Java is an object-oriented programming language that uses the concept of objects and classes to represent real-world entities and their interactions. Understanding these concepts is essential for developing robust and scalable Java applications.
In this tutorial, I will explain to you the fundamentals of objects and classes in Java, including their definition, creation, and usage. By the end of this tutorial, you will have a solid foundation in Java’s object-oriented programming paradigm and be able to create and manipulate objects and classes in your Java programs. So, let’s get started and empower your Java programming with objects and classes!
Classes in Java
A class in Java is a blueprint for creating objects. It is a user-defined data type that encapsulates data and methods into a single unit. In other words, a class defines the data and behavior of an object.
What is a class?
A class can be defined as a collection of data members (variables) and member functions (methods) that operate on the data members. The data members and methods are encapsulated within a class, meaning they can only be accessed by the members of that class.
In Java, classes are declared using the class keyword followed by the name of the class. For example, the following code declares a class named Person:
public class Person {
// data members
String name;
int age;
// member functions
void displayInfo() {
System.out.println("Name: " + name);
System.out.println("Age: " + age);
}
}
Types of Classes in Java
In Java, classes can be broadly classified into two categories: User-defined classes and Built-in classes.
User-defined Classes
User-defined classes are created by the programmer to suit the needs of the program. These classes can be further classified into three types: Concrete, Abstract, and Interface.
Concrete classes are regular classes that are instantiated to create objects. These classes can be further extended and modified by other classes to add more functionality.
public class Car {
// Class implementation goes here
}
Abstract classes are classes that cannot be instantiated. They are used as a blueprint for other classes and can contain both abstract and non-abstract methods. Abstract classes must be extended by other classes in order to be used.
public abstract class Shape {
// Abstract class implementation goes here
public abstract double area();
public abstract double perimeter();
}
Interface classes are similar to abstract classes in that they cannot be instantiated. They are used to define a set of methods that a class must implement if it wants to adhere to the interface. Multiple interfaces can be implemented by a single class.
public interface Drawable {
void draw();
}
Built-in Classes
Built-in classes are pre-defined classes that come with the Java programming language. These classes are part of the Java API and provide functionality for common tasks such as input/output, networking, and data manipulation.
Some examples of built-in classes are:
String– for manipulating stringsInteger– for working with integer valuesScanner– for reading input from the userFile– for working with files and directories
Defining a class in Java
A class is defined using the following syntax:
[access modifier] class ClassName {
// data members
[access modifier] dataType variableName;
// member functions
[access modifier] returnType functionName([parameters]) {
// code
}
}
Here, access modifier refers to the visibility of the class, data members, and member functions. The access modifier can be public, private, protected, or default. If no access modifier is specified, the default access modifier is used.
The dataType refers to the type of data stored in the data member, and the returnType refers to the type of data returned by the function. If a function doesn’t return any data, the returnType is void.
Class variables and methods
Class variables (also known as static variables) are shared among all the objects of a class. They are declared using the static keyword. For example:
public class Circle {
static final double PI = 3.14;
int radius;
static double area(int r) {
return PI * r * r;
}
double circumference() {
return 2 * PI * radius;
}
}
In this example, PI is a class variable and area() is a class method. The circumference() method is an instance method, which means it can only be called on an instance of the class.
Creating objects from classes
To create an object from a class, the new operator is used:
ClassName objectName = new ClassName();
Here, ClassName refers to the name of the class, and objectName is the name of the object being created. For example:
Person john = new Person();
In this example, john is an object of the Person class.
Once an object has been created, its data members and member functions can be accessed using the dot notation:
objectName.dataMember; objectName.memberFunction();
For example:
john.name = "John Doe"; john.age = 25; john.displayInfo();
This will display the following output:
Name: John Doe Age: 25
That’s it for this section ! In the next section, we’ll cover objects in Java.
Objects in Java
What is an object?
In Java, an object is a basic unit of the program that represents a real-world entity. An object consists of identity, behavior, and state.
- Identity: The identity of an object is defined by its memory address in the computer’s memory. It’s what makes each object unique, even if two objects have the same state and behavior.
- Behavior: The behavior of an object is defined by the methods it can perform, which are defined in its class. Methods are blocks of code that can be called to perform a specific action or return a value.
- State: The state of an object is defined by its properties or attributes, which are also defined in its class. These properties can be either primitive data types (such as integers, booleans, or characters) or other objects.
