see how to use abstract classes and interfaces in the next subsection.
4.2. Abstract Classes and Interfaces
In this subsection, you will learn how to use abstract classes and interfaces in Java. Abstract classes and interfaces are two types of classes that cannot be instantiated as objects, but can be used to define common behavior and functionality for other classes. Abstract classes and interfaces are useful for implementing abstraction and polymorphism in Java.
An abstract class is a class that has one or more abstract methods. An abstract method is a method that has no implementation, but only a declaration. An abstract class can also have concrete methods, which are methods that have an implementation. To create an abstract class in Java, you use the abstract keyword before the class name. For example, to create an abstract class named Animal, you write:
abstract class Animal {
// attributes and methods of the abstract class
}
To create an abstract method in Java, you use the abstract keyword before the method name and end the declaration with a semicolon. For example, to create an abstract method named makeSound that returns nothing (void) and takes no parameters, you write:
abstract void makeSound();
An abstract class cannot be instantiated as an object, but it can be extended by other classes. The subclasses of an abstract class must provide an implementation for all the abstract methods of the abstract class, unless they are also abstract. For example, to create a subclass named Dog that extends the abstract class Animal and provides an implementation for the makeSound method, you write:
class Dog extends Animal {
void makeSound() {
System.out.println("Woof!");
}
}
An interface is a class that has only abstract methods and constants. A constant is a variable that has a fixed value and cannot be changed. An interface can also have default methods, which are methods that have a default implementation that can be overridden by the subclasses. To create an interface in Java, you use the interface keyword before the class name. For example, to create an interface named Swimmable, you write:
interface Swimmable {
// constants and methods of the interface
}
To create a constant in an interface, you use the final keyword before the data type and assign a value to the variable. For example, to create a constant named WATER_DENSITY that has the value 1000, you write:
final int WATER_DENSITY = 1000;
To create an abstract method in an interface, you do not need to use the abstract keyword, as it is implied by default. For example, to create an abstract method named swim that returns nothing (void) and takes no parameters, you write:
void swim();
To create a default method in an interface, you use the default keyword before the method name and provide an implementation. For example, to create a default method named getSpeed that returns a double and takes no parameters, you write:
default double getSpeed() {
// code of the method
}
An interface cannot be instantiated as an object, but it can be implemented by other classes. A class can implement multiple interfaces, but it must provide an implementation for all the abstract methods of the interfaces. To implement an interface in Java, you use the implements keyword followed by the name of the interface. For example, to create a class named Fish that implements the interface Swimmable and provides an implementation for the swim method, you write:
class Fish implements Swimmable {
void swim() {
System.out.println("I swim by moving my fins and tail.");
}
}
Now that you know how to use abstract classes and interfaces in Java, let’s see how to perform dynamic binding and casting of objects in the next subsection.
4.3. Dynamic Binding and Casting
In this subsection, you will learn how to perform dynamic binding and casting of objects in Java. Dynamic binding and casting are two techniques that allow you to manipulate the type and behavior of objects at run time. Dynamic binding and casting are useful for implementing polymorphism and abstraction in Java.
Dynamic binding is the process of determining which method to invoke based on the type of the object at run time. Dynamic binding allows you to create generic and flexible code that can handle different types of objects. For example, if you have a superclass named Animal and a subclass named Dog that both have a method named makeSound, you can create an object of type Animal that refers to an object of type Dog and call the makeSound method. The compiler will decide which method to invoke based on the type of the object that the variable refers to, not the type of the variable itself. For example, you write:
Animal a = new Dog(); a.makeSound(); // this will invoke the makeSound method of the Dog class
Casting is the process of converting one type of object to another type of object. Casting allows you to access the attributes and methods of a specific type of object that are not available in the general type of object. For example, if you have a superclass named Animal and a subclass named Dog that has an attribute named breed, you can cast an object of type Animal that refers to an object of type Dog to access the breed attribute. To cast an object in Java, you use the name of the target type in parentheses before the object. For example, you write:
Animal a = new Dog(); ((Dog) a).breed = "Labrador"; // this will access the breed attribute of the Dog class
Now that you know how to perform dynamic binding and casting of objects in Java, let’s see how to conclude the blog in the next section.
5. Conclusion
In this blog, you have learned the basic concepts and principles of object-oriented programming (OOP) in Java. You have also learned how to create and use classes and objects, how to define and invoke constructors and methods, how to apply access modifiers and encapsulation, how to use inheritance and polymorphism, how to use abstract classes and interfaces, and how to perform dynamic binding and casting of objects in Java.
OOP is a powerful and widely used programming paradigm that allows you to design and implement flexible and modular code. OOP also supports the development of complex and large-scale applications that can be easily tested and debugged. Java is an object-oriented language that provides many features and tools to help you apply OOP principles and concepts in your code.
We hope that this blog has helped you to understand and appreciate the benefits and advantages of OOP in Java. We also hope that you have enjoyed the examples and code snippets that we have provided to illustrate the concepts and techniques that we have discussed. If you have any questions or feedback, please feel free to leave a comment below.
Thank you for reading this blog and happy coding!
