Class definition in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a class named "Car".

Java

public [1] Car {
    // class body
}

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

Apackage

Bclass

Cmethod

Dinterface

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'interface' instead of 'class'.

Using 'method' which is not a keyword for class definition.

The keyword class is used to define a class in Java.

2fill in blank

medium

Complete the code to declare a private integer field named "speed" inside the class.

Java

public class Car {
    [1] int speed;
}

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

Aprivate

Bpublic

Cprotected

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'public' which allows access from anywhere.

Using 'static' which is not an access modifier.

The keyword private restricts access to the field within the class only.

3fill in blank

hard

Fix the error in the constructor definition to properly initialize the "speed" field.

Java

public class Car {
    private int speed;

    public Car(int [1]) {
        speed = [1];
    }
}

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

AspeedValue

Bspeed

Cvalue

DspeedInput

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same name for parameter and field without 'this' keyword.

Not assigning the parameter to the field.

The constructor parameter should have a different name than the field to avoid confusion. Here, value is used, and then assigned to the field speed.

4fill in blank

hard

Fill both blanks to create a method named "getSpeed" that returns the speed.

Java

public class Car {
    private int speed;

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

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

Aint

Bvoid

CgetSpeed

DsetSpeed

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'void' as return type which means no value is returned.

Naming the method 'setSpeed' which is used to set a value.

The method getSpeed returns an integer value, so its return type is int.

5fill in blank

hard

Fill all three blanks to create a setter method named "setSpeed" that sets the speed field.

Java

public class Car {
    private int speed;

    public [1] [2]([3] newSpeed) {
        speed = newSpeed;
    }
}

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

Avoid

Bint

CsetSpeed

Dspeed

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'int' as return type for setter methods.

Using wrong method name like 'speed' instead of 'setSpeed'.

The setter method setSpeed does not return a value, so its return type is void. It takes an int parameter named newSpeed.

Practice

1. What is a class in Java?
class Car { }

easy

A. A blueprint to create objects with data and actions

B. A type of variable that stores numbers

C. A method that runs automatically

D. A special kind of loop

Solution

1. Step 1: Understand the role of a class

   A class defines a template or blueprint for creating objects that hold data and actions.

2. Step 2: Match the definition to options

   A blueprint to create objects with data and actions correctly describes a class as a blueprint for objects.

3. Final Answer:

   A blueprint to create objects with data and actions -> Option A

4. Quick Check:

   Class = blueprint for objects [OK]

Hint: Remember: class = blueprint for objects [OK]

Common Mistakes:

• Confusing class with variable
• Thinking class is a method
• Mixing class with loops

2. Which of the following is the correct way to define a class named Person in Java?

easy

A. Person class { }

B. class = Person { }

C. class Person { }

D. define class Person { }

Solution

1. Step 1: Recall Java class syntax

   In Java, a class is defined using the keyword class followed by the class name and braces.

2. Step 2: Check each option

   class Person { } matches the correct syntax: class Person { }. Others have incorrect order or keywords.

3. Final Answer:

   class Person { } -> Option C

4. Quick Check:

   Correct class syntax = class Name { } [OK]

Hint: Use 'class ClassName { }' to define a class [OK]

Common Mistakes:

• Swapping 'class' and class name
• Using '=' sign in class definition
• Using wrong keywords like 'define'

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

class Dog {
  String name = "Buddy";
}

public class Main {
  public static void main(String[] args) {
    Dog d = new Dog();
    System.out.println(d.name);
  }
}

medium

A. Buddy

B. null

C. Dog

D. Compilation error

Solution

1. Step 1: Understand object creation and field access

   The code creates a Dog object and accesses its field 'name' which is set to "Buddy".

2. Step 2: Predict the printed output

   Printing d.name outputs the string "Buddy" stored in the object.

3. Final Answer:

   Buddy -> Option A

4. Quick Check:

   Object field value = Buddy [OK]

Hint: Access object fields with dot notation: object.field [OK]

Common Mistakes:

• Expecting class name instead of field value
• Thinking uninitialized fields print 'null'
• Confusing syntax causing compile errors

4. Identify the error in this class definition:

class Animal {
  String type;
  void Animal() {
    type = "Mammal";
  }
}

medium

A. Class name should be lowercase

B. Constructor has void return type

C. Missing semicolon after type declaration

D. Field 'type' must be static

Solution

1. Step 1: Check constructor syntax

   Constructors in Java must not have a return type, not even void.

2. Step 2: Identify the error

   The method void Animal() is treated as a regular method, not a constructor, causing no constructor defined.

3. Final Answer:

   Constructor has void return type -> Option B

4. Quick Check:

   Constructor = no return type [OK]

Hint: Constructors never have a return type, not even void [OK]

Common Mistakes:

• Adding void to constructor
• Thinking semicolon needed after field
• Believing class names must be lowercase
• Assuming fields must be static

5. You want to create a class Book with a field title and a method printTitle() that prints the title. Which code correctly implements this?

hard

A. class Book {
String title;
void printTitle() {
System.out.println("title");
}
}

B. class Book {
String title;
void printTitle() {
print(title);
}
}

C. class Book {
String title;
void printTitle() {
System.out.print("title");
}
}

D. class Book {
String title;
void printTitle() {
System.out.println(title);
}
}

Solution

1. Step 1: Check method to print field value

   Method should use System.out.println with the field variable title to print its value.

2. Step 2: Evaluate each option

   Options printing the literal "title" (with or without newline) are incorrect. Calling undefined print(title) causes an error. Only System.out.println(title) correctly prints the field value.

3. Final Answer:

   class Book { String title; void printTitle() { System.out.println(title); } } -> Option D

4. Quick Check:

   Print field with System.out.println(field) [OK]

Hint: Use System.out.println(field) to print variable content [OK]

Common Mistakes:

• Using print() instead of println()
• Printing string literal instead of variable
• Calling undefined print() method