News! Feedback after Lecture 4! About Java and OOP!

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1 this Example: SignalGUI News Orphans Recursive References D0010E Object-Oriented Programming and Design Lecture 5 GUI Design Patterns Applets Model-View- Control Observer Hints for Lab 2 Use a Vector to store rooms that have been placed in Assignment 3. Assignment 4: Add code to the driver that creates a GUI that displays your level. Look-up the keyword this. Use it for the openings in rooms. Include dynamic variables in the room class that store the dimension and the color of the floor. Lecture 5 - Håkan Jonsson 1 Lecture 5 - Håkan Jonsson 2 Feedback after Lecture 4 Can a static method, like main, in one class be called from another class? Yes. Use ClassName.methodName( ) Is code declared public accessible to everyone, like over the Internet? No. Access is regulated with respect to code fragments [in the same project in Eclipse], not the programmers or the hardware. So, public means accessible from code everywhere in whatever package - in the same project. But in Internet-connected computers running Unix, each code fragment can be viewed as being stored at a unique location; URL + place in the file system. About Java and OOP All dynamic objects (like rooms in Lab 2) have state (data stored in dynamic variables) and behavior (effect caused by dynamic methods). Their behavior is triggered by someone calling their methods. The code in one method in an object (its behavior ) calls a method in another object (thereby causing it to behave). The result of a program is recorded in the states of all its objects. The final global result is a compilation of, and a computation using, data from local states. Usually just some subset of data is considered as the final result. Indeed, all data are the result of computations using data from states. A recursive explanation Lecture 5 - Håkan Jonsson 3 Lecture 5 - Håkan Jonsson 4

2 Keyword: this At run-time, the keyword this refers to the dynamic object that runs the code that contains the word this at the particular moment when this is encountered. NB Not the class. It is used like a reference variable but can not be assigned a new value. It makes it possible for dynamic objects to refer to themselves. It is useful to, for instance, send the dynamic object itself as an argument in a method call done in one of its methods, and to Compare with self in python differ between local and dynamic variables with the same name. public class Names { public Names(String first, String last) { this.first = first; this.last = last; } public String fullname() { return first + + last; } private String first, last; } (Parameters; local, automatic, variables) (Dynamic variables) Lecture 5 - Håkan Jonsson 5 Lecture 5 - Håkan Jonsson 6 Orphans An orphan is a dynamic object that no longer is possible to reach via a reference chain from main(). (In fact, not just main() but from any running thread.) Problem: Orphans still occupy memory space that can not be used for other purposes. Memory leakage. In some programming languages the memory occupied by orphans must be reclaimed by code in the program. All the responsibility on the programmer. Failure to reclaim leads to memory leakage and bugs that are hard (and time-consuming) to find. Out of memory. Orphans In Java, orphans are reclaimed automatically, when needed. Garbage collection. Advantage: Memory leakage can not occur However, memory is still finite so don t waste it Drawback: Garbage collection takes time and can kick in when not expected. It is possible to control the garbage collection process, but this is outside the scope of this course (there s no need). However, garbage collection works sufficiently good in applications that are not very time-critical. Lecture 5 - Håkan Jonsson 7 Lecture 5 - Håkan Jonsson 8

3 Recursive References public class Stuff { public Stuff morestuff; public int thing; public Stuff (int thing) { morestuff = null; this.thing = thing; } } (The following is executed in a static object with access to Stuff.) Stuff stuff = new Stuff(10); stuff.morestuff = new Stuff(2); stuff.morestuff.thing = 4; public class Stuff { public Stuff morestuff; public int thing; public Stuff (int thing) { morestuff = null; this.thing = thing; } } stuff.morestuff.morestuff = new Stuff(7); stuff.morestuff.morestuff = stuff; Lecture 5 - Håkan Jonsson 9 Lecture 5 - Håkan Jonsson 10 Stuff stuff = new Stuff(10); stuff.morestuff = new Stuff(2); morestuff == null thing == 10 morestuff thing == 10 A second object is created and referred to from the first object. stuff An object is created. Stuff stuff = new Stuff(10); stuff.morestuff = new Stuff(2); stuff.morestuff.thing = 4; stuff.morestuff.morestuff = new Stuff(7); stuff.morestuff.morestuff = stuff; Lecture 5 - Håkan Jonsson 11 stuff morestuff == null thing == 2 Stuff stuff = new Stuff(10); stuff.morestuff = new Stuff(2); stuff.morestuff.thing = 4; stuff.morestuff.morestuff = new Stuff(7); stuff.morestuff.morestuff = stuff; Lecture 5 - Håkan Jonsson 12

