L4 + L5 - Defining Classes

Question 1

What is the central concept in Java that makes object-oriented programming (OOP) possible?

Answer 1

Classes: Classes define types and form the basis for creating objects, methods, and overall structure of Java programs.

Question 2

How does Java programming typically organize software components?

Answer 2

Java programming consists of defining a number of classes. Each programmer-defined type in Java is implemented as a class, and even built in classes (like String and Scanner) are used to build software

Question 3

What is the role of a class in Java?

Answer 3

A class serves as a blueprint for creating objects. It defines the properties (fields) and behaviors (methods) that the objects instantiated from the class will have.

Question 4

Example of Predefined classes:

Answer 4

String and Scanner

Question 5

What does it mean when we say “a class is a type” in Java?

Answer 5

It means that classes in Java are not only blueprints for objects but also serve as data types. When you create an object from a class, you’re creating an instance of that type.

Question 6

How are programmer-defined types created in Java?

Answer 6

They are created by defining new classes. Every new type that a programmer defines in expressed as a class, which then can be used to instantiate objects.

Question 7

What is the significance of classes in the structure of a Java program?

Answer 7

Classes are fundamental because they encapsulate data and behavior, making it possible to organize, reuse, and modularize code effectively. All software components can be thought of as collections of classes.

Question 8

What is encapsulation?

Answer 8

The bundling of data (variables) and methods that operate on that data into a single unit, typically a class, while restricting direct access to some of the object’s details.

Question 9

Describe the connection between objects and classes.

Answer 9

Objects are instances of classes. While a class is the abstract definition or blueprint, an object is a concrete instance that occupies memory and has state and behavior defined by its class.

Question 10

Why is learning to define your own classes important for Java programming?

Answer 10

It allows you to create custom types that model real-world entities and problems. Defining your own classes is essential for building structured, modular, and maintainable programs.

Question 11

What is one of the key differences between using a predefined class and user-defined class in Java?

Answer 11

With predefined classes, you use the functionality provided by Java (like String or Scanner), whereas with user-defined classes, you design and implement your own structures tailored to your specific application needs.

Question 12

What is a class in Java?

Answer 12

A programmer-defined type that serves as a blueprint for creating objects, defining their attributes and behaviors

Question 13

What are the two key components of a class definition?

Answer 13

Instance variables (fields) and methods.

Question 14

How do you create an object in Java?

Answer 14

Using the ‘new’ operator. Example:

Classname obj - new Classname();

Question 15

What is an instance variable?

Answer 15

A variable defined in a class that holds data for an object

Question 16

How do you access an instance variable?

Answer 16

objectName.instanceVar;

Question 17

What are the two types of methods in Java?

Answer 17

Void methods - Perform an action but do not return a value.

Return methods - Compute and return a value.

Question 18

How do you invoke a method on an object?

Answer 18

Example:

objectName.methodName();

Question 19

What is the purpose of the main method in Java?

Answer 19

It serves as the entry point of a Java program and has the signature:

public static void main(String[] args)

Question 20

What is a constructor?

Answer 20

A special method used to initialize objects when they are created.

Question 21

What are they key characteristics of a constructor?

Answer 21

• Has the same name as the class
• Does not have a return type
• Often overloaded to provide multiple initialization options

Question 22

What happens if you do not define a constructor?

Answer 22

Java provides a default no-argument constructor.

Question 23

What is the ‘this’ keyword used for?

Answer 23

Refers to the calling object and is used to differentiate instance variables from parameters.

Question 24

What are accessor and mutator methods?

Answer 24

Accessor Methods - ( get methods ) retrieve instance variable values.

Mutator Methods ( set methods) modify instance variable values.

Question 25

What are 'public' and 'private' access modifiers?

Answer 25

'public' - Accessible from anywhere 'private' - Only accessible within the class

Question 26

What is method overloading?

Answer 26

Defining multiple methods with the same name but different parameter lists.

Question 27

What is the return statement used for?

Answer 27

It ends a method and specifies the value to return.

Question 28

What is the difference between local and instance variables?

Answer 28

Local variables - are declared inside methods and exist only within the method. Instance variables - are defined in the class and exist for the lifetime of an object.

Question 29

What is a formal parameter vs. an actual parameter?

Answer 29

Formal parameters - are declared in the method definition. Actual parameters - values passed to the method during invocation.

Question 30

What is an abstract data type (ADT)?

Answer 30

A data type that hides its implementation details and only exposes a well-defined interface.

Question 31

What is a static method?

Answer 31

A method that belongs to a class and can be used without an object.

Question 32

How do you declare a static method?

