Questions and Answers
What is created from classes in Java API?
How do you create a new Random object in Java?
Random rand = new Random();
Which of the following are important while working with objects? (Select all that apply)
The new operator in Java is used to create an object and its memory address is assigned to a variable.
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What are the fields of the Rectangle class?
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The UML class diagram includes a class name, fields, and _____.
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What is the file name convention for the Rectangle class in Java?
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What does a private access specifier mean?
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What is the purpose of the setLength method?
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In Java, the void keyword indicates that a method _____.
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What is the role of the rectangle variable in LengthDemo.java?
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Study Notes
Java Classes and Objects
- Classes in Java API serve as blueprints for creating objects. For example, a Scanner object is an instance of the Scanner class from the Java API.
Object Creation Examples
- A primitive variable is declared as follows:
int wholeNumber;
- An object is instantiated using
Random Rand = new Random();
.
Components for Working with Objects
- Two essential elements:
- The object itself, which must be created in memory.
- A reference variable that points to this object, enabling interaction with it (e.g.,
Random rand = new Random();
).
Understanding Object Instantiation
- The expression
Random Rand
declares a reference variablerand
for a Random object. -
new Random();
utilizes thenew
operator to create a Random object, returning its memory address. - The
=
operator assigns the returned address to therand
variable, allowing it to reference the Random object.
Class Structure Example
- Define a Rectangle class with properties for length and width, alongside methods like
setLength
,setWidth
,getLength
,getWidth
, andgetArea
for rectangle manipulation.
UML Overview
- UML diagram structure:
- Line 1: Class name
- Line 2: Listed fields of the object
- Line 3: Methods defined in the class
Code Implementation of a Class
- A simple class file should be named
Rectangle.java
with the declarationpublic class Rectangle
.
Access Specifiers Explained
-
private
: Restricts access to class members only within the same class. -
public
: Allows access to class members from outside the class.
Implementation of setLength
Method
- The
setLength
method enables external code to store a value in thelength
field of a Rectangle object, shown in the provided method body.
Method Definitions
-
void
: Indicates the method returns no value. -
setLength(double len)
: A method that accepts a double parameter to set the length of the Rectangle. - Instance methods operate on the class instances, lacking the
static
keyword.
Instance of a Class
- Each object created from a class is considered an instance, with method bodies typically manipulating instance data, like setting the length field.
LengthDemo Example
- The
LengthDemo
class illustrates object creation and method invocation:-
rectangle box = new Rectangle();
creates an instance and associates it with variablebox
. - The system outputs the invocation of
setLength(10.0)
.
-
Breakdown of LengthDemo.java
Statements
-
Rectangle box = new Rectangle();
: Declares a variablebox
of the Rectangle type, functioning as a reference variable that holds the memory address of the Rectangle object. - Upon calling
new Rectangle()
, an object is created and thebox
variable references this new instance.
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Description
This quiz covers essential concepts related to Java programming, including classes and object creation. Through flashcards, you'll learn about the Java API and how to effectively declare and utilize objects in your programs. Brush up on your Java knowledge and test your understanding of object-oriented programming principles.