4 stuff.morestuff.thing = 4; stuff.morestuff.morestuff = new Stuff(7); morestuff thing == 10 morestuff thing == 10 morestuff == null thing == 7 A third object is created and referred to from the second object. morestuff == null thing == 4 thing in the second object used to be 2 but is now assigned 4. morestuff thing == 4 stuff Stuff stuff = new Stuff(10); stuff.morestuff = new Stuff(2); stuff.morestuff.thing = 4; stuff.morestuff.morestuff = new Stuff(7); stuff.morestuff.morestuff = stuff; Lecture 5 - Håkan Jonsson 13 stuff Stuff stuff = new Stuff(10); stuff.morestuff = new Stuff(2); stuff.morestuff.thing = 4; stuff.morestuff.morestuff = new Stuff(7); stuff.morestuff.morestuff = stuff; Lecture 5 - Håkan Jonsson 14 stuff.morestuff.morestuff = stuff; morestuff thing == 10 stuff morestuff thing == 4 morestuff thing == 7 (An orphan) morestuff in the second object used to refer to the third object, but is now changed so it refers to the first object. This assignment orphans the third object. Stuff stuff = new Stuff(10); stuff.morestuff = new Stuff(2); stuff.morestuff.thing = 4; stuff.morestuff.morestuff = new Stuff(7); stuff.morestuff.morestuff = stuff; Lecture 5 - Håkan Jonsson 15 GUI:s in Java Everything on a computer screen has to be deliberately drawn. Text, geometric shapes, pictures, borders, scroll bars etc. Java provides means to make such drawings as Graphical User Interfaces (GUI:s) that consists of GUI components. With strange names like JPanel, JFrame and the like. Moreover, GUI:s in Java are event-driven. They react on events like mouse-clicks, keys pressed etc. When events occur, special methods are automatically called by the operating system. This might seem like magic but, no, that s not the case These methods are available for us to re-program, thereby making the program react in a suitable manner. We will soon look at two (2) examples to illustrate this. Lecture 5 - Håkan Jonsson 16

5 Again: Implementing" the Model-View-Control in Java 1) The model: Is observable. Automatically gets the methods setchange notifyobservers addobserver Must call " setchange notifyobservers in this order whenever it has changed. 2) The view: Is an observer. Must implement a method update The method update is called whenever the model calls setchange and notifyobservers. Should update the view. Must add itself as an observer of the model by calling" <model>.addobserver Again: Implementing" the Model-View in Java 3) The controller: Usually a listener for events. KeyListener, MouseListener, etc Automatically gets methods that are called when an event happens. A key is pressed, the mouse is moved, a mouse button is pressed etc. Different methods depending on the type of listener. Lecture 5 - Håkan Jonsson 17 Lecture 5 - Håkan Jonsson 18 Example: Counter This example shows a simple counter that can be changed by typing on the keyboard. The design is done using the design pattern Model-View-Control. The code is therefore split into three separate classes depending on function: Model View Control (In fact, two variants of controllers.) 5. I vyns update-metod ändrar vyn på sig för att överensstämma med det nya modelltillståndet. "Model" CounterView CounterModel "Control" CounterController "View" 4. Anropen i incdata resulterar i att update-metoden i (alla) observerare anropas. 3. incdata ökar räknaren (modellen ändrar sitt tillstånd) och anropar setchange samt notifyobservers. 2. I keypressed anropas incdata i modellen. (6. Baserat på den nya vyn kanske användaren trycker på ännu en knapp, och loopen sluts.) 1. På tangentbordet trycks "+" ned, vilket ger ett anrop till keypressed. Lecture 5 - Håkan Jonsson 19 Lecture 5 - Håkan Jonsson 20

