Creating a GUI Class An example of class design using inheritance and interfaces

GUI as an Example of I&P  The JFrame class  components and layout  inheriting from JFrame  The ActionListener interface  adding an action listener

Our Goal  Program opens up a window like this:  What can we do with it?  can enter numbers into the first and second number boxes  can click the “Calculate” button to put their sum into the result box  can click the “Done” button to end the program

The JFrame Class  javax.swing.JFrame is a window class  constructor takes window title  need to set its size & make it visible »otherwise it’s tiny and invisible JFrame win = new JFrame(“An Empty Window”); win.setSize(300, 200); win.setVisible(true);

Window Components  Want to add stuff to the window  labels, text fields, buttons win.add(new JLabel(“Hello!”), BorderLayout.NORTH); win.add(new JTextField(), BorderLayout.CENTER); win.add(new JButton(“OK”), BorderLayout.SOUTH); »there are other ways to lay out the window

Window Operations  Resizing: JFrame handles this  but don’t really need it, so we could get rid of it  Entering #s: JTextBox handles this  tho’ it will accept non-numbers….  Clicking buttons: JButtons “flash”  just do a tiny bit of colour change  nothing else happens!  we will need to do something about that!

Plan  Extend the class JFrame  BE the window! »have a main method that just opens the window  Have constructor set everything up  add all the labels/fields/buttons and set them up  make sure someone is listening to the buttons »has to be someone who knows how! »sounds like an interface….

New Window Classes  Create a class that extends JFrame  class is another window class public class AdderWindow extends JFrame { … }  Use constructor to add labels, buttons, etc.  laid out in a 5x2 grid this.setLayout(new GridLayout(5, 2));  three fields, two buttons, and four labels

Instance Variables  Window needs to track its pieces  except for the labels – they do nothing private JTextBox firstNumberBox; private JTextBox secondNumberBox; private JTextBox resultBox; private JButton calculateButton; private JButton doneButton;

Window Constructor Tasks  Constructor:  calls parent constructor: super(“Adding Numbers”);  sets the size and layout: setSize(…); setLayout(…);  creates and adds the required elements firstNumberBox = new JTextField(); … this.add(new JLabel(“First number:”)); this.add(firstNumberBox);…  (also need to “activate” the window–later )

Program to Open the Window  Create a variable for the AdderWindow AdderWindow d;  Create and save the AdderWindow object d = new AdderWindow();  Make it visible d.setVisible(true);  Or we can just create it and make it visible: (new AdderWindow()).setVisible(true);

Our Window (Inactive)  Here’s our window  not quite the same as what I showed you  Doesn’t do anything

Getting Some Action  Our buttons do nothing »well, they change colour a bit when they’re clicked  want them to do something  Button needs to tell someone it was clicked  that someone needs to listen for the click  it needs to know how to listen for a click  The ActionListener interface  “I can listen for events (including clicks)”

The ActionListener Interface  Knows how to listen for an event  event sent using actionPerformed method  Create a new class (“AL”) that implements the ActionListener interface public class AL implements ActionListener { public void actionPerformed(ActionEvent e) { … } public void actionPerformed(ActionEvent e) { … }}  objects of type AL can listen for clicks »and should know what to do when they get them

Adding ActionListeners  Need to tell the button who’s listening  then button can send messages to listeners »recall: “sending a message” = “calling a method”  the addActionListener method ActionListener al = new AL(); calculateButton.addActionListener(al);  click calculateButton  al’s actionPerformed method gets called

Problem  Needs to know how to respond to the click  needs to get/put numbers from/in text fields  needs to have access to those fields  those fields are private to our window class  So?  can add getters/setters in AdderWindow…  or we can make the window into the listener! »no need to make a separate class AL!

Listening to Own Components  Have window implement ActionListener public class AdderWindow extends JFrame implements ActionListener { … } »important: extends first, implements after  Add code to constructor doneButton.addActionListener(this); calculateButton.addActionListener(this); »says “this window is listening to you” »remember: “this” is Java for “me” / “I” / “my”  window must now implement actionPerformed

The actionPerformed Method  Argument has information about the event  including its “source”: which button it was »choose action based on button clicked public void actionPerformed(ActionEvent e) { Object clicked = e.getSource(); Object clicked = e.getSource(); if (clicked == doneButton) { if (clicked == doneButton) { System.exit(0);// end program System.exit(0);// end program } else if (clicked == calculateButton) { } else if (clicked == calculateButton) { addTheNumbers();// private method addTheNumbers();// private method }}

Adding the Numbers  Private method to add and report result  use getText and Integer.parseInt to read #s  use setText and Integer.toString to show result private void addTheNumbers() { int n1, n2; int n1, n2; n1 = Integer.parseInt(firstNumberBox.getText()); n1 = Integer.parseInt(firstNumberBox.getText()); n2 = Integer.parseInt(secondNumberBox.getText()); n2 = Integer.parseInt(secondNumberBox.getText()); resultBox.setText(Integer.toString(n1 + n2)); resultBox.setText(Integer.toString(n1 + n2));}

When Things Go Wrong  What could go wrong in AdderWindow?  user might put non-numbers into the boxes  what happens then? »“Calculate” button seems to do nothing »(but there are error messages in output window)  what should happen then? »maybe set result field to “Error”?  how can we get that to happen? »let’s look at that error message….