Creating objects in Java
In Java, objects are created from classes. Let’s use the previous Person class as an example to illustrate how to create an object in Java. To do that, you need to follow these steps:
- Declaration: Declare a variable of the
Personclass type using the class name. For example:
Person person1;
- Instantiation: Use the
newkeyword to create an instance of thePersonclass. This step creates a new object of thePersonclass and allocates memory for it. For example:
person1 = new Person();
- Initialization: Initialize the object using its constructor. In the case of the
Personclass, there is no explicit constructor defined, so the default constructor is used. You can set the object’s data members to their initial values. For example:
person1.name = "John Doe"; person1.age = 30;
Alternatively, you can combine the declaration, instantiation, and initialization into a single statement:
Person person2 = new Person(); person2.name = "Jane Doe"; person2.age = 25;
Note that the new keyword allocates memory for the object and returns a reference to it, which is stored in the variable person1 or person2. This reference can be used to access the object’s methods and properties.
In summary, to create an object in Java, you need to declare a variable of the class type, instantiate the class using the new keyword, and initialize the object’s data members. Once the object is created, you can access its member functions and data members using the dot notation.
What are the different ways of creating objects in Java?
In Java, there are several ways to create an object of a class. The most common way is to use the new keyword, which allocates memory for the object and calls its constructor. However, there are also other ways to create an object, including:
Using the new Keyword
The new keyword is the most common way to create an object in Java. It is used to allocate memory for the object and call its constructor. Here is an example:
MyClass obj = new MyClass();
In this example, the new keyword is used to create a new object of the MyClass class and assign it to the obj variable.
Using the Class.forName(String className) Method
Another way to create an object of a class is to use the Class.forName(String className) method. This method returns the Class object associated with the specified class name, and the newInstance() method of the Class object is then used to create a new object of the class. Here is an example:
try {
MyClass obj = (MyClass) Class.forName("com.example.MyClass").newInstance();
} catch (InstantiationException | IllegalAccessException | ClassNotFoundException e) {
// handle exception
}
In this example, the Class.forName(String className) method is used to get the Class object associated with the MyClass class, and the newInstance() method is used to create a new object of the class. Note that the newInstance() method returns an Object, so we need to cast it to the MyClass type.
Using the clone() Method
The clone() method is used to create a new object with the same state as an existing object. It is a protected method of the Object class, so the class that we want to clone must implement the Cloneable interface and override the clone() method. Here is an example:
class MyClass implements Cloneable {
// class definition
@Override
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}
}
// creating a new object using the clone() method
MyClass obj1 = new MyClass();
MyClass obj2 = null;
try {
obj2 = (MyClass) obj1.clone();
} catch (CloneNotSupportedException e) {
// handle exception
}
In this example, the MyClass class implements the Cloneable interface and overrides the clone() method. The clone() method is then used to create a new object obj2 with the same state as the existing object obj1.
Using Deserialization
Deserialization is the process of converting an object from its serialized form back into an object. We can use deserialization to create a new object of a class. Here is an example:
// serialization
try {
FileOutputStream fileOut = new FileOutputStream("myclass.ser");
ObjectOutputStream out = new ObjectOutputStream(fileOut);
MyClass obj = new MyClass();
out.writeObject(obj);
out.close();
fileOut.close();
} catch (IOException e) {
// handle exception
}
// deserialization
try {
FileInputStream fileIn = new FileInputStream("myclass.ser");
ObjectInputStream in = new ObjectInputStream(fileIn);
MyClass obj = (MyClass) in.readObject();
in.close();
fileIn.close();
} catch (IOException | ClassNotFoundException e) {
// handle exception
}
In this example, the MyClass object is serialized to a file using the ObjectOutputStream, and then deserialized from the file using the ObjectInputStream to create a new object of the class. Note that the MyClass class must implement the Serializable interface for it to be serialized and deserialized.
What are the different ways of initializing objects in Java?
In Java, there are three ways to initialize objects:
By Reference Variable
One way to initialize an object is by creating a reference variable that points to an existing object. This is done using the new keyword, which allocates memory for the object and returns a reference to it. Here’s an example:
// Creating an object of class MyClass using a reference variable MyClass obj = new MyClass();
In this example, we create a reference variable obj of type MyClass and initialize it to a new instance of the MyClass class.
By Method
Another way to initialize an object is by calling a method that returns an object. This is often used in factory design patterns, where a factory method is responsible for creating and returning an object. Here’s an example:
public class MyClass {
public static MyClass createObject() {
return new MyClass();
}
}
// Initializing an object using a method
MyClass obj = MyClass.createObject();
In this example, the createObject() method returns a new instance of the MyClass class, which we then assign to the obj reference variable.
