Chapter 6

Question 1

What is an object in Java?

Answer 1

An object is an entity in memory that performs specific tasks. It can:

Store data (fields).
Perform operations (methods).

Question 2

Name two capabilities of objects in Java.

Answer 2

Objects can store data (fields).
Objects can perform operations (methods).

Question 3

What is a class in Java?

Answer 3

A class is a blueprint for creating objects. It specifies:

The data an object can hold (fields).
The actions an object can perform (methods).

Question 4

What is an instance of a class?

Answer 4

An instance is a specific object created from a class.

Question 5

Give examples of objects you’ve used in Java.

Answer 5

Scanner: Reads input from users.
Random: Generates random numbers.
PrintWriter: Writes data to files.

Question 6

What are the fields in a Rectangle class?

Answer 6

length: Holds the rectangle’s length.
width: Holds the rectangle’s width.

Question 7

What are the methods in a Rectangle class?

Answer 7

setLength: Sets the length.
setWidth: Sets the width.
getLength: Returns the length.
getWidth: Returns the width.
getArea: Calculates and returns the area (length * width).

Question 8

What does UML stand for?

Answer 8

Unified Modeling Language. It is used to visually represent object-oriented systems.

Question 9

What are access specifiers in Java?

Answer 9

Java keywords that control how fields or methods can be accessed.

Question 10

What does the public access specifier do?

Answer 10

A public member can be accessed by any code, inside or outside the class.

Question 11

What does the private access specifier do?

Answer 11

A private member can only be accessed by methods within the same class, protecting it from external access.

Question 12

What is the purpose of the set and get methods in a class?

Answer 12

set methods assign values to fields.
get methods retrieve values from fields, ensuring controlled access.

Question 13

What does an object need to perform its actions?

Answer 13

An object needs a class as a blueprint to define its fields and methods.

Question 14

How many objects can be created from a single class?

Answer 14

As many as needed; each object is an independent instance of the class.

Question 15

Why are access specifiers important?

Answer 15

They help encapsulate and protect data within a class, controlling how it can be accessed and modified.

Question 16

What is Data Hiding?

Answer 16

Data hiding means fields in a class are private and cannot be directly accessed outside the class.
Access is controlled via public methods (accessors and mutators).

Question 17

What is an Accessor?

Answer 17

A method that allows acess to the value of a private field.
Example: public double getLength()

Question 18

What is a Mutator?

Answer 18

A method that modifies the value of a private field.
Example: public void setLength(double len)

Question 19

How does Data Hiding ensure integrity?

Answer 19

Only class methods can modify an object’s private fields, reducing unintended changes.
External code must use public methods, ensuring controlled updates.

Question 20

What is the purpose of dynamic calculations in methods?

Answer 20

Avoids stale data by calculating values (e.g., length × width) dynamically in a method.
Example:
java
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public double getArea() {
return length * width;
}

Question 21

What is UML?

Answer 21

Unified Modeling Language (UML) provides language-independent diagrams for:
Fields
Methods
Access modifiers
Return types

Question 22

What are Instance Fields and Methods?

Answer 22

Instance Fields: Each object gets its own copy of these fields.
Instance Methods: Operate on instance fields and require an object to be created.

Question 23

What is a Constructor?

Answer 23

A special method that initializes fields and is automatically called when an object is created.
Example:
java
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public Rectangle(double len, double w) {
length = len;
width = w;
}

Question 24

What is a Default Constructor?

Answer 24

Provided by Java if no constructor is written.
Sets numeric fields to 0, boolean fields to false, and object references to null.

Question 25

What is a No-Arg Constructor?

Answer 25

A constructor with no parameters.
Example:
java
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public Rectangle() {
length = 1.0;
width = 1.0;
}

Question 26

What happens if a reference variable is uninitialized?

Answer 26

It must reference an object before being used, or a compiler error will occur.
Example:
java
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Rectangle box;
box = new Rectangle(7.0, 14.0);

Question 27

What is Method Overloading?

Answer 27

Multiple methods in a class with the same name but different parameter lists.
Example:
java
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public void add(int a, int b) { }
public void add(String a, String b) { }

Question 28

What is a Method Signature?

Answer 28

The method’s name and the data types of its parameters (order matters).
Return type is not part of the signature.

Question 29

What is Shadowing?

Answer 29

A local variable in a method has the same name as an instance field, hiding the field’s value.
Shadowing is discouraged to avoid confusion.

Question 30

How are String objects created?

Answer 30

Using a constructor:
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String name = new String(“Michael Long”);
Shorthand:
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String name = “Michael Long”;

Question 31

What happens when an object is passed as an argument?

Answer 31

The object’s memory address is passed to the parameter.
The receiving method can access and modify the object.

Question 32

What are Explicit and Wildcard Imports?

Answer 32

Explicit Import: Imports a specific class.
java
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import java.util.Scanner;
Wildcard Import: Imports all classes in a package.
java
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import java.util.*;

Question 33

What are Responsibilities in Class Design?

Answer 33

Knowledge: What the class knows (fields).
Behavior: What the class does (methods).

Question 34

What is the Java Default Package?

Answer 34

The java.lang package is automatically available in any Java program.

Question 35

What is the purpose of Method Binding?

Answer 35

The compiler matches a method call with the correct overloaded method using the method signature.