Public access modifier in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a public class named Car.

Java

public class [1] {
    // class body
}

Drag options to blanks, or click blank then click option'

ACar

Bcar

CVehicle

DcarClass

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase names for class names.

Using a different name than the file name.

The class name must match the identifier after the public class keywords. Here, Car is the correct class name.

2fill in blank

medium

Complete the code to declare a public method named start inside the Car class.

Java

public class Car {
    public [1] start() {
        return "Car started";
    }
}

Drag options to blanks, or click blank then click option'

Avoid

BString

Cint

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using void when the method returns a value.

Using a numeric return type like int.

The method start returns a string, so its return type must be String.

3fill in blank

hard

Fix the error in the code by completing the access modifier for the variable speed.

Java

public class Car {
    [1] int speed;

    public void setSpeed(int s) {
        speed = s;
    }
}

Drag options to blanks, or click blank then click option'

Apublic

Bprotected

Cprivate

Ddefault

Attempts:

3 left

💡 Hint

Common Mistakes

Using public when the variable should be private for encapsulation.

Leaving out the access modifier causing default access.

The variable speed is accessed inside the class and should be private to encapsulate the data and allow controlled access through methods.

4fill in blank

hard

Fill both blanks to create a public constructor for the Car class that sets the speed.

Java

public class Car {
    int speed;

    public [1]([2] int speed) {
        this.speed = speed;
    }
}

Drag options to blanks, or click blank then click option'

ACar

Bvoid

Cint

DSpeed

Attempts:

3 left

💡 Hint

Common Mistakes

Using void as constructor name.

Using incorrect parameter types.

The constructor name must match the class name Car, and the parameter type is int for speed.

5fill in blank

hard

Fill all three blanks to create a public method named accelerate that increases speed by a given amount and returns the new speed.

Java

public class Car {
    int speed;

    public int [1](int [2]) {
        speed [3] amount;
        return speed;
    }
}

Drag options to blanks, or click blank then click option'

Aaccelerate

Bamount

C+=

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong method names.

Using subtraction instead of addition.

Using incorrect parameter names.

The method name is accelerate, the parameter is amount, and the speed is increased using +=.

Practice

1. What does the public access modifier mean in Java?

easy

A. The class, method, or variable is accessible from anywhere in the program.

B. The class, method, or variable is accessible only within its own class.

C. The class, method, or variable is accessible only within its own package.

D. The class, method, or variable is accessible only to subclasses.

Solution

1. Step 1: Understand the meaning of public

   The public modifier allows access from any other class in any package.

2. Step 2: Compare with other access levels

   Other modifiers like private or protected restrict access, but public is fully open.

3. Final Answer:

   The class, method, or variable is accessible from anywhere in the program. -> Option A

4. Quick Check:

   Public means open access [OK]

Hint: Public means accessible from anywhere in the program [OK]

Common Mistakes:

• Confusing public with private or protected
• Thinking public restricts access to package only
• Assuming public means only subclass access

2. Which of the following is the correct way to declare a public method in Java?

easy

A. void public myMethod() {}

B. public void myMethod() {}

C. private public void myMethod() {}

D. method public void myMethod() {}

Solution

1. Step 1: Recall Java method declaration syntax

   The access modifier comes first, then the return type, then the method name and parentheses.

2. Step 2: Check each option

   public void myMethod() {} follows correct syntax: public void myMethod() {}. Others have wrong order or invalid keywords.

3. Final Answer:

   public void myMethod() {} -> Option B

4. Quick Check:

   Access modifier first, then return type [OK]

Hint: Access modifier always comes before return type [OK]

Common Mistakes:

• Placing 'public' after 'void'
• Using invalid keywords like 'method'
• Combining multiple access modifiers incorrectly

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

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

medium

A. 5

B. 0

C. Compilation error

D. Runtime error

Solution

1. Step 1: Understand the public variable access

   The variable number is public and initialized to 5, so it can be accessed from main.

2. Step 2: Trace the program output

   The main method creates an object and prints obj.number, which is 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Public variable accessed correctly prints 5 [OK]

Hint: Public variables can be accessed directly from objects [OK]

Common Mistakes:

• Assuming default value 0 instead of initialized 5
• Thinking public causes errors
• Confusing static and instance variables

4. Identify the error in the following code snippet:

class Example {
    public int value;
}

public class Main {
    public static void main(String[] args) {
        Example ex = new Example();
        ex.value = 10;
        System.out.println(ex.value);
    }
}

medium

A. Class Example must be declared public to access its public variable.

B. Main class must not be public.

C. No error; code will compile and print 10.

D. Variable value must be private to be accessed.

Solution

1. Step 1: Check class and variable access

   Class Example is package-private but accessible within the same package. Its public variable value can be accessed.

2. Step 2: Verify main method usage

   Main creates Example object, assigns 10 to value, and prints it. This is valid.

3. Final Answer:

   No error; code will compile and print 10. -> Option C

4. Quick Check:

   Public variable accessible within same package [OK]

Hint: Public variables accessible within same package without public class [OK]

Common Mistakes:

• Thinking class must be public to access public members
• Confusing variable access with class access
• Assuming private needed for access

5. You want to create a Java class with a public method that can be called from any other class, but the class itself should only be accessible within its package. Which of the following is correct?

hard

A. Declare both the class and method as public.

B. Declare the class as private and the method as public.

C. Declare the class as public and the method as private.

D. Declare the class without public and the method as public.

Solution

1. Step 1: Understand class access modifiers

   Classes without public are package-private, so accessible only within the package.

2. Step 2: Understand method access modifiers

   Methods declared public inside a package-private class are accessible from other classes in the same package.

3. Step 3: Analyze options

   Declare the class without public and the method as public. matches the requirement: class package-private, method public. Declare both the class and method as public. makes class public (accessible everywhere). Declare the class as public and the method as private. hides method. Declare the class as private and the method as public. is invalid because classes cannot be private.

4. Final Answer:

   Declare the class without public and the method as public. -> Option D

5. Quick Check:

   Class package-private + public method = accessible in package [OK]

Hint: Class without public is package-private; method public means accessible inside package [OK]

Common Mistakes:

• Thinking class can be private
• Assuming public class needed for public method
• Confusing method access with class access