Answer 32

Use the 'static' keyword in the method header. Example: public static int myMethod(int x) { return x * 2; }

Question 33

How do you call a static method?

Answer 33

Use the class name instead of an object: int result = MyClass.myMethod(5);

Question 34

Can a static method invoke an instance method?

Answer 34

No. A static method cannot access instance variables or methods because it has not 'this'

Question 35

What is a static variable?

Answer 35

A variable shared by all objects of a class. Only one copy exists. Example: private static in count = 0;

Question 36

How do you declare a static constant?

Answer 36

Use 'public static final' Example: public static final int MAX_VALUE = 100;

Question 37

What is special about the Math class?

Answer 37

It contains static mathematical methods and constants. Like Math.PI

Question 38

How do you generate a random number between 0.0 and 1.0?

Answer 38

Use: double randomNum = Math.random();

Question 39

What are wrapper classes?

Answer 39

Wrapper classes provide a way to use primitive data types as objects. (e.g., Integer, Double, char)

Question 40

What is automatic boxing and unboxing?

Answer 40

Boxing: Converting a primitive into an object automatically. Example: Integer num = 10; //auto-boxing Unboxing: Converting an object into a primitive automatically. Example: int x = num;

Question 41

What are two types of memory in a computer?

Answer 41

1. Main Memory (RAM) - Stores variables during execution. 2. Secondary memory (Disk) - Stores files permanently.

Question 42

How is a class-type variable stored?

Answer 42

It stores a reference (memory address), not the actual object.

Question 43

What does 'null' represent in Java?

Answer 43

A reference that does not point to any object.

Question 44

What does 'new' do in Java?

Answer 44

It creates an object and returns its memory reference.

Question 45

What is an anonymous object?

Answer 45

An object that is created but not assigned to a variable. Example: new myClass().someMethod();

Question 46

What is a copy constructor?

Answer 46

A constructor that creates a new object as a copy of another object.

Question 47

What is the difference between deep and shallow copies?

Answer 47

Shallow copy - Copies references (not independent objects) Deep copy - Creates new independent objects.

Question 48

How can privacy leaks occur in classes?

Answer 48

Returning mutable instance variables directly instead of creating copies.

Question 49

What is an immutable class?

Answer 49

A class whose objects cannot be changed after creation (e.g., String)

Question 50

Why override the equals method?

Answer 50

To compare object contents instead of memory addresses.

Question 51

What does 'toString()' do?

Answer 51

Converts an object into a readable string representation.

Question 52

What is a package?

Answer 52

A collection of related classes grouped in a directory.

Question 53

How do you import a package in Java?

Answer 53

import java.util.Scanner;

Question 54

What happens if no package is declared?

Answer 54

The class belongs to the default package.

Question 55

How do you document a class using javadoc?

Answer 55

Use special comments: /** * @param name The name of the user * @return A greeting message */

Question 56

How do you generate documentation for a class?

Answer 56

Run: javadoc myClass.java

Question 57

What is a class invariant?

Answer 57

A condition that is always true for a valid object.

Question 58

What does CLASSPATH do?

Answer 58

Specifies where Java looks for user-defined packages.

Question 59

What is the difference between '==' and equals()?

Answer 59

- '==' checks memory addresses - equals() checks content.

Question 60

What happens when you assign one object reference to another using '='?

Answer 60

Both variables point to the same object, meaning changes to one affect the other. Example: person p1 = new Person("Alice"); person p2 = p1; //p2 now refers to the same object as p1

Question 61

Why should you not use '==' to compare objects?

Answer 61

'==' checks memory addresses, not content. Instead, use the equals() method. Example: String s1 = new String("Hello"); String s2 = new String("Hello"); System.out.println(s1 == s2); // false (different objects) System.out.println(s1.equals(s2)); // true (same content)

Question 62

How do assignment operators behave with class-type variables?

Answer 62

They copy the reference, not the actual object. Both references point to the same memory location. Example: Person p1 = new Person("Bob"); Person p2 = p1; // Both p1 and p2 now refer to the same person.

Question 63

What is the key difference between passing primitive and class-type parameters?

Answer 63

Primitive Types - A copy of the value is passed (modifications do not affect the original) Class Types - A copy of the reference is passed (modification affect the original object). Example: void modifyPrimitive(int x) { x = 10; } // No effect on original variable void modifyObject(Person p) { p.setName("John"); } // Changes original object.

Question 64

What makes a class mutable or immutable?

Answer 64

Immutable class: No methods can modify instance variables after creation (e.g., String) Mutable class: provides methods that change internal state.