Real-world modeling in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a class named Car.

Java

public class [1] {

}

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

AEngine

BDrive

CCar

DVehicle

Attempts:

3 left

💡 Hint

Common Mistakes

Using a name that does not represent the object, like Engine or Drive.

The class name should be Car to model a car.

2fill in blank

medium

Complete the code to add a field for the car's color.

Java

public class Car {
    private [1] color;
}

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

Aint

Bboolean

Cdouble

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Using numeric types like int or double for color.

The color of a car is best represented as a String because it is text.

3fill in blank

hard

Fix the error in the constructor to set the car's color.

Java

public class Car {
    private String color;

    public Car(String [1]) {
        this.color = color;
    }
}

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

Acolor

Bpaint

Cshade

Dhue

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different parameter name than the one assigned to the field.

The constructor parameter must be named color to match the assignment this.color = color;.

4fill in blank

hard

Fill both blanks to add a method that returns the car's color.

Java

public class Car {
    private String color;

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

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

AgetColor

Bcolor

Cthis.color

DfetchColor

Attempts:

3 left

💡 Hint

Common Mistakes

Returning the parameter name or a wrong variable.

Using a method name that does not follow the getter naming convention.

The method name to get the color is getColor, and it returns this.color to access the field.

5fill in blank

hard

Fill all three blanks to add a method that sets the car's color.

Java

public class Car {
    private String color;

    public void [1](String [2]) {
        this.[3] = [2];
    }
}

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

AsetColor

BnewColor

Ccolor

DupdateColor

Attempts:

3 left

💡 Hint

Common Mistakes

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

Using incorrect method or parameter names.

The setter method is named setColor, it takes a parameter newColor, and assigns it to the field color.

Practice

1. What is the main purpose of real-world modeling in Java programming?

easy

A. To avoid using variables in the program

B. To write code that runs faster on computers

C. To make programs use less memory

D. To create classes that represent real-life objects with properties and actions

Solution

1. Step 1: Understand real-world modeling concept

   Real-world modeling means making classes that represent things from real life, like a Car or Person.

2. Step 2: Identify the purpose of these classes

   These classes have properties (data) and methods (actions) to organize code and make it easier to understand.

3. Final Answer:

   To create classes that represent real-life objects with properties and actions -> Option D

4. Quick Check:

   Real-world modeling = Classes for real-life objects [OK]

Hint: Think: real-world objects become classes with data and actions [OK]

Common Mistakes:

• Confusing performance optimization with modeling
• Thinking it means avoiding variables
• Believing it reduces memory usage automatically

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

easy

A. class Book {}

B. Book class {}

C. class = Book {}

D. class Book() {}

Solution

1. Step 1: Recall Java class declaration syntax

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

2. Step 2: Check each option

   class Book {} uses correct syntax: class Book {}. Others have wrong order, symbols, or parentheses.

3. Final Answer:

   class Book {} -> Option A

4. Quick Check:

   Java class declaration = class Name {} [OK]

Hint: Remember: class keyword + name + curly braces [OK]

Common Mistakes:

• Putting parentheses after class name
• Writing 'Book class' instead of 'class Book'
• Using '=' sign in class declaration

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

class Car {
  String color;
  void displayColor() {
    System.out.println("Color: " + color);
  }
}

public class Main {
  public static void main(String[] args) {
    Car myCar = new Car();
    myCar.color = "Red";
    myCar.displayColor();
  }
}

medium

A. Color: null

B. Color: Red

C. Compilation error

D. No output

Solution

1. Step 1: Understand object creation and property assignment

   A new Car object is created, and its color property is set to "Red".

2. Step 2: Analyze method call output

   The displayColor() method prints "Color: " plus the color property, which is "Red".

3. Final Answer:

   Color: Red -> Option B

4. Quick Check:

   Property set to "Red" prints "Color: Red" [OK]

Hint: Check property value before method prints it [OK]

Common Mistakes:

• Assuming default null prints instead of assigned value
• Thinking code has syntax errors
• Missing object creation step

4. Identify the error in this Java class modeling a Person:

public class Person {
  String name;
  int age;

  void Person(String n, int a) {
    name = n;
    age = a;
  }
}

medium

A. Constructor has void return type, so it's a method, not a constructor

B. Missing semicolon after variable declarations

C. Class name should be lowercase

D. Variables should be private

Solution

1. Step 1: Check constructor syntax

   Constructors in Java do not have a return type. Here, void Person(...) is a method, not a constructor.

2. Step 2: Understand impact of error

   Because of void, this method won't initialize the object when created, causing default values.

3. Final Answer:

   Constructor has void return type, so it's a method, not a constructor -> Option A

4. Quick Check:

   Constructor = no return type [OK]

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

Common Mistakes:

• Thinking void is needed for constructors
• Ignoring constructor syntax rules
• Confusing methods with constructors

5. You want to model a Library that contains many Book objects. Which design correctly represents this real-world relationship in Java?

hard

A. class Book { Library library; }

B. class Library { Book book; }

C. class Library { List<Book> books = new ArrayList<>(); }

D. class Library { int bookCount; }

Solution

1. Step 1: Understand the relationship between Library and Books

   A library contains many books, so it should hold a collection (like a list) of Book objects.

2. Step 2: Analyze each option

   class Library { List<Book> books = new ArrayList<>(); } uses a List<Book> to hold many books, correctly modeling the relationship. class Library { Book book; } holds only one Book, whereas class Book { Library library; } reverses the relationship, and class Library { int bookCount; } only counts books without storing them.

3. Final Answer:

   class Library { List<Book> books = new ArrayList<>(); } -> Option C

4. Quick Check:

   Many books = collection in Library class [OK]

Hint: Use collections to model 'many' relationships [OK]

Common Mistakes:

• Using single object instead of collection for many items
• Confusing ownership direction between classes
• Using only counters without storing objects