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.