Programming with Java standard classes

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:

JOptionPane: an I/O class Output: display something on an output device (i.e. the computer’s 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 we’ll begin by looking at just output

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

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, “It’s my window”); –the second argument, a string literal, indicates the text we want to display in the window

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.setSize (300, 200); myWindow.setVisible(true); JOptionPane.showMessageDialog(myWindow, “It’s my window”); JOptionPane.showMessageDialog(null, “and I’ll cry \n if I want to”); }

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, we’ll introduce another JOptionPane class method that allows us to take keyboard input from the user

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

The String class We can declare and create a String object just like we do other types of objects, for example: String 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”;

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 don’t exist within the String object

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”

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

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 message’s argument; examples: bigRiver.indexOf(“Miss”) // returns 0 bigRiver.indexOf(“is”) // returns 1 bigRiver.indexOf(“sis”) // returns 3

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”

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.showMessageDialog(null, "Full Name: " + fullName); JOptionPane.showMessageDialog(null, "First: " + firstName); JOptionPane.showMessageDialog(null, "Last: " + lastName); JOptionPane.showMessageDialog(null, "Your last name has "+lastName.length( )+" characters."); }

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.

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, “It’s my window”); JOptionPane.showMessageDialog(null, “and I’ll cry \n if I want to”); In general, the syntax for both showMessageDialog and showInputDialog is: JOptionPane.methodName (WindowObject, MessageObject);

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.

Example: showInputDialog The following code fragment produces a dialog window like the one shown below: String daddy; daddy = JOptionPane.showInputDialog (null, “Who’s your daddy?”);