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Method Overloading Review

Method Overloading Review. Method Overloading (1/4 ). Can define multiple methods of same name within a class, as long as method signatures are different Example: java.lang.Math static method max takes in two numbers and returns the greater of the two

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Method Overloading Review

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  1. Method Overloading Review

  2. Method Overloading (1/4) • Can define multiple methods of same name within a class, as long as method signatures are different • Example: java.lang.Math • static method max takes in two numbers and returns the greater of the two • There are actually three max methods-- one for ints, one for floats, one for doubles // this is an approximation of what Math’s // three max methods look like public class Math { // other code elided public static int max(int a, int b) { // return max of two ints } public static float max(float a, float b) { // return max of two floats } public static double max(double a, double b){ // return max of two doubles } }

  3. Method Overloading (2/4) • Methods in same class with same name but different parameter lists • When callingan overloaded method, Java infers which method you mean based on parmeter list • Thus cannot have two methods with identical signatures but different return types—compiler error becauseJava can’t determine which to call! // this is an approximation of what Math’s // three max methods look like public class Math { // other code elided public static int max(int a, int b) { // return max of two ints } public static float max(float a, float b) { // return max of two floats } public static double max(double a, double b){ // return max of two doubles } }

  4. Method Overloading (3/4) • Be careful not to confuse overloading and overriding! • override an inherited method in a subclass: the signatures (name; number, types, and order of parameters) must be the same • overload methods in the same class: names are same, but signatures must be different // this is an approximation of what Math’s // three max methods look like public class Math { // other code elided public static int max(int a, int b) { // return max of two ints } public static float max(float a, float b) { // return max of two floats } public static double max(double a, double b){ // return max of two doubles } }

  5. Method Overloading (4/4) • If neither overriding nor overloading: Can use same name, indeed signatures for methods in different classes • Java can differentiate by type of instance on which method is called during method resolution • for example: if classes Car and Dog both have a move method that takes in one parameter of type int, Java knows to use the Dog’s move when called on an instance of Dog: • Dog dog = new Dog(); • Car car = new Car(); • dog.move(1); //Dog’s move method is called. • car.move(1); //Car’s move method is called.

  6. Method Overloading: Constructors • Even constructors can be overloaded! Wardrobe class has multiple constructors • A String (java.lang.String) is a sequence of alphanumeric characters, including space! • Example: String s = “CS15 Rocks!”; • Can use System.out.println to print any string you want: System.out.println(s); public class Wardrobe { private String _top; private String _bottom; public Wardrobe(String top, String bottom) { _top = top; _bottom = bottom; } public Wardrobe(String top) { _top = top; _bottom = “Jeans”; } public Wardrobe() { _top = “T-Shirt”; _bottom = “Jeans”; } }

  7. Method Overloading: Example • An overloaded method can call other overloaded methods • public class Plastic { • public Plastic(BurnBookmyBurnBook) { • DirtySecret secret = myBurnBook.getDirtySecret(); • this.backStab(secret); • } • public void backStab(DirtySecret secret) { • String victim = secret.getVictim(); //find out whose secret it is • this.backStab(victim, secret); • } • public void backStab(String victim, DirtySecret secret) { • //code to backstab elided (we don’t teach you how to backstab!) • } • //other methods elided • }

  8. Lecture 9 Intro to Swing 0 of 88

  9. What is Swing? • Swing is an API (Application Programming interface) • part of core Java library • used to create applications with 2D graphics • Frequently used classes in javax.swing package • javax.swing.JFrame– the main window used by your app • javax.swing.JPanel-- a canvas that contains graphical components • javax.swing.JButton-- a clickable button JFrame JPanel JButtons 1 of 88

  10. Creating Applications from Scratch • Until now, TAs took care of graphical components for you • Support code created JFrame, JPanels, all graphical components • From now on, you’re in charge of this! 2 of 88

  11. Graphical User Interfaces (GUIs) • GUIs provide a user-controlled (i.e., graphical) way to send messages to a system of objects • You’ll use Swing to create your own GUIs throughout the semester 3 of 88

  12. Pixels and Coordinate System X (0, 0) • The screen is a grid of pixels (tiny dots) • Integer Cartesian plane with: • origin in upper-left corner • y-axis increasing downward • corresponds to Latin-based languages’ writing order Y 4 of 88 pixels

  13. Pixels and Coordinate System X (0, 0) • When working with Swing, you’ll be able to set the size and location of different components (panels, buttons, etc.) in pixels • The “location” of each component will be the location of its top left corner Y 5 of 88 pixels

  14. Creating GUIs with Swing • Pattern: instantiate the Swing components you need and add them to graphical containers • Start with JFrame JFrame JPanel JLabel Hello, World! • Add JPanelto JFrame JButtons • Add JButtons, JLabels, etc. to JPanel 6 of 88

  15. Creating GUIs with Swing • Swing uses a default layout to arrange components in a JFrameor JPanel • We’ll learn how to customize layout later in lecture • For now, let Swing do it for us JFrame JPanel JLabel Hello, World! JButtons 7 of 88

  16. Our First Swing Application JFrame • Spec: app that contains text reading “CS15 Rocks!” and button that randomly changes the text’s color with every click • Useful classes: JFrame, JPanel, JLabel, JButton, and ActionListener JLabel JPanel JButton 8 of 88

  17. DEMO: ColorTextApp JFrame JLabel JPanel JButton 9 of 88

  18. Process JFrame • Create a top-level App class that contains an instance of JFrame • Create a subclass of JPanelthat contains an instance of JButtonand an instance of JLabel • Add an instance of your JPanelsubclass to the JFrame • Set up an ActionListenerthat changes the label’s color each time the button is clicked JLabel JPanel JButton 10 of 88

