Private access modifier in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the private access modifier do in Java?

It restricts access to the member (variable or method) so that it can only be accessed within the same class where it is declared.

beginner

Can a private variable be accessed directly from another class?

No, a private variable cannot be accessed directly from outside its own class.

intermediate

Why use <code>private</code> access modifier for class members?

To protect data by hiding it from outside classes and to control how it is accessed or modified through methods.

intermediate

How can other classes access private members safely?

By using public methods called getters and setters that control access to the private members.

beginner

Example: What will happen if you try to access a private variable from another class?

The compiler will give an error saying the variable is not visible because it is private.

What is the scope of a private member in Java?

A Anywhere in the project

B Within the package

C Only in subclasses

D Only within the class it is declared

A private member can only be accessed inside the class it is declared.

Which of these can access a private variable?

A Any class in the project

B Any class in the same package

C A method inside the same class

D A subclass in a different package

Only methods inside the same class can access private variables.

Why do programmers use private variables with public getters and setters?

A To hide data and control how it is accessed or changed

B To make variables accessible everywhere

C To speed up the program

D To allow direct access from other classes

Using private variables with getters/setters protects data and controls access.

What error occurs if you try to access a private variable from another class?

A Runtime error: NullPointerException

B Compile-time error: variable not visible

C No error, it works fine

D Syntax error: missing semicolon

The compiler prevents access to private variables from outside the class.

Which keyword is used to declare a member accessible only within its own class?

A private

B public

C protected

D default

The private keyword restricts access to the declaring class only.

Explain what the private access modifier does and why it is important in Java.

Think about keeping things safe inside a box only you can open.

Describe how you can allow other classes to read or change a private variable safely.

Imagine a secure door with a key only you give to trusted friends.

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