Class definition in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a class in Java?

A class in Java is a blueprint or template that defines the structure and behavior (data and methods) of objects. It groups related variables and functions together.

beginner

How do you define a simple class named <code>Car</code> in Java?

You define it using the <code>class</code> keyword followed by the class name and curly braces:<br><pre>public class Car {
  // fields and methods
}</pre>

beginner

What are fields and methods inside a class?

Fields are variables that hold data about the object. Methods are functions that define actions or behaviors the object can perform.

beginner

What does the keyword <code>public</code> mean when used before a class or method?

<code>public</code> means the class or method can be accessed from other classes anywhere in the program.

beginner

Why do we use classes in programming?

Classes help organize code by grouping related data and actions. They make code reusable, easier to understand, and model real-world things.

Which keyword is used to define a class in Java?

A object

B struct

C define

D class

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

What is inside a class in Java?

A Only methods

B Fields and methods

C Only variables

D Functions only

A class contains fields (variables) and methods (functions) that define data and behavior.

What does public before a class mean?

A Class is private

B Class is static

C Class can be accessed anywhere

D Class is abstract

public means the class can be accessed from any other class.

Which of these is a valid class name in Java?

A Car123

B 123Car

C car-name

D class

Class names must start with a letter or underscore and cannot be Java keywords. Car123 is valid.

Why do programmers use classes?

A To organize data and behavior together

B To write code faster

C To avoid using variables

D To make programs run slower

Classes organize related data and actions, making code easier to manage and reuse.

Explain what a class is and why it is useful in Java programming.

Think about how a class models real-world things.

Describe the parts inside a class and their roles.

What does a class hold to represent an object?

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