Encapsulation best practices in Java - Mini Project: Build & Apply

Encapsulation Best Practices in Java

📖 Scenario: You are creating a simple Java class to represent a bank account. You want to protect the account's balance so that it cannot be changed directly from outside the class. This is a common real-world need to keep data safe and controlled.

🎯 Goal: Build a Java class called BankAccount that uses encapsulation best practices: private fields, public getter and setter methods, and validation inside the setter.

📋 What You'll Learn

Create a class called BankAccount with a private double field named balance

Add a public method getBalance() that returns the current balance

Add a public method setBalance(double amount) that sets the balance only if the amount is zero or positive

Do not allow direct access to the balance field from outside the class

💡 Why This Matters

🌍 Real World

Encapsulation is used in real-world software to protect sensitive data like bank balances, passwords, or personal information.

💼 Career

Understanding encapsulation is essential for writing safe, maintainable, and professional Java code in software development jobs.

1

Create the BankAccount class with a private balance field

Create a public class called BankAccount and inside it declare a private double variable named balance initialized to 0.0.

Java

public class BankAccount {
    // Your code here
}

Hint

Use the keyword private before the double balance variable to hide it from outside the class.

2

Add a public getter method for balance

Inside the BankAccount class, add a public method called getBalance() that returns the value of the balance variable.

Java

public class BankAccount {
    private double balance = 0.0;

    // Your code here
}

Hint

The getter method should be public and return the private balance value.

3

Add a public setter method with validation

Inside the BankAccount class, add a public method called setBalance(double amount) that sets the balance only if amount is greater than or equal to 0. Otherwise, do not change the balance.

Java

public class BankAccount {
    private double balance = 0.0;

    public double getBalance() {
        return balance;
    }

    // Your code here
}

Hint

Use an if statement to check if amount is not negative before setting balance.

4

Test the BankAccount class by setting and getting balance

Create a main method inside the BankAccount class. Inside it, create a BankAccount object called account. Use setBalance(100.0) to set the balance, then print the balance using getBalance().

Java

public class BankAccount {
    private double balance = 0.0;

    public double getBalance() {
        return balance;
    }

    public void setBalance(double amount) {
        if (amount >= 0) {
            balance = amount;
        }
    }

    // Your code here
}

Hint

Use System.out.println(account.getBalance()); to print the balance.

Practice

1.

What is the main purpose of encapsulation in Java?

easy

A. To hide the internal data of a class and control access to it

B. To make all variables public so they can be accessed anywhere

C. To allow direct access to class variables without methods

D. To write code faster by skipping method definitions

Solution

1. Step 1: Understand encapsulation concept

   Encapsulation means hiding data inside a class to protect it from outside access.

2. Step 2: Identify the purpose of encapsulation

   It controls how data is accessed or changed using getter and setter methods.

3. Final Answer:

   To hide the internal data of a class and control access to it -> Option A

4. Quick Check:

   Encapsulation = Data hiding and controlled access [OK]

Hint: Encapsulation means hiding data and controlling access [OK]

Common Mistakes:

• Thinking encapsulation means making variables public
• Confusing encapsulation with inheritance
• Believing encapsulation allows direct variable access

2.

Which of the following is the correct way to declare a private variable in a Java class?

class Person {
    ? String name;
}

easy

A. private

B. public

C. protected

D. static

Solution

1. Step 1: Recall Java access modifiers

   Private variables are declared with the keyword private to hide them inside the class.

2. Step 2: Check the options

   Only private hides the variable from outside access, others allow wider access.

3. Final Answer:

   private -> Option A

4. Quick Check:

   Private keyword hides variables [OK]

Hint: Use 'private' to hide variables inside class [OK]

Common Mistakes:

• Using public instead of private for encapsulation
• Confusing protected with private
• Using static which controls memory, not access

3.

What will be the output of the following code?

class Car {
    private String model = "Tesla";
    public String getModel() {
        return model;
    }
}

public class Test {
    public static void main(String[] args) {
        Car car = new Car();
        System.out.println(car.getModel());
    }
}

medium

A. Runtime error

B. Tesla

C. Compilation error

D. null

Solution

1. Step 1: Understand private variable access

   The variable model is private but accessed via the public getter getModel().

2. Step 2: Check the output of getModel()

   The getter returns the string "Tesla", so printing it outputs "Tesla".

3. Final Answer:

   Tesla -> Option B

4. Quick Check:

   Getter returns private variable value [OK]

Hint: Private data accessed via public getter returns value [OK]

Common Mistakes:

• Expecting direct access to private variable
• Thinking code causes compilation error
• Confusing output with null or error

4.

Identify the error in the following code related to encapsulation:

class BankAccount {
    private double balance;
    public void setBalance(double balance) {
        balance = balance;
    }
    public double getBalance() {
        return balance;
    }
}

medium

A. The balance variable should be public

B. The getter method should be private

C. The setter method does not update the class variable correctly

D. The setter method should return a value

Solution

1. Step 1: Analyze the setter method

   The setter uses balance = balance; which assigns the parameter to itself, not the class variable.

2. Step 2: Understand correct assignment

   To update the class variable, use this.balance = balance; to refer to the instance variable.

3. Final Answer:

   The setter method does not update the class variable correctly -> Option C

4. Quick Check:

   Use 'this' to assign parameter to instance variable [OK]

Hint: Use 'this' to assign setter parameter to class variable [OK]

Common Mistakes:

• Forgetting 'this' keyword in setter
• Making getter private which breaks access
• Expecting setter to return a value

5.

You want to create a class Student with a private variable grade that can only be set if the value is between 0 and 100. Which is the best way to implement this using encapsulation?

hard

A. Make grade public and check the value before assigning

B. Make grade static and assign directly

C. Use a protected grade variable and no setter

D. Use a private grade variable with a setter that validates the value

Solution

1. Step 1: Understand encapsulation for validation

   Encapsulation allows controlling how variables are set by using private variables and setters with checks.

2. Step 2: Choose the best practice

   Using a private variable with a setter that validates the input ensures grade stays between 0 and 100.

3. Final Answer:

   Use a private grade variable with a setter that validates the value -> Option D

4. Quick Check:

   Setters with validation keep data safe [OK]

Hint: Validate data inside setter to protect private variables [OK]

Common Mistakes:

• Making variables public and trusting external code
• Skipping validation in setter
• Using static which shares data across all instances