Object interaction in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Java code involving object interaction?

Consider the following Java classes and main method. What will be printed when the program runs?

Java

class Engine {
    int power;
    Engine(int power) {
        this.power = power;
    }
    int getPower() {
        return power;
    }
}

class Car {
    Engine engine;
    Car(Engine engine) {
        this.engine = engine;
    }
    int totalPower() {
        return engine.getPower() + 50;
    }
}

public class Main {
    public static void main(String[] args) {
        Engine e = new Engine(100);
        Car c = new Car(e);
        System.out.println(c.totalPower());
    }
}

A 150

B 0

C 50

D 100

💡 Hint

Look at how Car uses the Engine object to calculate total power.

The Engine object has power 100. The Car adds 50 to this power in totalPower(). So the output is 150.

❓ Predict Output

intermediate

What is the output when two objects interact with method calls?

Given these classes, what will be printed?

Java

class Wallet {
    private int money;
    Wallet(int money) {
        this.money = money;
    }
    void spend(int amount) {
        money -= amount;
    }
    int getMoney() {
        return money;
    }
}

class Person {
    Wallet wallet;
    Person(Wallet wallet) {
        this.wallet = wallet;
    }
    void buySomething(int price) {
        wallet.spend(price);
    }
}

public class Main {
    public static void main(String[] args) {
        Wallet w = new Wallet(200);
        Person p = new Person(w);
        p.buySomething(70);
        System.out.println(w.getMoney());
    }
}

A 70

B 200

C 130

D -70

💡 Hint

Think about how the Person uses the Wallet to spend money.

The wallet starts with 200. The person spends 70, so 200 - 70 = 130 remains.

🔧 Debug

advanced

What error does this code produce when objects interact incorrectly?

Examine the code below. What error will occur when running it?

Java

class Book {
    String title;
    Book(String title) {
        this.title = title;
    }
}

class Library {
    Book book;
    Library() {
        // book not initialized
    }
    void printTitle() {
        System.out.println(book.title);
    }
}

public class Main {
    public static void main(String[] args) {
        Library lib = new Library();
        lib.printTitle();
    }
}

A ArrayIndexOutOfBoundsException

B Compilation error: book not initialized

C Prints empty string

D NullPointerException

💡 Hint

Think about what happens if you try to access a field of an object that was never set.

The book field is never assigned an object, so it is null. Accessing book.title causes a NullPointerException.

📝 Syntax

advanced

Which option causes a compilation error due to incorrect object interaction?

Look at these code snippets. Which one will NOT compile?

A class A { void foo() {} } class B { void bar() { A a; a.foo(); } }

B class A { void foo() {} } class B { A a = null; void bar() { a.foo(); } }

C class A { void foo() {} } class B { A a = new A(); void bar() { a.foo(); } }

D } } ;)(oof.a { )(rab diov ;)(A wen = a A { B ssalc } }{ )(oof diov { A ssalc

💡 Hint

Check if the object a is initialized before calling its method.

Option A declares a but does not initialize it. Calling a.foo() causes a compile error because a might not have been initialized.

🚀 Application

expert

How many objects are created and what is the final output?

Analyze the code below. How many objects are created and what is printed?

Java

class Node {
    int value;
    Node next;
    Node(int value) {
        this.value = value;
    }
    void setNext(Node next) {
        this.next = next;
    }
    int sum() {
        if (next == null) return value;
        return value + next.sum();
    }
}

public class Main {
    public static void main(String[] args) {
        Node n1 = new Node(1);
        Node n2 = new Node(2);
        Node n3 = new Node(3);
        n1.setNext(n2);
        n2.setNext(n3);
        System.out.println(n1.sum());
    }
}

A 1 object created; output: 6

B 3 objects created; output: 6

C 3 objects created; output: 3

D 2 objects created; output: 6

💡 Hint

Count each new Node(...) call and understand the recursive sum method.

Three Node objects are created (n1, n2, n3). The sum method adds 1 + 2 + 3 = 6.

Practice

1.

What does it mean when two objects in Java interact?

Choose the best explanation.

easy

A. One object calls a method of another object to perform a task.

B. Two objects share the same memory location.

C. Objects are created using the same class.

D. Objects are stored in the same variable.

Solution

1. Step 1: Understand object interaction meaning

   Object interaction means objects communicate by calling each other's methods to work together.

2. Step 2: Evaluate options

   Only One object calls a method of another object to perform a task. describes calling methods between objects, which is how interaction happens.

