Chapter Five: Objects & Classes Quiz

Questions and Answers

What are classes and what are objects?

Classes are factories for creating objects, while objects are collections of data and behaviors that represent some entity.

What are instance variables and instance methods?

Instance variables and instance methods are non-static variables and methods declared in a class.

What does null mean in Java?

Null is a special value that indicates a variable does not point to any object.

What is a constructor?

A constructor is a special subroutine in a class called with the new operator to create new objects. Signup and view all the answers

What does the statement 'fruit = new Kumquat();' do?

It creates a new object of type Kumquat and stores a reference to it in the variable fruit. Signup and view all the answers

What is meant by instance variable and instance method?

They are non-static variables and methods in a class representing the data and behaviors of objects. Signup and view all the answers

What are subclass and superclass?

A subclass inherits properties and behaviors from a superclass. Signup and view all the answers

How to modify a class to make instance variables private and add getter/setter methods?

Add 'private' in front of each variable and include getter and setter methods for each. Signup and view all the answers

Why does class Player have a toString() method?

Player inherits the toString() method from the Object class. Signup and view all the answers

What is polymorphism?

Polymorphism allows different objects to respond to the same method differently. Signup and view all the answers

What is garbage collection?

Garbage collection identifies and disposes of objects that can no longer be used. Signup and view all the answers

What is an abstract class in Java?

An abstract class cannot be instantiated and is used as a basis for subclasses. Signup and view all the answers

What is 'this' in Java?

'this' is a special variable that refers to the current object. Signup and view all the answers

Write a complete definition for a simple Counter class.

public class Counter { private int value = 0; public void increment() { value++; } public int getValue() { return value; } } Signup and view all the answers

Fill in the blanks to simulate tossing a coin 100 times: If (Math.random() < 0.5) ; else ;

headCount.increment() ; tailCount.increment() ; Signup and view all the answers

Print the number of heads and tails: System.out.println('There were ' + __________ + ' heads.');

headCount.getValue() Signup and view all the answers

Print the number of tails: System.out.println('There were ' + __________ + ' tails.');

tailCount.getValue() Signup and view all the answers

Study Notes

Objects and Classes in Object-Oriented Programming

Classes and Objects: Classes serve as blueprints for creating objects, defining structures and behaviors. An object is an instance of a class, encapsulating data and functions.
Example: In a Dog class, Lassie is a specific Dog object with individual characteristics.

Instance Variables and Methods

Definition: Instance variables and methods are non-static members of a class, specific to each object created.
Storage: They become part of each instantiated object, providing unique states and behaviors.

Null in Java

Definition: Null indicates that a variable of object type does not point to any object. It serves as a placeholder for objects stored in the heap memory.

Constructors

Purpose: A constructor initializes a new object and has the same name as the class. It is invoked with the new keyword to create instances.

Creating Objects

Memory Allocation: The statement fruit = new Kumquat(); allocates memory for a new Kumquat object, invokes its constructor, and stores the reference in the variable fruit.

Subclass and Superclass

Inheritance: Subclass inherits properties and behaviors from a superclass, which it can extend or modify. Syntax for inheritance in Java is class Subclass extends Superclass.

Getter and Setter Methods

Encapsulation: Making instance variables private and providing getters and setters enables controlled access to them. This pattern enhances data encapsulation and integrity.

Inherited Methods

toString() Method: If a class does not extend any other class, it inherits from Object class by default, which includes a pre-defined toString() method.

Polymorphism

Concept: Polymorphism allows objects of different classes to respond to the same method call in varied ways, often through method overriding in subclasses.

Garbage Collection

Memory Management: Garbage collection in Java automatically reclaims memory by identifying and disposing of unreachable objects, in contrast to manual memory management requiring the programmer’s oversight.

Abstract Classes

Definition: An abstract class cannot create objects and serves as a template for subclasses. It is tagged with the abstract modifier to enforce this restriction.

The "this" Keyword

Functionality: The this keyword references the current object instance within its class, helping to distinguish between instance variables and parameters of methods.

Counter Class Example

Implementation: A class Counter tracks a numeric value with private access. It includes methods increment() to increase the counter's value and getValue() to return the current count.

Coin Toss Simulation

Design: Utilize two Counter objects to count heads and tails from simulated coin flips, demonstrating class usage and method invocation for counting.

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Description

Test your knowledge on object-oriented programming concepts with this quiz focused on classes and objects. Discover the intricacies of how they interact and their roles in software development. Perfect for students diving into programming fundamentals.