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Lesson 2: User Interfaces Revisited

In Java Programming Language Basics, Part 1, you learned how to use Java Foundation Classes (JFC) Project Swing (Project Swing) components to build a simple user interface with very basic backend functionality. You also learned how to use the Remote Method Invocation (RMI) application programming interface (API) to send data from a client program to a server program on the net where the data can be accessed by other client programs.

This lesson takes the RMI application from Part 1, Lesson 8: Remote Method Invocation, creates a more involved user interface, and uses a different layout manager. These changes give you the beginnings of a very simple electronic-commerce application that consists of two types of client programs: one lets end users place purchase orders and the other lets order processors view the orders.

About the Example

This is a very simple electronic commerce example for instructional purposes only. It consists of three programs: two client programs, one for ordering fruit and another for viewing the order, and one server program that makes order information available to clients that view the orders.

Fruit Order Client

The FruitClient program presents a user interface and prompts the end user to order apples, peaches, and pears at $1.25 each.

After the end user enters the number of each item to order, he or she presses the Return key to commit the order and update the running total.

The Tab key or mouse moves the cursor to the next field. At the bottom, the end user provides a credit card number and customer ID.

When the end user clicks Purchase, all values entered into the form are sent to the server program.

The end user must press the Return key for the total to update. If the Return key is not pressed, an incorrect total is sent across the net with the order. The end of this lesson asks you to change the code so there is no danger incorrect totals are sent across the net because the end user did not press the Return key.

Server Program

The RemoteServer program provides remotely accessible send and get methods. Fruit order clients call send methods to send data to the server, and view order clients call the get methods to retrieve the data. In this example, the server program has no user interface.

View Order Client

The OrderClient program presents a user interface, and when the end user clicks View Order, the program gets the order data from the server program and puts it on the screen.

Compile and Run the Example

See Part 1, Lesson 8: Remote Method Invocation, for information on how to run the example. Use the Part 1, Lesson 8 instructions, but use the source code provided in this lesson. Here is a summarized version of those steps:

Compile: These instructions assume development is in the zelda home directory.

cd /home/zelda/classes
rmic -d . RemoteServer
cp RemoteServer*.class /home/zelda/public_html/classes
cp Send.class /home/zelda/public_html/classes

cd \home\zelda\classes
rmic -d . RemoteServer
copy RemoteServer*.class \home\zelda\public_html\classes
copy Send.class \home\zelda\public_html\classes

Start rmi Registry:

cd /home/zelda/public_html/classes
unsetenv CLASSPATH
rmiregistry &

cd \home\zelda\public_html\classes
start rmiregistry 

Start Remote Server:

cd /home/zelda/public_html/classes
-Djava.rmi.server.codebase=http://kq6py/~zelda/classes RemoteServer

cd \home\zelda\public_html\classes
java -Djava.rmi.server.codebase=
  file:c:\home\zelda\public_html\classes RemoteServer

Start RMIClient1:

cd /home/zelda/classes

java -Djava.rmi.server.codebase=
  http://kq6py/~zelda/classes/ RMIClient1

cd \home\zeldzeldaa\classes

java -Djava.rmi.server.codebase=
  file:c:\home\zelda\classes\ RMIClient1\home\zelda\public\html

Start RMIClient2:

cd /home/zelda/classes
java -Djava.rmi.server.codebase=
  http://kq6py/~zelda/classes RMIClient2

cd \home\zelda\classes
java -Djava.rmi.server.codebase=
  file:c:\home\zelda\public_html\classes RMIClient2

Fruit Order Client Code

The code uses label, text field, text area, and button components to create the user interface for ordering fruit.

On the display, user interface components are arranged in a 2-column grid with labels in the left column, and the input and output data fields (text fields and text areas) aligned in the right column.

The end user enters his or her apples, peaches, and pears order into the text fields and presses the Return key after each fruit entry. When the Return key is pressed, the text field behavior updates the item and cost totals displayed in the text areas.

The Reset button behavior clears the display, and the underlying total cost and items variables. The Purchase button behavior sends the order data to the server program. If the Reset button is clicked before the Purchase button, null values are sent over the network.

Instance Variables

These next lines declare the Project Swing component classes the SwingUI class uses. These are instance variables that can be accessed by any method in the instantiated class. In this example, they are built in the SwingUI constructor and accessed in the actionPerformed method implementation.

  JLabel col1, col2; 
  JLabel totalItems, totalCost; 
  JLabel cardNum, custID;
  JLabel applechk, pearchk, peachchk;

  JButton purchase, reset;
  JPanel panel;

  JTextField appleqnt, pearqnt, peachqnt;
  JTextField creditCard, customer;
  JTextArea items, cost;

  static Send send;
  int itotal=0; 
  double icost=0;


The constructor is fairly long because it creates all the components, sets the layout to a 2-column grid, and places the components in the grid on a panel. A panel is a container component that holds other components.

