Download Intro to Objects and Random number generation

Chapter 3
Using Classes and Objects
Creating Objects
 A variable holds either a primitive type or a
reference to an object
 A class name can be used as a type to declare
an object reference variable
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Remember…
Primitive Data Types
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X
int x = 5;
A primitive data type
variable contains the
value itself, but an
object variable
contains the address
of the object
Primitive Data Type
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References to Objects

An object reference variable holds the address of an object
String s = “Hello”;
//same as: String s = new String(“Hello”);
“Hello”
s
An object
variable
contains the
address of
the object
Memory
location
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Declaring the Object
 The object itself must be created separately
String s;
s
Memory
location
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Creating the Object itself
 The object itself must be created separately
s
String s;
s = “Hello”;
“Hello”
Memory
location
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Is this assignment?
String s;
String t;
s = “hello”;
t = s;
t
“Hello”
s
Memory
location
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Creating Objects, the formula
 Generally, we use the new operator to create an object
Object
objectReference = new
Class name
Object
Class name
(
0 or more
parameters
);
The identifier
or variable name
Specifications for the
newly created object
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Creating Objects
 Generally, we use the new operator to create an object
String name = new String (”Blanca J. Polo");
This keyword new calls the String constructor
That is the one that creates/builds the object
Creating an object is called instantiation
An object is an instance of a particular class
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Invoking Methods
 We've seen that once an object has been instantiated, we can
use the dot operator to invoke its methods
int nameLength = 0;
nameLength = name.length()
 A method may return a value
 The returned value can be used in an assignment or expression
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The main method is void
A void method
doesn’t return
anything
Return types
can be of any
Primitive Data
Type
Return types can be
of any Object kind,
either pre-made by
JAVA or made by
you
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Review: The difference
int num
38
Srting name
”Blanca Polo"
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Primitive Data Type
Assignment Review
 The act of assignment takes a copy of a value and stores it in a
variable
 For primitive types:
Before:
num1
38
num2
96
num2 = num1;
After:
num1
38
num2
38
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Object Assignment
Review
 For object references, assignment copies the address:
Before:
name1
”Tom Hanks"
name2
”Kurt Russell"
name2 = name1;
name1
After:
”Tom Hanks"
name2
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Aliases
 Two or more references that refer to the same object are called
aliases of each other
 That creates an interesting situation: one object can be
accessed using multiple reference variables
 Aliases can be useful, but should be managed carefully
 Changing an object through one reference changes it for all of
its aliases, because there is really only one object
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Garbage Collection
 When an object no longer has any valid references to it, it can
no longer be accessed by the program
 The object is useless, and therefore is called garbage
 Java performs automatic garbage collection periodically,
returning an object's memory to the system for future use
 In other languages, the programmer is responsible for
performing garbage collection
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Class Libraries
 A class library is a collection of classes that we can use when
developing programs
 The Java standard class library is part of any Java development
environment
 Other class libraries can be obtained through third party
vendors, or you can create them yourself
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What is in a Package
 A package is a directory of files that contain utilities.
 All classes of the java.lang package are imported
automatically into all programs.
 The String class, and the System.out class are part of this
package.
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Packages
 The classes of the Java standard class library are organized into
packages
 Some of the packages in the standard class library are:
Package
Purpose
java.lang
java.applet
java.awt
javax.swing
java.net
java.util
javax.xml.parsers
General support
Creating applets for the web
Graphics and graphical user interfaces
Additional graphics capabilities
Network communication
Utilities
XML document processing
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The import Declaration
 When you want to use a class from a package, you could use its
fully qualified name
java.util.Random
 Or you can import the class, and then use just the class name
import java.util.Random;
 To import all classes in a particular package, you can use the *
wildcard character
import java.util.*;
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The import Declaration
 Import statements are always at the top of the program.
import java.lang.*;
 Many of the classes that we will use in this class will need to be
imported.
 Different JAVA programming environments have different
libraries
 We will use the standard libraries that can be found in any
programming environment.
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Counting
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The Random Class
 The Random class is part of the java.util package
 It provides methods that generate pseudorandom numbers
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Random Class Methods
 Constructor:
Random( )
Random r = new Random( );
• float nextFloat( )
Returns a random number between
0.0 and 1.0 inclusive
float f;
f = r.nextFloat( );
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More Random Class Methods
• int nextInt( )
Returns a random number that
ranges over all possible int values
positive and negative
int i;
i = r.nextInt( );
• int nextInt( int num )
Returns a random number between
0 and num-1 (inclusive)
int i;
i = r.nextInt(5 );
// generates a number between 0 and 4
See
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Java/Numbers/GenOneRandom.java
Java/Numbers/OneRandom.java
Questions
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Questions?
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