Real-world modeling in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of class inheritance and method overriding

What is the output of this Java program that models animals and their sounds?

Java

class Animal {
    void sound() {
        System.out.println("Some sound");
    }
}

class Dog extends Animal {
    void sound() {
        System.out.println("Bark");
    }
}

public class Main {
    public static void main(String[] args) {
        Animal a = new Dog();
        a.sound();
    }
}

A Bark

B Compilation error

C Some sound

D Runtime error

💡 Hint

Think about which method is called when an Animal reference points to a Dog object.

In Java, the method called depends on the actual object type, not the reference type. Since 'a' refers to a Dog object, Dog's sound() method runs, printing 'Bark'.

🧠 Conceptual

intermediate

Choosing the right data structure for modeling a library

You want to model a library system where each book has a unique ISBN and you need to quickly find a book by its ISBN. Which Java collection is best suited for this?

A LinkedList<Book>

B ArrayList<Book>

C HashMap<String, Book>

D TreeSet<Book>

💡 Hint

Consider which collection allows fast lookup by a unique key.

HashMap allows fast lookup by key (ISBN), making it ideal for quickly finding books by their unique identifier.

🔧 Debug

advanced

Identify the error in this class modeling a bank account

What error will this Java code produce when compiled or run?

Java

public class BankAccount {
    private double balance;

    public BankAccount(double initialBalance) {
        balance = initialBalance;
    }

    public void withdraw(double amount) {
        if (balance >= amount) {
            balance = balance - amount;
        } else {
            System.out.println("Insufficient funds");
        }
    }

    public double getBalance() {
        return balance;
    }

    public static void main(String[] args) {
        BankAccount acc = new BankAccount(100);
        acc.withdraw(150);
        System.out.println("Balance: " + acc.getBalance());
    }
}

A Output: Insufficient funds\nBalance: 100.0

B Runtime error: negative balance allowed

C Output: Balance: -50.0

D Compilation error: missing return type

💡 Hint

Check what happens when withdrawal amount is more than balance.

The withdraw method checks if balance is enough before subtracting. If not, it prints a message and does not change balance. So balance stays 100.

📝 Syntax

advanced

Identify the syntax error in this Java class modeling a vehicle

Which option correctly identifies the syntax error in this code?

Java

public class Vehicle {
    private String model;

    public Vehicle(String model) {
        this.model = model;
    }

    public String getModel() {
        return model;
    }
}

A Missing return type in getModel method

B Missing semicolon after 'this.model = model' in constructor

C Class name should be lowercase

D Constructor name does not match class name

💡 Hint

Look carefully at the constructor's assignment line.

Java statements must end with a semicolon. The line 'this.model = model' is missing a semicolon, causing a syntax error.

🚀 Application

expert

Modeling a simple employee hierarchy with abstract classes

Given the abstract class Employee and two subclasses Manager and Developer, which code snippet correctly implements the abstract method and prints the correct output when run?

Java

abstract class Employee {
    String name;
    Employee(String name) {
        this.name = name;
    }
    abstract void work();
}

class Manager extends Employee {
    Manager(String name) {
        super(name);
    }
    void work() {
        System.out.println(name + " manages the team.");
    }
}

class Developer extends Employee {
    Developer(String name) {
        super(name);
    }
    void work() {
        System.out.println(name + " writes code.");
    }
}

public class Company {
    public static void main(String[] args) {
        Employee e1 = new Manager("Alice");
        Employee e2 = new Developer("Bob");
        e1.work();
        e2.work();
    }
}

A Compilation error: cannot instantiate abstract class Employee

B Alice writes code.\nBob manages the team.

C Runtime error: abstract method not implemented

D Alice manages the team.\nBob writes code.

💡 Hint

Check how abstract methods are implemented in subclasses and which method runs.

Both Manager and Developer implement the abstract method work(). The output matches their respective implementations.

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