The Error Messages  Look at the error messages:  “java.lang.NumberFormatException” »there was a problem with the format of the numbers  at … AdderDialog.java:81 »line 81 of your program java.lang.NumberFormatException: For input string: "ten" at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65) at java.lang.Long.parseLong(Long.java:589) at java.lang.Long.parseLong(Long.java:631) at AdderDialog.addTheNumbers(AdderDialog.java:81) at AdderDialog.actionPerformed(AdderDialog.java:73) …

Exceptions  Objects that represent problems in program  problems that the program itself might be able to do something about  Throw and Catch  method with problem throws the exception »Integer.parseInt threw a NumberFormatException  method that can deal with it will catch it »addTheNumbers can deal with it… »…so it needs to be ready to catch it

Getting Ready to Catch  Tell Java you can deal with the exception  a “catch block” (catch has a parameter) catch (WhatKindOfException e) { … } »body of catch says what to do for that exception  But need to warn Java it might happen  a “try block” (comes before the catch block) try { … } »must include the command the exception was thrown from

Dealing with the Problem  Try to add the numbers; catch the exception private void addTheNumbers() { int n1, n2; int n1, n2; try { try { n1 = Integer.parseInt(firstNumberBox.getText()); n1 = Integer.parseInt(firstNumberBox.getText()); n2 = Integer.parseInt(secondNumberBox.getText()); n2 = Integer.parseInt(secondNumberBox.getText()); resultBox.setText(Integer.toString(n1 + n2)); resultBox.setText(Integer.toString(n1 + n2)); } catch (NumberFormatException nfe) { } catch (NumberFormatException nfe) { resultBox.setText(“ERROR”); resultBox.setText(“ERROR”); }} »no exception: numbers get added up »exception: result gets set to “ERROR”

Try-Catch Control  Try block contains the code we want to do  but might throw an exception we can deal with  Catch block has code to deal with problem  says what kind of problem it deals with »e.g. NumberFormatException  If try block works, catch block gets skipped  If exception happens, jump to catch block  remainder of try block gets skipped

If There’s No Problem  Both numbers are OK try { try { n1 = Integer.parseInt(firstNumberBox.getText()); n1 = Integer.parseInt(firstNumberBox.getText()); n2 = Integer.parseInt(secondNumberBox.getText()); n2 = Integer.parseInt(secondNumberBox.getText()); resultBox.setText(Integer.toString(n1 + n2)); resultBox.setText(Integer.toString(n1 + n2)); } catch (NumberFormatException nfe) {// skipped } catch (NumberFormatException nfe) {// skipped resultBox.setText(“ERROR”); resultBox.setText(“ERROR”); } ? n1 ? n2 100 n1 First number: Second number: Result: 200 n2

But If There Is a Problem  Suppose second number is bad try { try { n1 = Integer.parseInt(firstNumberBox.getText()); n1 = Integer.parseInt(firstNumberBox.getText()); n2 = Integer.parseInt(secondNumberBox.getText()); n2 = Integer.parseInt(secondNumberBox.getText()); resultBox.setText(Integer.toString(n1 + n2));// skipped! resultBox.setText(Integer.toString(n1 + n2));// skipped! } catch (NumberFormatException nfe) { } catch (NumberFormatException nfe) { resultBox.setText(“ERROR”); resultBox.setText(“ERROR”); } 100 2oo ERROR ? n1 ? n2 100 n1 First number: Second number: Result:

Input Exceptions  Exceptions may happen in reading as well  user enters word where number expected »InputMismatchException  file runs out of data early »NoSuchElementException  Can deal with these as well  but don’t need to

Input Exceptions  Read numbers from file »may be words in file; may be too few numbers for (int i = 0; i < N; ++i) { try { try { arr[i] = in.nextInt(); arr[i] = in.nextInt(); } catch (InputMismatchException ime) { } catch (InputMismatchException ime) { arr[i] = -1;// indicates bad input arr[i] = -1;// indicates bad input in.next();// skip bad input in.next();// skip bad input } catch (NoSuchElementException nse) { } catch (NoSuchElementException nse) { break;// no sense trying to read any more! break;// no sense trying to read any more! }}

Multiple Catch Blocks  Can have as many catch blocks as you like  each for a different kind of exception  Catch order may be important  some exception classes inherit from others »InputMismatchExc n is a NoSuchElementExc n »if you ask for an int, and the next thing is a word, then there’s no such thing as what you asked for  must put more specific exceptions first »IME must come before NSEE

Never Catch Exception  Exception is most general exception class  if have catch(Exception e), it must come last  it’ll catch any exception  But DON’T CATCH IT!  what could you possibly do that would deal with every possible kind of problem?

The throws Clause  If your code may throw a checked exception and your method doesn’t deal with it, then you need to tell Java you won’t deal with it »that’s what being “checked” means  add throws clause to method header private static Scanner open(String name) throws FileNotFoundException { return new Scanner(new File(name)); return new Scanner(new File(name));}

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