The Reset and Purchase buttons and the appleQnt, pearQnt, and peachQnt text fields are added as action listeners. This means when the end user clicks one of the buttons or presses Return in one of the text fields, an action event occurs that causes the platform to call the FruitClient.actionPerformed method where the behaviors for these components are defined.

As explained in Part1, Lesson 4: Building a User Interface, a class declares the ActionListener interface and implements the actionPerformed method if it needs to handle action events such as button clicks and text field Returns. Other user interface components generate some different action events, and as a result, require you to implement different interfaces and methods.

//Create left and right column labels
col1 = new JLabel("Select Items");
col2 = new JLabel("Specify Quantity");

//Create labels and text field components
applechk = new JLabel("   Apples");
appleqnt = new JTextField();

pearchk = new JLabel("   Pears");
pearqnt = new JTextField();

peachchk = new JLabel("   Peaches");
peachqnt = new JTextField();

cardNum = new JLabel("   Credit Card:");
creditCard = new JTextField();

customer = new JTextField();
custID = new JLabel("   Customer ID:");

//Create labels and text area components
totalItems = new JLabel("Total Items:");
totalCost = new JLabel("Total Cost:");
items = new JTextArea();
cost = new JTextArea();

//Create buttons and make action listeners
purchase = new JButton("Purchase");

reset = new JButton("Reset");

The example in Part 1, Lesson 4: Building a User Interface, used the BorderLayout layout manager. This example uses the GridLayout layout manager, which arranges components in a grid or the number of rows and columns you specify. The example uses a 2-column grid with an unlimited number of rows as indicated by the zero (unlimited rows) and two (two columns) in the statement panel.setLayout(new GridLayout(0,2));.

The layout manager and color are set on the panel, and the panel is added to the content pane with a call to the getContentPane method of the JFrame class. A content pane lets different types of components work together in Project Swing.

//Create a panel for the components
  panel = new JPanel();

//Set panel layout to 2-column grid 
//on a white background
  panel.setLayout(new GridLayout(0,2));

//Add components to panel columns
//going left to right and top to bottom









Event Handling

The actionPerformed method provides behavior for each of the following possible application events:

  • The mouse is clicked on the Purchase or Reset button.
  • The Return key is pressed inside the appleQnt, peachQnt, or pearQnt field.

Purchase Button: The Purchase button behavior involves retrieving data from the text fields and text areas, and sending that data to the server program. The server program is available to the FruitClient program through its Send interface, which declares the remote server methods for sending and getting data.

The send variable is an instance of the Send interface. This instance is created in the FruitClient program's main method. The send variable is declared static and global in the FruitClient program so the static main method can instantiate it, and to make it accessible to the actionPerformed method.

if(source == purchase){
  cardnum = creditCard.getText();
  custID = customer.getText();
  apples = appleqnt.getText();
  peaches = peachqnt.getText();
  pears = pearqnt.getText();
  } catch (Exception e) {
     System.out.println("Cannot send data to server"); 
pearQnt Text Field pearQnt
if(source == pearqnt){
  number = pearqnt.getText();
  if(number.length() > 0){
    pearsNo = Integer.valueOf(number);
    itotal += pearsNo.intValue();
  } else {
    itotal += 0;

Cursor Focus

End users can use the Tab key to move the cursor from one component to another within the user interface. The default Tab key movement steps through all user interface components including the text areas.

Because the end user does not interact with the text areas, there is no reason for the cursor to go there. The example program includes a call in its constructor to pearqnt.setNextFocusableComponent to make the cursor move from the pearqnt text field to the creditcard text field bypassing the total cost and total items text areas when the Tab key is pressed.

  applechk = new JLabel("   Apples");
  appleqnt = new JTextField();

  pearchk = new JLabel("   Pears");
  pearqnt = new JTextField();

  peachchk = new JLabel("   Peaches");
  peachqnt = new JTextField();

  cardNum = new JLabel("   Credit Card:");
  creditCard = new JTextField();
//Make cursor go to creditCard component

  customer = new JTextField();
  custID = new JLabel("   Customer ID:");

Converting Strings to Numbers and Back

To calculate the items ordered and their cost, the string values retrieved from the appleQnt, peachQnt, and pearQnt text fields have to be converted to their number equivalents.

The string value is returned in the number variable. To be sure the user actually entered a value, the string length is checked. If the length is not greater than zero, the end user pressed Return without entering a value. In this case, the else statement adds zero to the running total and the cursor focus is set for the creditCard text field. Adding zero is not really necessary, but does make the code more understandable for someone reading it.

If the length is greater than zero, an instance of the java.lang.Integer class is created from the string. Next, the Integer.intValue() method is called to produce the integer ( int) equivalent of the string value so it can be added to the items total kept in the itotal integer variable.

if(number.length() > 0){
  pearsNo = Integer.valueOf(number);
  itotal += pearsNo.intValue();
} else {
  itotal += 0;

To display the total items, a java.lang.Integer object is created from the itotal integer variable. The Integer.toString method is called to produce the String equivalent of the integer ( int). This string is passed to the call to this.cost.setText(text2) to update the Total Cost field in the display.

