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Week 3: Introduction

Week 3: Introduction. Last meeting we discussed Advantages of using subprograms How subprograms work How you write them How you call them What happens when they are called Using parameters with subprograms Allow "communication" with the subprogram Recursive subprograms.

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Week 3: Introduction

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  1. Week 3: Introduction • Last meeting we discussed • Advantages of using subprograms • How subprograms work • How you write them • How you call them • What happens when they are called • Using parameters with subprograms • Allow "communication" with the subprogram • Recursive subprograms University of Pittsburgh Computer Science

  2. Week 3: Introduction • Ex. Towers of Hanoi • Procedural abstraction • Can use a subprogram without having to know details of its implementation • Data abstraction • Can use a new data type without having to know details of its implementation • Object-oriented programming • Encapsulation of data and operations • Inheritance • Polymorphism University of Pittsburgh Computer Science

  3. Week 3: Adding a Graphical Interface • In the past two weeks we have seen how to make things easier for the programmer • But the USER of the program is the main concern • Early programs interacted with the user through the command line and the console • User runs program by typing some command • Input is typed from the keyboard onto console • Output is shown on console • Not very user-friendly! University of Pittsburgh Computer Science

  4. Week 3: Adding a Graphical Interface • As more "regular" people started using computers, the user-interface had to be improved • People at Xerox and Apple began developing ways for a "mouse" to be used with a program • Graphics abilities on computers were improved, and could be used in conjunction with the "mouse" • Now most production software has a GUI (Graphical User Interface) to go with it • Let's see a little bit of how this is done University of Pittsburgh Computer Science

  5. Week 3: Graphical Components • Graphical Components • Objects in a program that work primarily in a graphical way • Windows • Used to contain and manipulate the other graphical components, and for drawing • Buttons • User "pushes" button by clicking the mouse • Button responds by executing a subprogram • Dialog Boxes • Some graphical information is shown to the user • User types in a response • User's response is processed University of Pittsburgh Computer Science

  6. Week 3: Java Buttons • Let's look in more detail at a Button • What do we need in order to use one? • Must be in some window • Must be created and initialized • A subprogram to handle the "click" must be implemented and associated with the button • Download SimpleButton.java • Compile and run it • Let's look at the code University of Pittsburgh Computer Science

  7. Week 3: Java Buttons • Remember what we needed • Must be in some window • In Java a JFrame is a window • Note that the main class extends JFrame • The button will be inside this window • Must be created and initialized • Button variable is declared in the data area • Button object is created in the constructor • A subprogram to handle the "click" must be implemented and associated with the button • Let's talk more about this one University of Pittsburgh Computer Science

  8. Week 3: Event-Driven Programming • Older computer programs were the initiators of activities • Program prompts, user responds • Event-driven programs have the opposite approach • Program waits for user to initiate an activity, then responds to it • If user does nothing, program just idles • Programmer must code ways that program will react to user's actions University of Pittsburgh Computer Science

  9. Week 3: Event-Driven Programming • In Java, event-driven programming is handled in three parts: • Events: • These are generated due to the user's actions • ActionEvent: some action has occurred • MouseEvent: mouse has done something • WindowEvent: something has happened to window • Event generators: • The components/activities that cause the events to be generated • Button has been clicked • Mouse has been moved • Window has been closed University of Pittsburgh Computer Science

  10. Week 3: Event-Driven Programming • Event handlers (listeners) • Subprograms that execute in response to an event • Programmer indicates here what program should do when event occurs • Must be linked to the event-generator University of Pittsburgh Computer Science

  11. Week 3: Event-Driven Programming Button is clicked in window, generating an ActionEvent ActionEvent is sent as a parameter to the handling subprogram to process ActionEvent actionPerformed(ActionEvent e) { code to respond to event } Handling subprogram executes, responding to the event that occurred University of Pittsburgh Computer Science

  12. Week 3: Event-Driven Programming • Look again at SimpleButton.java • Event: ActionEvent • Event generator: button click • Event listener: ButtonHandler (ActionListener) • Note where it is linked to the button • Note what is done to handle click University of Pittsburgh Computer Science

  13. Week 3: Java Buttons • Let's add another button to our program • Let's have this button change the title of our window to whatever we want it to be • What do we need to do to do this? • Create another button variable and object • Call it button2 • Add the variable right after button1 (same line) • Create the object below button1: button2 = new JButton("Change Title"); • Link our new button to a listener (handler) • Let's use the same one as for the first button button2.addActionListener(bhandler); University of Pittsburgh Computer Science

