Download Jan 28

Objectives
► Become
familiar with the class String
► Learn how to use input and output dialog boxes
in a program
► Learn how to tokenize the input stream
► Explore how to format the output of decimal
numbers with the class DecimalFormat
Object and Reference Variables
► Declare
a reference variable of a class type
► Use the operator new to:
 Allocate memory space for data
 Instantiate an object of that class type
► Store
the address of the object in a reference
variable
The Operator new
► Statement:
Integer num;
num = new Integer(78);
► Result:
Garbage Collection
► Change
value of num:
num = new Integer(50);
► Old memory space reclaimed
Using Predefined Classes and
Methods in a Program
► Many
predefined packages, classes, methods in
Java
► Library: Collection of packages
► Package: Contains several classes
► Class: Contains several methods
► Method: Set of instructions
Using Predefined Classes and
Methods in a Program
► To
use a method you must know:
 Name of class containing method (Math)
 Name of package containing class (java.lang)
 Name of method (pow), its parameters (int a, int b), and
the function or purpose of the method (a^b)
int i=2 j=4, k;
k= Math.pow( i , j );
The value of k will be 2^4=16
Using Predefined Classes and Methods
in a Program
► Example
method call:
import java.io.*;
//imports package
OR
import java.io.BufferReader: //imports class
Math.pow(2,3); //calls power method in
//class Math or package java.lang
Package java.lang is automatically imported
► (Dot)
. Operator: used to access the method in the class
The class String
► String
variables are reference variables
► Given String name;
 Equivalent Statements:
name = new String("Lisa Johnson");
name = “Lisa Johnson”;
Commonly Used String Methods
► String(String str)
► char
charAt(int index)
► int indexOf(char ch)
► int indexOf(String str, int pos)
► int compareTo(String str)
Commonly Used String Methods
► String
concat(String str)
► boolean equals(String str)
► int length()
► String replace(char ToBeReplaced, char
ReplacedWith)
► String toLowerCase()
► String toUpperCase()
What is tokenizing
► Consider
the following string
 “This is a test of string tokenizing”
► A token
is a substring of this string that is selected as
follows:
 A particular delimeter or separation character is chosen, in
this case choose whitespace (blank or tab)
 Starting at the beginning of the string characters are added to
the substring until a delimeter is encountered
 The token is the substring (the delimeter is not added to the
substring)
What is tokenizing
► The
next token is selected as follows:
 The next character in the string that is not a delimeter is
located. This character is the first character of the next token.
 Starting at this character, additional characters are added to
the substring until a delimeter is encountered
 The token is the substring (the delimeter is not added to the
substring)
► For
this example the second token would be “is”
► The pattern continues until all characters in the string
have been processed.
Tokenizing a String
► class StringTokenizer
 Contained in package java.util
 Tokens usually delimited by whitespace characters
(space, tab, newline, etc)
 Contains methods:
►public StringTokenizer(String str, String delimits)
►public int countTokens()
►public boolean hasMoreTokens()
►public String nextToken(String delimits)
StringTokenizer tokenizer;
String inputLine;
String name;
String number;
inputLine = “Judy 3250”;
tokenizer = new StringTokenizer(inputLine);
name = tokenizer.nextToken();
num = Integer.parseInt( tokenizer.nextToken() );
Using Dialog Boxes for Input/Output
► Use
a graphical user interface (GUI)
► class JOptionPane
 Contained in package javax.swing
 Contains methods: showInputDialog and showMessageDialog
► Syntax:
str = JOptionPane.showInputDialog(strExpression)
► Program must end with System.exit(0);
Parameters for the Method
showMessageDialog
JOptionPane Options for the
Parameter messageType
JOptionPane Example
Formatting the Output of Decimal
Numbers
► Type
float: defaults to 6
decimal places
► Type double: defaults to
15 decimal places
class Decimal Format
► Import package java.text
► Create
DecimalFormat object and initialize
► Use method format
► Example:
DecimalFormat twoDecimal =
new DecimalFormat("0.00");
twoDecimal.format(56.379);
► Result:
56.38