Note: The cost text area variable is referenced as this.cost because the actionPerformed method also has a cost variable of type Double. To reference the global text area and not the local Double by the same name, you have to reference it as this.cost.

  num = new Integer(itotal);
  text = num.toString();

  icost = (itotal * 1.25);
  cost = new Double(icost);
  text2 = cost.toString();

The int data type contains a single whole 32-bit integer value that can be positive or negative. You can use the standard arithmetic operators (+, -, *, and /) to perform arithmetic operations on the integer.

The Integer class, not only contains a whole 32-bit integer value that can be positive or negative, but provides methods for working on the value. For example, the Integer.intValue method lets you convert an Integer to an int to perform arithmetic operations.

The double data type contains a 64-bit double-precision floating point value. The Double class not only contains a 64-bit double-precision floating point value, but provides methods for working on the value. for example, the Double.doubleValue method lets you convert a Double to a double to perform arithmetic operations.

Server Program Code

The server program consists of the class that implements the methods declared in the interface. These classes are described in Part 1, Lesson 8: Remote Method Invocation with the only difference being in this lesson there are many more sendXXX and getXXX methods to declare and implement. Here is the list:

  • public void sendCreditCard(String creditcard){cardnum = creditcard;}
  • public String getCreditCard(){return cardnum;}
  • public void sendCustID(String cust){custID = cust;}
  • public String getCustID(){return custID;}
  • public void sendAppleQnt(String apps){apples = apps;}
  • public String getAppleQnt(){return apples;}
  • public void sendPeachQnt(String pchs){ peaches = pchs;}
  • public String getPeachQnt(){return peaches;}
  • public void sendPearQnt(String prs){pears = prs;}
  • public String getPearQnt(){return pears;}
  • public void sendTotalCost(double cst){cost = cst;}
  • public double getTotalCost(){return cost; }
  • public void sendTotalItems(int itm){items = itm;}
  • public int getTotalItems(){return items;}

The important thing to note is data of any type and size can be easily passed from one client through the server to another client using the RMI API. No special handling is needed for large amounts of data or special considerations for different data types, which can sometimes be issues when using socket communications.

View Order Client Code

The class uses text areas and buttons to display the order information.

The code is very similar to the class so rather than repeat much of what you have read above, this section highlights two parts of the actionPerformed method behavior for viewing an order.

The first part retrieves the credit card number, and the number of apples, peaches, and pears ordered from the server and sets those values in the corresponding text areas.

The second part retrieves the cost and item totals, which are double and integer, respectively. It then converts the total cost to a java.lang.Double object, and the total items to a java.lang.Integer object, and calls the toString method on each to get the string equivalents. Finally, the strings can be used to set the values for the corresponding text areas.

if(source == view){
//Retrieve and display text
  text = send.getCreditCard();

  text = send.getCustID();

  text = send.getAppleQnt();

  text = send.getPeachQnt();

  text = send.getPearQnt();

//Convert Numbers to Strings
  cost = send.getTotalCost();
  price = new Double(cost);
  unit = price.toString();

  items = send.getTotalItems();
  itms = new Integer(items);
  i = itms.toString();

 } catch (Exception e) {
  System.out.println("Cannot send data to server");

Program Improvements

The example program as it is currently written has two major design flaws in the fruit order client. The first one involves the need to press the Return key for calculations to happen, and the second involves handling the error condition if the end user enters a character that is not a number when ordering apples, peaches, and pears.

Calculations and Pressing Return: If the end user enters a value for apples, peaches, or pears and moves to the next field without pressing the Return key, no calculation is made. This means when the end user clicks the Purchase key, the order is sent, but the item and cost totals will be incorrect. So, in this particular application relying on the Return key action event is not good design.

Modify the actionPerformed method so this does not happen. Here is one possible solution. Give it a try before taking a look.

Non-Number Errors: If the end user enters a non-number value for apples, peaches, or pears the program will present a stack trace indicating an illegal number format. A good program will catch and handle the error, rather than produce a stack trace.

Hint: You need to figure out which part of the code throws the error and enclose it in a try and catch block. try and catch blocks were first introduced in Part 1, Lesson 6: File Access and Permissions. The error you need to catch is java.lang.NumberFormatException.

Give it a try before taking a look at the solution.

More Information

You can find more information on event listening in the Writing Event Listeners lesson in The Java Tutorial.

The Variables and Data Types trail in The Java Tutorial provides more information on primitive data types.

See The JFC Swing Tutorial: A Guide to Constructing GUIs for more information on Project Swing.

*As used on this web site, the terms "Java virtual machine" or "JVM" mean a virtual machine for the Java platform.