  14. Week 3: Java Buttons • Tell our window about the new button so it can handle its display • Let's make the buttons WEST and EAST • Change button1 from CENTER to WEST, then add button2: c.add(button2, BorderLayout.EAST); • Write the code to handle the button • We must test to see if button2 generated the event • If so, execute the code to handle it else if (e.getSource() == button2) { String newTitle = JOptionPane.showInputDialog( SimpleButton.this, "New Title?"); SimpleButton.this.setTitle(newTitle); } University of Pittsburgh Computer Science

  15. Week 3: Java Buttons • What else can a button do? • How about a toggle button • Click it once to "turn something on" • Click it again to "turn it off" • Similar to the power switch on your remote • Let's use it to turn on and off the button that we just made University of Pittsburgh Computer Science

  16. Week 3: Java Buttons • How can we make a toggle button? • We need to keep track of the button's current "state" • Is the button on or off? • We can do this with a boolean variable, titleOn • Remember that boolean variables have two values, true or false • Each time button is clicked, change to the opposite state and execute the appropriate action • If on, turn off; If off, turn on • We'll initialize it to on (true) • Add new button variable and create new object • For text, we'll put "Turn off Title Changer" initially • Link it to same listener as other two buttons • Put it in SOUTH of the window's layout University of Pittsburgh Computer Science

  17. Week 3: Java Buttons if (titleOn) { button2.setVisible(false); button3.setText("Turn Title On"); titleOn = false; } else { button2.setVisible(true); button3.setText("Turn Title Off"); titleOn = true; } • Let's look at and run ToggleButton.java • Add code to handler for new button • What do we do here? • If titleOn, turn it off and change button text • If not titleOn, turn it on and change button text University of Pittsburgh Computer Science

  18. Week 3: Other Java Components • Java has MANY GUI Components • Menu • Allows selection from list of options • Pull-down and pop-up menus can be created • CheckBox • Allows something to be selected or not • Label • Allows text to be displayed in different fonts and colors • TextArea • Allows user to input text and can detect changes University of Pittsburgh Computer Science

  19. Week 3: Mouse Events • Key to all GUIs is the mouse • What are some useful things to know about the mouse? • Where is it? • X and Y coordinates • Is a mouse button pressed? • If so is it over anything that requires an action? • Is it moving/has it moved? • Is it "dragging" anything along with it? University of Pittsburgh Computer Science

  20. Week 3: Mouse Events • Java events involving the mouse are MouseEvents • Two different listeners for ease of handling • MouseListener handles/implements • mousePressed • mouseReleased • mouseClicked • mouseEntered • mouseExited • MouseMotionListener handles/implements • mouseMoved • mouseDragged University of Pittsburgh Computer Science

  21. Week 3: Mouse Events • Let's look at an example • Download, compile and run Mousey.java • What does the code do? • Getting a bit more complicated • A separate Panel class is created for the graphics • It is in this class that most of the work is done • We have theShape to store the rectangle • We have theColors to store the different background colors • We add a MouseListener and a MouseMotionListener University of Pittsburgh Computer Science

  22. Week 3: Mouse Events • Look at the subprograms • mousePressed • Indicate the mouse has been pressed • Get the current location of the mouse • If it's within the shape, select the shape • If not, change the background color (and unselect the shape, if it had been selected) • mouseReleased • Indicate the mouse has been released University of Pittsburgh Computer Science

  23. Week 3: Mouse Events • mouseMoved • Show current location of the mouse • Note that this event occurs each time mouse is moved even the slightest bit • Thus it occurs hundreds/thousands of times as you are using a GUI program • However in many applications it is ignored • mouseDragged • Mouse is move WHILE button is held down • Note when this occurs mouseMoved does not • If the shape has been selected • Get the new location of the mouse • Reposition the shape to the new location University of Pittsburgh Computer Science

  24. Week 3: Simple Java Graphics • What is going on with the shape? • It is a Rectangle2D, predefined in Java • This shape can be drawn graphically in various ways (ex. filled or just outline) • To "move" it we really just change it's "frame" with a new upper left corner • There are some interesting things we can do • See if it contains a point • See if it intersects another shape • Can be useful for games, other graphical applications • There are other shapes that we can use as well University of Pittsburgh Computer Science

  25. Week 3: Summary • This week we discussed • Graphical interfaces • Simple graphical components in Java • Implementing Java buttons • Events and event-driven programming • Using the mouse and its related events • Simple graphics in Java University of Pittsburgh Computer Science

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