Class definition in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Java class instantiation?

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

Java

public class Car {
    String brand;
    int year;

    public Car(String brand, int year) {
        this.brand = brand;
        this.year = year;
    }

    public void printInfo() {
        System.out.println(brand + " " + year);
    }

    public static void main(String[] args) {
        Car myCar = new Car("Toyota", 2020);
        myCar.printInfo();
    }
}

A Toyota 2020

B Car@15db9742

C null 0

D Compilation error

💡 Hint

Look at the constructor and the printInfo method to see what values are printed.

The constructor sets the brand to "Toyota" and year to 2020. The printInfo method prints these values separated by a space.

❓ Predict Output

intermediate

What happens if you try to access a private field directly?

Given this Java class, what will happen if you try to compile and run the main method?

Java

public class Person {
    private String name;

    public Person(String name) {
        this.name = name;
    }

    public static void main(String[] args) {
        Person p = new Person("Alice");
        System.out.println(p.name);
    }
}

A Runtime NullPointerException

B Prints: null

C Compilation error: name has private access in Person

D Prints: Alice

💡 Hint

Check the access modifier of the field and where it is accessed.

Private fields are accessible within the same class, including from static methods via an instance. The program compiles and prints 'Alice'.

❓ Predict Output

advanced

What is the output of this Java class with static and instance variables?

Analyze the following Java code and determine what it prints when run.

Java

public class Counter {
    static int count = 0;
    int id;

    public Counter() {
        count++;
        id = count;
    }

    public void printId() {
        System.out.println("Counter id: " + id);
    }

    public static void main(String[] args) {
        Counter c1 = new Counter();
        Counter c2 = new Counter();
        c1.printId();
        c2.printId();
        System.out.println("Total count: " + Counter.count);
    }
}

A

Counter id: 0
Counter id: 0
Total count: 0

B

Counter id: 2
Counter id: 2
Total count: 2

C

Counter id: 1
Counter id: 2
Total count: 2

D Compilation error

💡 Hint

Static variables are shared among all instances, instance variables are unique per object.

Each new Counter increments the static count and assigns it to the instance id. So c1 has id 1, c2 has id 2, and total count is 2.

❓ Predict Output

advanced

What error does this Java class produce?

Look at this Java class code. What error will occur when compiling?

Java

public class Animal {
    String type;

    public Animal() {
        this.type = type;
    }
}

A Runtime NullPointerException

B No error, compiles successfully

C Compilation error: variable type might not have been initialized

D Compilation error: cannot assign a variable to itself

💡 Hint

Check what happens when you assign a field to itself in the constructor.

Assigning 'this.type = type;' assigns the field to itself because the parameter 'type' is missing, so it assigns the field to its own value (null). This is allowed and compiles without error.

🧠 Conceptual

expert

How many objects are created in this Java code?

Consider the following Java code snippet. How many objects are created when main runs?

Java

public class Box {
    int size;

    public Box(int size) {
        this.size = size;
    }

    public static void main(String[] args) {
        Box b1 = new Box(5);
        Box b2 = b1;
        Box b3 = new Box(10);
    }
}

A 2

B 3

C 1

D 0

💡 Hint

Remember that assigning one object reference to another does not create a new object.

Two new Box objects are created with 'new Box(5)' and 'new Box(10)'. The assignment 'Box b2 = b1;' does not create a new object, just another reference.

Practice

1. What is a class in Java?
class Car { }

easy

A. A blueprint to create objects with data and actions

B. A type of variable that stores numbers

C. A method that runs automatically

D. A special kind of loop

Solution

1. Step 1: Understand the role of a class

   A class defines a template or blueprint for creating objects that hold data and actions.

2. Step 2: Match the definition to options

   A blueprint to create objects with data and actions correctly describes a class as a blueprint for objects.

3. Final Answer:

   A blueprint to create objects with data and actions -> Option A

4. Quick Check:

   Class = blueprint for objects [OK]

Hint: Remember: class = blueprint for objects [OK]

Common Mistakes:

• Confusing class with variable
• Thinking class is a method
• Mixing class with loops

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

easy

A. Person class { }

B. class = Person { }

C. class Person { }

D. define class Person { }

Solution

1. Step 1: Recall Java class syntax

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

2. Step 2: Check each option

   class Person { } matches the correct syntax: class Person { }. Others have incorrect order or keywords.

3. Final Answer:

   class Person { } -> Option C

4. Quick Check:

   Correct class syntax = class Name { } [OK]

Hint: Use 'class ClassName { }' to define a class [OK]

Common Mistakes:

• Swapping 'class' and class name
• Using '=' sign in class definition
• Using wrong keywords like 'define'

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

class Dog {
  String name = "Buddy";
}

public class Main {
  public static void main(String[] args) {
    Dog d = new Dog();
    System.out.println(d.name);
  }
}

medium

A. Buddy

B. null

C. Dog

D. Compilation error

Solution

1. Step 1: Understand object creation and field access

   The code creates a Dog object and accesses its field 'name' which is set to "Buddy".

2. Step 2: Predict the printed output

   Printing d.name outputs the string "Buddy" stored in the object.

3. Final Answer:

   Buddy -> Option A

4. Quick Check:

   Object field value = Buddy [OK]

Hint: Access object fields with dot notation: object.field [OK]

Common Mistakes:

• Expecting class name instead of field value
• Thinking uninitialized fields print 'null'
• Confusing syntax causing compile errors

4. Identify the error in this class definition:

class Animal {
  String type;
  void Animal() {
    type = "Mammal";
  }
}

medium

A. Class name should be lowercase

B. Constructor has void return type

C. Missing semicolon after type declaration

D. Field 'type' must be static

Solution

1. Step 1: Check constructor syntax

   Constructors in Java must not have a return type, not even void.

2. Step 2: Identify the error

   The method void Animal() is treated as a regular method, not a constructor, causing no constructor defined.

3. Final Answer:

   Constructor has void return type -> Option B

4. Quick Check:

   Constructor = no return type [OK]

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

Common Mistakes:

• Adding void to constructor
• Thinking semicolon needed after field
• Believing class names must be lowercase
• Assuming fields must be static

5. You want to create a class Book with a field title and a method printTitle() that prints the title. Which code correctly implements this?

hard

A. class Book {
String title;
void printTitle() {
System.out.println("title");
}
}

B. class Book {
String title;
void printTitle() {
print(title);
}
}

C. class Book {
String title;
void printTitle() {
System.out.print("title");
}
}

D. class Book {
String title;
void printTitle() {
System.out.println(title);
}
}

Solution

1. Step 1: Check method to print field value

   Method should use System.out.println with the field variable title to print its value.

2. Step 2: Evaluate each option

   Options printing the literal "title" (with or without newline) are incorrect. Calling undefined print(title) causes an error. Only System.out.println(title) correctly prints the field value.

3. Final Answer:

   class Book { String title; void printTitle() { System.out.println(title); } } -> Option D

4. Quick Check:

   Print field with System.out.println(field) [OK]

Hint: Use System.out.println(field) to print variable content [OK]

Common Mistakes:

• Using print() instead of println()
• Printing string literal instead of variable
• Calling undefined print() method