Encapsulation in Java [In-Depth Tutorial]


Written by - Bashir Alam
Reviewed by - Deepak Prasad

Encapsulation is one of the fundamental concepts of object-oriented programming (OOP) and is considered essential for building robust and scalable applications. Encapsulation in Java is the process of hiding the internal implementation details of a class from the outside world and providing a well-defined public interface for interacting with the class.

By encapsulating the internal data and methods of a class, developers can control access to these elements and ensure that they are used in a way that preserves the integrity of the class. This is achieved by defining private access modifiers for the internal data and methods, which restricts direct access to these elements from outside the class. Instead, public methods, also known as getters and setters, are provided to manipulate the data and perform operations on it.

In Java, encapsulation can be implemented using access modifiers such as private, public, protected, and default. Private access modifier restricts access to the declared member within the class only, while public access modifier allows access to the declared member from anywhere in the application. Protected access modifier provides access within the class and its subclasses, while default access modifier provides access within the same package only.

Implementing encapsulation in Java involves careful design and implementation of classes and their interfaces. Proper use of access modifiers, getters, and setters can help in achieving encapsulation, which ultimately leads to a more maintainable, reusable, and extensible codebase.

 

Steps to Implement Encapsulation in Java

To implement encapsulation in Java, follow these steps:

 

Step-1. Declaring Private Data Members

The first step in implementing encapsulation is to declare the instance variables of a class as private. This means that they can only be accessed and modified from within the class itself.

For example:

public class Person {
    private String name;
    private int age;
}

In this example, the name and age instance variables are declared as private.

 

Step-2. Creating Getter and Setter Methods

Next, create public getter and setter methods to access and modify the private instance variables. Getter methods are used to retrieve the values of the private variables, while setter methods are used to set new values for them.

For example:

public class Person {
    private String name;
    private int age;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

In this example, we have created getter and setter methods for the name and age instance variables.

 

Step-3. Implementing Encapsulation in a Class

Finally, implement encapsulation in the class by using the getter and setter methods to access and modify the private instance variables. This ensures that the internal state of the object is protected from external interference.

For example:

public class Person {
    private String name;
    private int age;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        if (age >= 0) {
            this.age = age;
        }
    }
}

In this example, we have added a check in the setAge() method to ensure that the age is always a positive integer.

 

Example of Encapsulation in Java

Here is an example of Encapsulation in Java programming language.

public class BankAccount {
    private double balance;
    private String accountNumber;
    private String customerName;

    public BankAccount(String accountNumber, String customerName, double balance) {
        this.accountNumber = accountNumber;
        this.customerName = customerName;
        this.balance = balance;
    }

    public void deposit(double amount) {
        balance += amount;
    }

    public void withdraw(double amount) {
        if (balance >= amount) {
            balance -= amount;
        } else {
            System.out.println("Insufficient funds!");
        }
    }

    public double getBalance() {
        return balance;
    }

    public String getAccountNumber() {
        return accountNumber;
    }

    public String getCustomerName() {
        return customerName;
    }
}

In this example, we have defined a class called "Employee" that encapsulates the data fields (name, age, salary) and the methods to access and modify them.

The data fields are marked as private, which means they can only be accessed within the same class. To access or modify these fields, we have defined public getter and setter methods. These methods provide a controlled way of accessing and modifying the data fields.

For example, to get the name of an Employee object, we can call the getName() method. Similarly, to set the name of an Employee object, we can call the setName() method.

 

How do you define private data members and access them from outside the class?

Private data members are those data members that can only be accessed within the class where they are declared. To access private data members from outside the class, you need to define public getter and setter methods.

A getter method is a public method that allows the user to retrieve the value of a private data member. It typically has a return type that matches the type of the private data member and a name that starts with "get" followed by the name of the data member. For example, if you have a private data member called "age," you would define a getter method called "getAge()".

A setter method is a public method that allows the user to set the value of a private data member. It typically has a void return type and a name that starts with "set" followed by the name of the data member. For example, if you have a private data member called "age," you would define a setter method called "setAge()".

By using getter and setter methods, you can control how the private data members are accessed and modified from outside the class. This helps ensure that the internal state of the class is protected and that it can only be accessed and modified in a controlled manner.

Here is an example implementation of encapsulation in Java, with comments inside the code to explain what's happening:

public class Person {
    // Declare private data members
    private String name;
    private int age;

    // Define getter method for name
    public String getName() {
        return name;
    }

    // Define setter method for name
    public void setName(String newName) {
        name = newName;
    }

    // Define getter method for age
    public int getAge() {
        return age;
    }

    // Define setter method for age
    public void setAge(int newAge) {
        if (newAge >= 0) { // Validate input to ensure age is non-negative
            age = newAge;
        }
    }
}

In this example, we have a class called Person with two private data members: name (a String) and age (an int). To encapsulate these data members, we define public getter and setter methods for each.