By Constructor
The most common way to initialize an object in Java is by using a constructor. A constructor is a special method that is called when an object is created using the new keyword. Here’s an example:
public class MyClass {
public MyClass() {
// Constructor code here
}
}
// Initializing an object using a constructor
MyClass obj = new MyClass();
In this example, we define a constructor for the MyClass class that initializes the object’s state when it is created using the new keyword. We then create a new instance of the MyClass class and assign it to the obj reference variable.
These are the three main ways to initialize objects in Java. Each method has its own use cases and benefits, so it’s important to understand how they work and when to use them.
Object variables and methods
Objects have their own set of data members and member functions. These are accessed using the dot operator (.).
For example, suppose the Person class has a member variable called name and a member function called sayHello(). We can access these as follows:
person.name = "John"; person.sayHello();
This sets the name variable of the person object to “John” and calls the sayHello() function on the person object.
Accessing object variables and methods
Object variables and methods can be accessed from within the class itself or from outside the class. To access them from outside the class, we must create an object of the class first.
For example, suppose we have the following Person class:
public class Person {
String name;
public void sayHello() {
System.out.println("Hello, my name is " + name);
}
}
To create an object of this class and access its variables and methods, we would use the following code:
Person person = new Person(); person.name = "John"; person.sayHello();
This creates a new object of class Person, sets its name variable to “John”, and calls its sayHello() method.
In summary, objects in Java are instances of classes that have their own set of data members and member functions. They are created using the new operator followed by a call to the constructor of the class, and their variables and methods are accessed using the dot operator (.).
What is an anonymous object?
An anonymous object in Java is an object that is created without assigning it to a variable. Instead, it is used to call a method or perform an operation on the fly. Anonymous objects are typically used for one-off operations and are not stored for later use.
Here’s an example of an anonymous object:
int result = new Calculator().add(3, 5);
In this example, we create an anonymous object of the Calculator class and immediately call its add method with the parameters 3 and 5. The result is assigned to the result variable.
Note that the anonymous object is created using the new keyword, followed by the name of the class and any constructor arguments (in this case, there are none). The method call is then chained onto the end of the object creation statement using dot notation.
Anonymous objects are useful for short, one-off operations that don’t require a separate variable or instance of a class. They can help to make your code more concise and readable, but should be used judiciously to avoid cluttering your code with unnecessary objects.
Objects vs Classes in Java
Objects and classes are two fundamental concepts in Java programming. An object is an instance of a class, and a class is a blueprint or a template for creating objects. In other words, a class is a definition or a specification of the properties and behaviors of an object, while an object is an instance or a realization of a class.
To put it simply, a class is like a recipe for a cake, while an object is an actual cake baked from that recipe. The class defines the ingredients, the instructions, and the shape of the cake, while the object is the cake itself that you can see, touch, and eat.
In Java, you use classes to define the structure and behavior of your program, and you create objects from those classes to perform specific tasks or operations. By using objects, you can encapsulate data and behavior into modular and reusable units, which can make your code more organized, maintainable, and scalable.
To learn more about the difference between objects and classes in Java, as well as other fundamental concepts of object-oriented programming, you can check out the following tutorial Object vs Class in Java: A Beginner’s Guide.
Conclusion
In conclusion, understanding the difference between objects and classes is crucial for writing effective Java programs. Objects represent instances of classes, which are blueprints or templates for creating objects. By defining classes and creating objects from them, you can structure your code in a modular and reusable way, and leverage the power of object-oriented programming.
In this tutorial, we’ve covered the basics of classes, objects, and their relationship in Java. While there’s much more to explore in Java programming and object-oriented design, this tutorial should give you a solid understanding of the fundamentals of objects and classes in Java. Don’t forget to check out the Java tutorials for beginners page for more tutorials and resources to help you master Java programming.
Frequently asked questions
- What is the main advantage of using objects and classes in Java programming?
The main advantage of using objects and classes in Java programming is that they provide a way to structure code in a modular and reusable manner. By defining classes and creating objects from them, you can encapsulate data and behavior, and hide the implementation details of your code from other parts of the program. This makes the code more maintainable, scalable, and flexible. - Can a class be an object in Java?
No, a class cannot be an object in Java. A class is a blueprint or a template for creating objects, but it is not an object itself. In other words, a class is a specification of the properties and behaviors that an object should have, while an object is an instance or a realization of a class. When you create an object in Java, you are creating a new instance of a class. - What is the purpose of a constructor in Java?
The purpose of a constructor in Java is to initialize the state of an object when it is created. Constructors are special methods that have the same name as the class and are called when an object is instantiated. They can take parameters to set the initial values of the object’s fields, or they can have no parameters and use default values. Constructors are essential for creating objects with specific properties and behaviors, and can also help enforce the rules and constraints of a class.