Download The Random and String Classes

The
Random
and
String
Classes
The import statement
Creating Objects
A variable holds either a primitive type or a reference to an
object
Object objectName = new Object (0 or more parameters);
Object o;
In the above declaration, what will be the value of o?
Object x;
x=o;
Explain the above situation…
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Question
Object o;
In the above declaration, what will be the value of o?
Object x;
x=o;
Explain the above situation…
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3
Invoking Methods
returnValue =Object.method(parameters);
Will I always have a return value?
Are parameters always present?
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Aliases
• Two or more references that refer to the same object are
called aliases of each other
Before:
name1
”Tom Hanks"
name2
”Kurt Russell"
name2 = name1;
”Tom Hanks"
name1
After:
name2
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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
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See
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Java/Numbers/GenOneRandom.java
Java/Numbers/OneRandom.java
String Methods
• Once a String object has been created, neither its value
nor its length can be changed. Thus we say that an object
of the String class is immutable
• However, several methods of the String class return
new String objects that are modified versions of the
original
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String Indexes
String s = “Hello world”;
H
0
e
1
l
2
l
3
o
4
5
w
6
o
7
r
8
l
9
d
10
•The length of s is 11
•It has positions from 0 to 10
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String Methods
String toUpperCase( )
String toLowerCase( )
int length( )
String substring(int begin)
String substring(int begin, int end)
String replace( char oldChar, char newChar)
String trim( )
int indexOf(char anyChar)
int indexOf(String anyString)
char charAt(int position)
String concat(String anotherString)
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See java/String/StringMethods.java
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Questions?
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