Programming with Java standard classes

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Programming with Java standard classes
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Java API

• Application Programming Interface
• Provides hundreds of standard classes that can be
  incorporated into your programs readymade
  solutions for a variety of tasks
• Detailed documentation of Java standard classes
  freely available on web http//java.sun.com/j2se/
  1.5.0/docs/api/index.html

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JOptionPane an I/O class

• Output display something on an output device
  (i.e. the computers monitor)
• display results of computation
• communicate with the user
• Most Java programs employ a graphical user
  interface (GUI) with two types of windows
• frame window the type of window created by our
  first example program
• dialog window allows communication with user
  most commonly used to prompt for receive input,
  but well begin by looking at just output

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JOptionPane output example
JOptionPane.showMessageDialog(null, Look at
me!)

• Displays a small window in the center of the
  screen containing the words in the string literal
  and a button labeled OK
• This is an example of a message that calls a
  class method of the JOptionPane class we are
  not making a request to an object, but rather to
  the class itself

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JOptionPane.showMessageDialog example

• Arguments to the method
• the first argument, null, indicates that there is
  no existing frame object upon which this dialog
  should appear if we want the dialog to appear in
  an existing frame window, we would pass the name
  of the frame object as the first argument,
  instead of null for example 
• JFrame myWindow
• // review what goes here?
• JOptionPane.showMessageDialog (myWindow, Its my
  window)
• the second argument, a string literal, indicates
  the text we want to display in the window

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Example program
// Sample program 2 displaying messages //
using JOptionPane.showMessageDialog   import
javax.swing.   class Example2 public
static void main (String args) JFrame
myWindow   myWindow new JFrame() myWindow.se
tSize (300, 200) myWindow.setVisible(true)   J
OptionPane.showMessageDialog(myWindow, Its my
window) JOptionPane.showMessageDialog(null,
and Ill cry \n if I want to)
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Some notes on the example

• Displaying multiple lines of text the special
  character \n represents the control character
  you get when you press the Enter key on the
  keyboard (n new line)
• Later on, well introduce another JOptionPane
  class method that allows us to take keyboard
  input from the user

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The String class

• We have already seen several instances of string
  literal values a.k.a. string constants that
  is, sets of characters enclosed within double
  quotation marks
• The Java API contains the String class, which
  allows us to manipulate string data through the
  use of String objects

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The String class

• We can declare and create a String object just
  like we do other types of objects, for
  exampleString bigRiver new String
  (Mississippi)
• In fact, the String class is an exception to the
  new rule you can create a String object
  without it, as in this example
• String bigRiver Amazon
• But you may find it less confusing to stick to
  the form you know for other classes, like the
  first example, above
• Bear in mind, though, that it is quite common to
  assign a new string literal to a String object
  without using the new operator, as in the
  following lines of code
• String bigRiver new String (Amazon)
• // some other stuff happens
• bigRiver Mississippi

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Operations on String objects

• The Java API defines many operations on String
  objects we will review just a few of them here
• substring takes 2 arguments representing the
  beginning and ending positions of a String within
  a String returns the resulting substring
• Note that the first position in a String in Java
  is designated position 0 so a 4-letter word
  would start at 0 and end at 3
• An error will result if you attempt to call the
  method using positions that dont exist within
  the String object

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Examples using substring

• If the String object bigRiver contains the
  literal value Mississippi
• bigRiver.substring(6, 8) // returns sip
• bigRiver.substring (0, 2) // returns Mis
• bigRiver.substring (4, 5) // returns is

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Examples using substring

• The method calls in the example would return the
  literal values indicated, but they would neither
  be stored nor displayed anywhere therefore,
  these method calls would usually occur within the
  context of an assignment statement or another
  method call for example
• String sub new String (bigRiver.substring(6,
  8))
• // returns sipp and assigns it to new object
  sub
• JOptionPane.showMessageDialog(bigRiver.substring
  (4, 5))
• // displays is in a small window with an OK
  button

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More String methods

• The length method returns the length (in
  characters) of the String object for example, if
  String bigRiver contains the value Mississippi
  then
• bigRiver.length() // returns 11
• The indexOf method returns a number indicating
  the position of the beginning of the first
  occurrence of the substring specified in the
  messages argument examples
• bigRiver.indexOf(Miss) // returns 0
• bigRiver.indexOf(is) // returns 1
• bigRiver.indexOf(sis) // returns 3

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String concatenation

• String concatenation an operation that makes one
  String from two, using the operator for
  example
• String phrase, word
• phrase new String (old man river)
• word new String ( that )
• phrase phrase word phrase
• // phrase now contains old man river that old
  man river

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Program example
import javax.swing. class Ch2StringProcessing
public static void main( String args )
String fullName, firstName, lastName,
space   fullName new String("Decafe
Latte") space new String(" ")
firstName fullName.substring(0,
fullName.indexOf(space)) lastName
fullName.substring(fullName.indexOf(space) 1,

fullName.length()) JOptionPane.showMessag
eDialog(null, "Full Name " fullName)
JOptionPane.showMessageDialog(null, "First "
firstName) JOptionPane.showMessageDialog(
null, "Last " lastName)
JOptionPane.showMessageDialog(null, "Your last
name has "lastName.length( )" characters.")

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Reading input

• In order to receive input data from a user, we
  need two things
• a mechanism by which to read the data
• a place to store the data
• We can use another method, showInputDialog, of
  the JOptionPane class for the first part we can
  use a String object for the second part.

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Using showInputDialog for reading input

• The syntax for showInputDialog is almost
  identical to the previous JOptionPane method we
  looked at, showMessageDialog. You may recall
  from a previous example program the following
  lines of code
• JOptionPane.showMessageDialog(myWindow, Its my
  window)
• JOptionPane.showMessageDialog(null, and Ill
  cry \n if I want to)
• In general, the syntax for both showMessageDialog
  and showInputDialog is
• JOptionPane.methodName (WindowObject,
  MessageObject)

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I/O dialog windows

• We know from the examples that the first
  argument, the WindowObject, can either be a
  JFrame object we have declared and initialized or
  null we also know that the second argument can
  be a String literal value.
• The difference between the two methods is that
  showMessageDialog merely displays a window
  containing the specified message, but
  showInputDialog method displays the message as a
  prompt, followed by a space for the user to enter
  input.

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Example showInputDialog
The following code fragment produces a dialog
window like the one shown below String
daddy daddy JOptionPane.showInputDialog (null,
Whos your daddy?)