Private access modifier in Java - Mini Project: Build & Apply

Understanding the Private Access Modifier in Java

📖 Scenario: Imagine you are creating a simple Java class to represent a bank account. You want to keep the account balance safe so that it cannot be changed directly from outside the class.

🎯 Goal: You will create a Java class with a private variable to store the balance, then add methods to safely access and update the balance.

📋 What You'll Learn

Create a class called BankAccount

Add a private variable called balance of type double

Add a public method getBalance() to return the current balance

Add a public method deposit(double amount) to add money to the balance

Add a public method withdraw(double amount) to subtract money from the balance if enough funds exist

Print the balance after deposits and withdrawals

💡 Why This Matters

🌍 Real World

In real banking software, sensitive data like account balances must be protected from direct changes to avoid errors or fraud.

💼 Career

Understanding access modifiers like private is essential for writing secure and maintainable code in professional Java development.

1

Create the BankAccount class with a private balance variable

Create a class called BankAccount with a private double variable named balance initialized to 0.0.

Java

public class BankAccount {
    // Your code here
}

Hint

Use the keyword private before the variable balance to restrict access.

2

Add a public method to get the balance

Add a public method called getBalance() that returns the value of the private variable balance.

Java

public class BankAccount {
    private double balance = 0.0;

    // Your code here
}

Hint

The method should be public so other classes can call it.

3

Add deposit and withdraw methods

Add two public methods: deposit(double amount) to add amount to balance, and withdraw(double amount) to subtract amount from balance only if balance is enough.

Java

public class BankAccount {
    private double balance = 0.0;

    public double getBalance() {
        return balance;
    }

    // Your code here
}

Hint

Use an if statement in withdraw to check if there is enough balance.

4

Create a main method to test the BankAccount class

Add a main method inside BankAccount that creates an object called account, deposits 100.0, withdraws 30.0, and then prints the balance using System.out.println.

Java

public class BankAccount {
    private double balance = 0.0;

    public double getBalance() {
        return balance;
    }

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

    public void withdraw(double amount) {
        if (balance >= amount) {
            balance -= amount;
        }
    }

    // Your code here
}

Hint

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

Practice

1. What does the private access modifier do in Java?

easy

A. It restricts access to the member only within its own class.

B. It allows access from any class in the same package.

C. It allows access from any class in any package.

D. It allows access only from subclasses.

Solution

1. Step 1: Understand the meaning of private

   The private modifier restricts access to the member so that only code inside the same class can use it.

2. Step 2: Compare with other access levels

   Other modifiers like public or protected allow wider access, but private is the most restrictive.

3. Final Answer:

   It restricts access to the member only within its own class. -> Option A

4. Quick Check:

   Private means only inside class [OK]

Hint: Private means only accessible inside the same class [OK]

Common Mistakes:

• Confusing private with protected or public
• Thinking private allows subclass access
• Assuming private allows package access

2. Which of the following is the correct way to declare a private integer variable named count in a Java class?

easy

A. private count int;

B. int private count;

C. private int count;

D. count private int;

Solution

1. Step 1: Recall Java variable declaration syntax

   In Java, the access modifier comes first, then the type, then the variable name.

2. Step 2: Check each option

   private int count; follows the correct order: private int count;. Others have wrong order or syntax.

3. Final Answer:

   private int count; -> Option C

4. Quick Check:

   Access modifier + type + name [OK]

Hint: Access modifier comes before type and variable name [OK]

Common Mistakes:

• Placing 'private' after the type or variable name
• Mixing order of keywords
• Missing semicolon

3. What will be the output of the following Java code?

class Test {
  private int number = 5;
  public void printNumber() {
    System.out.println(number);
  }
}
public class Main {
  public static void main(String[] args) {
    Test t = new Test();
    t.printNumber();
  }
}

medium

A. Runtime error

B. Compilation error

C. 0

D. 5

Solution

1. Step 1: Understand private member access inside class

   The variable number is private but accessed inside its own class method printNumber(), which is allowed.

2. Step 2: Check main method output

   The main method creates an object and calls printNumber(), which prints the value 5.

3. Final Answer:

   5 -> Option D

4. Quick Check:

   Private accessible inside class method [OK]

Hint: Private fields can be used inside their own class methods [OK]

Common Mistakes:

• Thinking private prevents access even inside class
• Expecting compilation error due to private
• Confusing default value with actual initialization

4. Identify the error in the following Java code:

class Person {
  private String name = "Alice";
}
public class Main {
  public static void main(String[] args) {
    Person p = new Person();
    System.out.println(p.name);
  }
}

medium

A. Missing constructor in Person class.

B. Cannot access private field name from outside the class.

C. Syntax error in class declaration.

D. No error; code runs fine.

Solution

1. Step 1: Check access to private field from outside class

   The field name is private in Person class, so it cannot be accessed directly from Main class.

2. Step 2: Understand Java access rules

   Accessing p.name from Main causes a compilation error due to private access.

3. Final Answer:

   Cannot access private field name from outside the class. -> Option B

4. Quick Check:

   Private fields block outside access [OK]

Hint: Private fields can't be accessed outside their class [OK]

Common Mistakes:

• Assuming private fields are accessible everywhere
• Thinking missing constructor causes error here
• Ignoring access modifiers

5. You want to keep a user's password private in a Java class but allow other classes to check if a password matches. Which is the best way to do this?

hard

A. Make the password field private and provide a public method to check the password.

B. Make the password field public so other classes can read it directly.

C. Make the password field protected and access it from other classes.

D. Make the password field private and provide a public method to return the password.

Solution

1. Step 1: Protect sensitive data with private fields

   Passwords should be private to prevent direct access and protect security.

2. Step 2: Provide controlled access via public method

   A public method can check if a given password matches without revealing the actual password.

3. Final Answer:

   Make the password field private and provide a public method to check the password. -> Option A

4. Quick Check:

   Private data + public check method = secure design [OK]

Hint: Keep data private; use public methods to interact safely [OK]

Common Mistakes:

• Making password public exposes sensitive data
• Returning password directly breaks security
• Using protected exposes data to subclasses unnecessarily