3. Final Answer:

   One object calls a method of another object to perform a task. -> Option A

4. Quick Check:

   Object interaction = method calls between objects [OK]

Hint: Objects interact by calling methods on each other [OK]

Common Mistakes:

• Thinking objects share memory to interact
• Confusing object creation with interaction
• Assuming variables store multiple objects

2.

Which of the following is the correct syntax to call a method start() on an object car in Java?

easy

A. start(car);

B. car.start();

C. car->start();

D. start.car();

Solution

1. Step 1: Recall Java method call syntax

   In Java, to call a method on an object, use objectName.methodName();.

2. Step 2: Check each option

   car.start(); matches correct syntax. Options A, B, and C are invalid Java syntax.

3. Final Answer:

   car.start(); -> Option B

4. Quick Check:

   Method call = object.method() [OK]

Hint: Use objectName.methodName() to call methods [OK]

Common Mistakes:

• Using arrow (->) like in C++
• Reversing method and object order
• Calling method like a function with object as argument

3.

Consider the following Java code:

class Light {
    boolean isOn = false;
    void toggle() {
        isOn = !isOn;
    }
    boolean status() {
        return isOn;
    }
}

class Room {
    Light light = new Light();
    void switchLight() {
        light.toggle();
    }
    boolean lightStatus() {
        return light.status();
    }
}

public class Main {
    public static void main(String[] args) {
        Room room = new Room();
        System.out.println(room.lightStatus());
        room.switchLight();
        System.out.println(room.lightStatus());
    }
}

What is the output when this program runs?

medium

A. false
true

B. true
false

C. false
false

D. true
true

Solution

1. Step 1: Check initial light status

   Initially, isOn is false, so room.lightStatus() prints false.

2. Step 2: Toggle light and check status again

   Calling room.switchLight() toggles isOn to true. Then room.lightStatus() prints true.

3. Final Answer:

   false true -> Option A

4. Quick Check:

   Initial false, toggled true = false then true [OK]

Hint: Track boolean changes step-by-step [OK]

Common Mistakes:

• Assuming toggle sets true first without initial check
• Confusing method calls and variable values
• Ignoring initial value of isOn

4.

Find the error in this Java code snippet involving object interaction:

class Printer {
    void print(String message) {
        System.out.println(message);
    }
}

public class Main {
    public static void main(String[] args) {
        Printer printer;
        printer.print("Hello World");
    }
}

medium

A. The print statement syntax is incorrect.

B. The method 'print' is not defined in the Printer class.

C. The object 'printer' is declared but not initialized before use.

D. The class Printer should be public.

Solution

1. Step 1: Check object declaration and initialization

   The object 'printer' is declared but never assigned a new Printer instance.

2. Step 2: Understand consequences of uninitialized object

   Calling a method on an uninitialized object causes a NullPointerException at runtime.

3. Final Answer:

   The object 'printer' is declared but not initialized before use. -> Option C

4. Quick Check:

   Uninitialized object causes runtime error [OK]

Hint: Always initialize objects before calling methods [OK]

Common Mistakes:

• Assuming declaration equals initialization
• Thinking method absence causes error here
• Ignoring runtime NullPointerException

5.

You have two classes, BankAccount and Customer. A Customer has a BankAccount object. You want to add a method transferTo in BankAccount that transfers money to another BankAccount. Which of the following best shows how objects interact to perform this transfer?

class BankAccount {
    double balance;
    void deposit(double amount) { balance += amount; }
    void withdraw(double amount) { balance -= amount; }
    void transferTo(BankAccount other, double amount) {
        // Fill in this method
    }
}

class Customer {
    BankAccount account = new BankAccount();
}

hard

A. deposit(amount); withdraw(amount);

B. other.withdraw(amount); deposit(amount);

C. balance -= amount; other.balance += amount;

D. withdraw(amount); other.deposit(amount);

Solution

1. Step 1: Understand transfer logic

   To transfer money, withdraw from current account and deposit into the other account.

2. Step 2: Check method calls for interaction

   Calling withdraw(amount) on this object and deposit(amount) on the other object shows interaction.

3. Final Answer:

   withdraw(amount); other.deposit(amount); -> Option D

4. Quick Check:

   Transfer = withdraw from one, deposit to another [OK]

Hint: Transfer = withdraw from self, deposit to other [OK]

Common Mistakes:

• Withdrawing from the wrong account
• Directly changing balance without methods
• Depositing before withdrawing