Private access modifier in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of accessing private field within the same class

What is the output of this Java program?

Java

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

A 0

B 5

C Compilation error: number has private access in Test

D Runtime error

💡 Hint

Private fields can be accessed inside the same class.

The private field 'number' is accessed inside the class method 'printNumber', so it prints 5.

❓ Predict Output

intermediate

Output when accessing private field from another class

What happens when you try to compile and run this code?

Java

public class A {
    private int secret = 10;
}

public class B {
    public static void main(String[] args) {
        A a = new A();
        System.out.println(a.secret);
    }
}

A 10

B Runtime error: IllegalAccessException

C 0

D Compilation error: secret has private access in A

💡 Hint

Private fields are not accessible outside their own class.

The field 'secret' is private in class A, so class B cannot access it directly, causing a compilation error.

🔧 Debug

advanced

Why does this code cause a compilation error?

Identify the reason for the compilation error in this code snippet.

Java

class Person {
    private String name;
    public Person(String name) {
        this.name = name;
    }
}

class Employee extends Person {
    public Employee(String name) {
        super(name);
    }
    public void printName() {
        System.out.println(name);
    }
}

A Missing return type in printName method

B Constructor call super(name) is invalid

C Cannot access private field 'name' in subclass Employee

D No compilation error

💡 Hint

Private fields are not inherited directly accessible in subclasses.

The private field 'name' in Person is not accessible in subclass Employee, so accessing it in printName causes a compilation error.

📝 Syntax

advanced

Which option correctly declares a private method and calls it within the same class?

Select the code snippet that compiles and runs correctly, printing "Hello".

A

public class Demo {
    private void greet() {
        System.out.println("Hello");
    }
    public void call() {
        greet();
    }
    public static void main(String[] args) {
        new Demo().call();
    }
}

B

public class Demo {
    private void greet() {
        System.out.println("Hello");
    }
    public void call() {
        this.greet();
    }
    public static void main(String[] args) {
        Demo.call();
    }
}

C

public class Demo {
    private void greet() {
        System.out.println("Hello");
    }
    public static void call() {
        greet();
    }
    public static void main(String[] args) {
        call();
    }
}

D

public class Demo {
    private static void greet() {
        System.out.println("Hello");
    }
    public void call() {
        greet();
    }
    public static void main(String[] args) {
        new Demo().call();
    }
}

💡 Hint

Private methods can be called inside the same class instance methods.

Option A correctly declares a private instance method and calls it from a public instance method. The main method creates an instance and calls call(), which prints "Hello".

🚀 Application

expert

How many private fields are accessible in this nested class?

Given the code below, how many private fields of Outer are accessible directly inside Inner?

Java

public class Outer {
    private int a = 1;
    private int b = 2;
    int c = 3;
    public class Inner {
        public int sum() {
            return a + b + c;
        }
    }
}

A 2

B 1

C 3

D 0

💡 Hint

Inner classes can access all members of the outer class, including private ones.

Inner class can access all fields of Outer, including private fields 'a' and 'b' (and package-private 'c'), so both private fields are accessible directly inside Inner.

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