6 2. Signals from the last lecture Our GUI (the view) is separate from the signal (the model). The model is represented by the class Signal. Holds the current state (on or off). The view is represented by the class SignalGUI. Keeps a drawing of the model up-to-date and reacts to user input (events). The controller has been included into the view. The model and the view/controller are both members of the package signal. This close relationship is beneficial since the view needs to be able to change the model (via methods the model provides) whenever the user requests this and access to the state of the model to be able draw a picture of the model after it has been change. SignalGUI Example of a GUI programmed in Java. 4 classes: Signal SignalGUI Driver A main program Separates the model from the view. The model is a signal. The view is a 2D picture with a colored rectangle in a pop-up window. Apart from model and view there is a third part called control; here it is bundled with the view. Lecture 5 - Håkan Jonsson 21 Lecture 5 - Håkan Jonsson 22 A GUI for Signals Lecture 5 - Håkan Jonsson 23 Lecture 5 - Håkan Jonsson 24

7 Again: GUITest package f5.ex1; public class GUITest { public static void main(string[] args) { Driver driver = new Driver(); driver.run(); } } Lecture 5 - Håkan Jonsson 25 Driver - NEW package f5.ex2; import f5.ex2.signal.signal; import f5.ex2.signal.signalgui; import f5.ex2.signal.signaltui; public class Driver { public void run() { // a signal s1 Signal s1 = new Signal(); // 1a. a graphical GUI for s1 SignalGUI sgs1a = new SignalGUI(s1, 100, 100); // 1b. a second graphical GUI for s1 // SignalGUI sgs1b = new SignalGUI(s1, 100, 100); // 2. a textual UI for s1 // SignalTUI stui = new SignalTUI(s1, 400, 100); // 3. another signal s2 // Signal s2 = new Signal(); // 4. a graphical GUI for s2 // SignalGUI sgs2 = new SignalGUI(s2, 100, 100) } } Lecture 5 - Håkan Jonsson 26 Again: Signal package f5.ex1.signal; import java.util.observable; // public class Signal extends Observable { public Signal() {} boolean on = false; boolean ison() { return on; } void seton() { on = true; setchanged(); // notifyobservers(); // } void setoff() { on = false; setchanged(); // notifyobservers(); // } package f5.ex1.signal; import java.awt.color; import java.awt.dimension; import java.awt.graphics; import java.awt.event.keyevent; import java.awt.event.keylistener; import java.util.observable; import java.util.observer; import javax.swing.jframe; import javax.swing.jpanel; public class SignalGUI implements Observer { private Signal s; private Display d; public SignalGUI(Signal s, int x, int y) { this.s = s; JFrame frame = new JFrame("A Signal"); frame.setdefaultcloseoperation(jframe.exit_on_close); d = new Display(); frame.getcontentpane().add(d); frame.pack(); frame.setlocation(x, y); frame.setvisible(true); s.addobserver(this); Again: SignalGUI:1 } } Lecture 5 - Håkan Jonsson 27 Lecture 5 - Håkan Jonsson 28