The getName() method simply returns the value of the private name data member. The setName() method takes a String parameter and sets the value of name to that parameter.

The getAge() method returns the value of the private age data member. The setAge() method takes an int parameter and sets the value of age to that parameter, but only if the parameter is non-negative. This is an example of how encapsulation can be used to control access to the internal state of an object and ensure that it is used in a safe and consistent manner.

 

What is the difference between public and private access modifiers in encapsulation?

Public and private are access modifiers that are commonly used in encapsulation to control the visibility of data members and methods in a class. The main difference between public and private access modifiers is in their level of visibility:

  1. Private: Private members can only be accessed within the class where they are declared. This means that they are hidden from all other classes, including subclasses and other classes in the same package.
  2. Public: Public members can be accessed from anywhere in the program, including other classes, subclasses, and classes in different packages.

In encapsulation, private access modifiers are typically used to hide the internal implementation details of a class from the outside world, while public access modifiers are used to provide a well-defined interface for interacting with the class. By using private access modifiers to protect the internal state of a class and public access modifiers to provide controlled access to that state, developers can ensure that their code is robust, maintainable, and extensible.

It is important to note that there are also other access modifiers in Java, such as protected and default, that can be used in encapsulation to control the visibility of members within a class hierarchy and a package, respectively.

public class Person {
    // Private data members
    private String name;
    private int age;

    // Public constructor
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    // Public getter method for name
    public String getName() {
        return name;
    }

    // Private setter method for age
    private void setAge(int age) {
        if (age >= 0) {
            this.age = age;
        }
    }

    // Public method to update age
    public void birthday() {
        setAge(age + 1);
    }
}

In this example, we have a class called Person with two private data members: name (a String) and age (an int). We have a public constructor that allows us to create new Person objects with a name and age, and a public getter method for the name data member. We also have a private setter method for the age data member, which ensures that the age is non-negative before setting it. Finally, we have a public method called birthday() that increments the age of the Person object by 1.

By making the name data member and the getName() method public, we provide controlled access to the name of the Person object from outside the class. However, by making the age data member private and the setAge() method private, we hide the internal implementation details of how age is set and validated. This helps ensure that the age of the Person object is only modified in a controlled and safe way.

The birthday() method is public, but it does not directly modify the age data member. Instead, it calls the private setAge() method to update the age, which ensures that the age is non-negative before setting it. This helps protect the internal state of the Person object and ensures that it is modified in a consistent and safe way.

 

Why is encapsulation important in object-oriented programming?

In Java, encapsulation is implemented by using access modifiers, such as private, public, and protected, to control the visibility and accessibility of class members, such as fields and methods. Here are some of the benefits of encapsulation in Java:

  • Data Hiding: Encapsulation allows hiding the implementation details of a class from the outside world. This means that the internal state of an object is protected and cannot be accessed or modified directly by other classes. Instead, access to the data is provided through public methods that enforce the proper behavior and validity of the data.
  • Modularity: Encapsulation promotes modularity by breaking down a complex system into smaller, more manageable components. Each class encapsulates its own data and behavior, making it easier to understand and maintain the code. Changes to one class do not affect the other classes, as long as the interface between them remains the same.
  • Security: Encapsulation provides security by preventing unauthorized access to sensitive data. By using access modifiers, you can restrict access to critical data and ensure that it can only be modified through controlled methods.
  • Code Reusability: Encapsulation promotes code reusability by making it possible to create objects that can be reused in different contexts. A well-encapsulated class can be used as a building block to create more complex systems without exposing their internal details.
  • Better Testing: Encapsulation makes it easier to test classes in isolation, without the need to know the implementation details of other classes. This promotes more effective unit testing and reduces the risk of unintended side effects.

 

Summary

Encapsulation is an important concept in object-oriented programming that helps protect the internal state of an object, promote modularity, support abstraction, and facilitate code reuse. It involves bundling data and methods that operate on that data into a single unit, known as a class, and controlling access to that data through access modifiers like public and private.

Private access modifiers are used to hide the internal implementation details of a class from the outside world, while public access modifiers are used to provide a well-defined interface for interacting with the class. By using private access modifiers to protect the internal state of a class and public access modifiers to provide controlled access to that state, developers can ensure that their code is robust, maintainable, and extensible.

Overall, encapsulation is a fundamental principle in object-oriented programming that plays a crucial role in promoting software quality, maintainability, and scalability.

 

Further Reading

Java encapsulation

 

Bashir Alam

He is a Computer Science graduate from the University of Central Asia, currently employed as a full-time Machine Learning Engineer at uExel. His expertise lies in OCR, text extraction, data preprocessing, and predictive models. You can reach out to him on his Linkedin or check his projects on GitHub page.

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