Notes, Wu chapter 2 part 2 - Kirkwood Community College
GUI methods for I/O
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 input/output class
• output: display something on an output device (i.e. the computer’s monitor)
o display results of computation
o communicate with the user
• Most Java programs employ a graphical user interface (GUI) with two types of windows:
o frame window: the type of window created by our first example program
o dialog window: allows communication with user; most commonly used to prompt for & receive input, but we’ll begin by looking at just 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
o recall the syntax we saw before for a message:
objectName.methodName(argument(s));
o this new syntax is a variation:
className.methodName(argument(s));
o when we start defining our own classes and methods, you will see how these two mechanisms work
• arguments to the method:
o 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:
o the second argument, a string literal, indicates the text we want to display in the window
• the following program example employs both types of dialogs (within a frame and in its own frame):
// Sample program 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”);
}
}
• 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
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.
The showInputDialog method:
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);
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. For example, the following code fragment produces a dialog window like the one shown below:
String daddy;
daddy = JOptionPane.showInputDialog (null, “Who’s your daddy?”);
[pic]
Reading Numeric Input
• We have already seen that we can read Strings via a showInputDialog window
• It turns out we can use the same mechanism to read other kinds of data, but it takes a little more work
• In order to transform the String returned by showInputDialog into a number, we must perform a type conversion
• Type conversion of a string to a numeric type is accomplished using various parse methods, which are members of the numeric wrapper classes
• Wrapper classes exist as a bridge between simple types (like int or double) and object types (like String)
• The following code fragment illustrates the mechanism:
String input;
int userAge;
input = JOptionPane.showInputDialog
(null, “Enter your age in years”);
userAge = Integer.parseInt(input);
• In the example above, the wrapper class is Integer, and the parse method is parseInt
• Some other useful wrapper classes include Long, Float and Double, and their corresponding parse methods, parseLong, parseFloat and parseDouble
• The general syntax for conversion of a String to a number is:
WrapperClassName.parseMethod(String)
where WrapperClassName is Integer, Double, etc. and parseMethod is parseInt or parseDouble, etc. and String is a String object containing the text to be converted
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