Object interaction in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an object of the class Car.

Java

Car myCar = new [1]();

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

ACar

BObject

CVehicle

Dcar

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase class name like 'car' instead of 'Car'.

Using a different class name like 'Vehicle' which is not defined.

To create an object, use the class name with the new keyword. Class names in Java start with uppercase letters.

2fill in blank

medium

Complete the code to call the method startEngine on the car object.

Java

myCar.[1]();

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

AbeginEngine

BrunEngine

CengineStart

DstartEngine

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect method names like 'runEngine' or 'engineStart'.

Forgetting the parentheses after the method name.

To call a method, use the object name followed by a dot and the method name exactly as defined.

3fill in blank

hard

Fix the error in the code to access the speed attribute of the car object.

Java

int currentSpeed = myCar.[1];

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

Aspeed

BcurrentSpeed

CgetSpeed()

DSpeed

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase 'Speed' instead of 'speed'.

Trying to call an attribute as a method like 'getSpeed()' if it does not exist.

Attributes are accessed by their exact name with correct case. Here, the attribute is 'speed' with lowercase 's'.

4fill in blank

hard

Fill both blanks to create a new Engine object and assign it to the car's engine attribute.

Java

myCar.engine = new [1]();
[2] engine = myCar.engine;

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

AEngine

BCar

DVehicle

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Car' or 'Vehicle' instead of 'Engine' for the object type.

Mismatching the variable type and the object assigned.

The class name for the engine is 'Engine'. We create a new Engine object and assign it to the car's engine attribute. Then we declare a variable of type Engine to refer to it.

5fill in blank

hard

Fill all three blanks to define a method in the Car class that returns the engine's horsepower.

Java

public int [1]() {
    return this.engine.[2];
}

Car myCar = new Car();
int hp = myCar.[3]();

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

AgetHorsepower

Bhorsepower

DstartEngine

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong method names like 'startEngine' instead of 'getHorsepower'.

Using incorrect attribute names.

The method name is 'getHorsepower'. It returns the 'horsepower' attribute of the engine. We call the method on the car object to get the horsepower value.

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