8 public void update(observable obs, Object arg1) { d.repaint(); } private class Display extends JPanel { public Display() { addkeylistener(new Listener()); setbackground(color.white); setpreferredsize(new Dimension(200, 200)); setfocusable(true); } public void paintcomponent(graphics g) { super.paintcomponent(g); if (s.ison()) { g.setcolor(color.green); } else { g.setcolor(color.red); } g.fillrect(50, 50, 100, 100); g.setcolor(color.black); g.drawrect(50, 50, 100, 100); } Again: SignalGUI:2 } } private class Listener implements KeyListener { public void keypressed(keyevent arg0) {} } Again: SignalGUI:3 public void keyreleased(keyevent arg0) {} public void keytyped(keyevent event) { switch (event.getkeychar()) { case '1': s.seton(); break; case '2': s.setoff(); break; default: break; } } Lecture 5 - Håkan Jonsson 29 Lecture 5 - Håkan Jonsson 30 package f5.ex2.signal; import java.awt.textarea; import java.awt.event.keyevent; import java.awt.event.keylistener; import java.util.observable; import java.util.observer; import javax.swing.jframe; import javax.swing.jscrollpane; public class SignalTUI implements Observer { public SignalTUI(Signal s, int x, int y) { this.s = s; s.addobserver(this); JFrame frame = new JFrame("Text UI for a Signal"); text = new TextArea(20,40); text.seteditable(false); text.addkeylistener(new Listener()); JScrollPane scrollp = new JScrollPane(text); frame.getcontentpane().add(scrollp); printstatus(); frame.pack(); frame.setlocation(x,y); frame.setvisible(true); } SignalTUI:1- NEW Lecture 5 - Håkan Jonsson 31 } public void update(observable obs, Object arg1) { printstatus(); } void printstatus() { if (s.ison()) { text.append("on\n"); } else { text.append("off\n"); } } private Signal s; private TextArea text; /* instead of a Display */ private class Listener implements KeyListener { public void keypressed(keyevent arg0) {} public void keyreleased(keyevent arg0) {} } public void keytyped(keyevent event) { switch (event.getkeychar()) { case '1' : s.seton(); break; case '2' : s.setoff(); break; default : break; } } SignalTUI:2 Lecture 5 - Håkan Jonsson 32

9 Applets Coding an Applet An applet is a kind of Java program that can be included on a web page. The code of the applet is fetched from the web server by the browser along with HTML-code that describes the web page. The applet is then started in the browser and its graphical output is shown in a dedicated rectangular region on the page. It is possible to program applets to interact with users but applets have very restricted access to the local host for security reasons. An applet is essentially a computer program that you download and run without inspecting the code You program an applet pretty much as a regular program with a GUI. However, since an applet runs as part of another program (the browser) no main-method is needed. Let s take a look at an example: AppletExample.java Here is also an example of a mouse listener. Lecture 5 - Håkan Jonsson 33 Lecture 5 - Håkan Jonsson 34 package f5.ex1; import java.awt.*; import java.awt.event.*; AppletExample:1 import javax.swing.*; public class AppletExample extends JApplet { public void init() { MyDisplay d = new MyDisplay(); d.addmouselistener(new MyListener(d)); getcontentpane().add(d); } class MyDisplay extends JPanel { public MyDisplay() { setbackground(color.black); setforeground(color.yellow); setfont(new Font("SansSerif", Font.BOLD, 14)); } } public void paintcomponent(graphics g) { Color c; super.paintcomponent(g); g.setcolor(color.yellow); if (pressed) { g.drawstring(" The mouse button IS being pressed", 10, 20); g.setcolor(color.green); g.fillrect(150,50,20,20); } else { g.drawstring("the mouse button is NOT being pressed", 10, 20); g.setcolor(color.red); g.filloval(150,50,20,20); } } void userpressed() { pressed = true; } void userreleased() { pressed = false; } private boolean pressed = false; AppletExample:2 Lecture 5 - Håkan Jonsson 35 Lecture 5 - Håkan Jonsson 36

10 } class MyListener implements MouseListener { private MyDisplay mydisplay; } public MyListener(MyDisplay md) { mydisplay = md; } public void mousepressed(mouseevent arg0) { mydisplay.userpressed(); mydisplay.repaint(); } public void mousereleased(mouseevent arg0) { mydisplay.userreleased(); mydisplay.repaint(); } AppletExample:3 public void mouseentered(mouseevent arg0) { /* not in use */ } public void mouseexited(mouseevent arg0) { /* not in use */ } public void mouseclicked(mouseevent arg0) { /* not in use */ } Lecture 5 - Håkan Jonsson 37