  19. Top-level Class: ColorTextApp • This is our top-level class • It contains the JFrame(window in which the app will display) as a component public class ColorTextApp { • public ColorTextApp() { JFrame frame = new JFrame(); frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE); frame.setVisible(true); } public static void main(String[] args) { new ColorTextApp(); } } 11 of 88

  20. Top-level Class: ColorTextApp • First we instantiate a JFrame, which we store in local variable frame • Note: these slides will omit import statements at top of file public class ColorTextApp { • public ColorTextApp() { JFrame frame = new JFrame(); frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE); frame.setVisible(true); } public static void main(String[] args) { new ColorTextApp(); } } 12 of 88

  21. Top-level Class: ColorTextApp • Next we call setDefaultCloseOperationon our JFrame • This method specifies what happens when user hits “x” in corner of the JFrame • Pass in constant JFrame.EXIT_ON_CLOSEas an argument so app will quit when we press the “x” • Swing magic keeps JFrames and their contents alive as long as the “x” is not pressed, so they will not be “garbage- collected!” (Java GC’s variables when they go out of scope – cease to exist) public class ColorTextApp { • public ColorTextApp() { JFrame frame = new JFrame(); frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE); frame.setVisible(true); } public static void main(String[] args) { new ColorTextApp(); } } 13 of 88

  22. Top-level Class: ColorTextApp • Finally, we call the method setVisibleon our JFrame • Pass in booleanvalue trueas an argument • This makes the JFrameshow up! • Pro tip: Always call this method last to avoid nasty layout bugs public class ColorTextApp { public ColorTextApp() { JFrame frame = new JFrame(); frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE); frame.setVisible(true); } public static void main(String[] args) { new ColorTextApp(); } } 14 of 88

  23. Process JFrame • Create a top-level App class that contains an instance of JFrame • Create a subclass of JPanel that contains an instance of JButton and an instance of JLabel • Add an instance of your JPanel subclass to the JFrame • Set up an ActionListener that changes the label’s color each time the button is clicked JLabel JPanel JButton 15 of 88

  24. JPanelSubclass: ColorTextPanel • The class ColorTextPanel“is a” JPanel: our canvas that may contain other Swing GUI components • We’ve specialized this class to contain and display a JLabeland a Jbutton (local vars) public class ColorTextPanel extends JPanel { public ColorTextPanel() { JLabel label = new JLabel("CS15 Rocks!"); JButton button = new JButton("Random Color"); Dimension panelSize = new Dimension(150, 75); this.setPreferredSize(panelSize); this.add(label); this.add(button); } } 16 of 88

  25. JPanelSubclass: ColorTextPanel First, we instantiate a JLabel and JButton In each case, we pass in a String as an argument This String is the text we want the label/button to display Here we pass the string as a “literal”; could also pass a string variable public class ColorTextPanel extends JPanel { public ColorTextPanel() { JLabel label = new JLabel("CS15 Rocks!"); JButton button = new JButton("Random Color"); Dimension panelSize = new Dimension(150, 75); this.setPreferredSize(panelSize); this.add(label); this.add(button); } } 17 of 88

  26. JPanelSubclass: ColorTextPanel • Now we set the “preferred size” of our panel • First, instantiate a Dimension(object that represents a width and height, in pixels) • First argument is width, second is height • Then call method setPreferredSizeon panel, passing in the Dimensionas an argument public class ColorTextPanel extends JPanel { public ColorTextPanel() { JLabel label = new JLabel("CS15 Rocks!"); JButton button = new JButton("Random Color"); Dimension panelSize = new Dimension(150, 75); this.setPreferredSize(panelSize); this.add(label); this.add(button); } } 18 of 88

  27. The ColorTextPanelclass • Finally, we add the label and button to the panel by calling the panel’s addmethod • This is a necessary step for the label and button to show up! public class ColorTextPanel extends JPanel { public ColorTextPanel() { JLabel label = new JLabel("CS15 Rocks!"); JButton button = new JButton("Random Color"); Dimension panelSize = new Dimension(150, 75); this.setPreferredSize(panelSize); this.add(label); this.add(button); } } 19 of 88

  28. Process JFrame • Create a top-level App class that contains an instance of JFrame • Create a subclass of JPanel that contains an instance of JButton and an instance of JLabel • Add an instance of your JPanel subclass to the JFrame • Set up an ActionListener that changes the label’s color each time the button is clicked JLabel JPanel JButton 20 of 88

  29. The ColorTextAppclass public class ColorTextApp { public ColorTextApp() { JFrame frame = new JFrame(); frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE); JPanel panel = new ColorTextPanel(); frame.add(panel); frame.pack(); frame.setVisible(true); } public static void main(String[] args) { new ColorTextApp(); } } • First, instantiate a new ColorTextPanel • Then, add it to the JFrameby calling the addmethod • Finally, call the method packon the JFrame • “pack” tells frame to resize itself based on the preferred sizes of the components inside it 21 of 88

  30. Process JFrame • Create a top-level App class that contains an instance of JFrame • Create a subclass of JPanel that contains an instance of JButton and an instance of JLabel • Add an instance of your JPanel subclass to the JFrame • Set up an ActionListener that changes the label’s color each time the button is clicked JLabel JPanel JButton 22 of 88

  31. Responding to User Input • Need a way to respond to stimulus of button being clicked • We refer to this as event handling • A source generates an event (like a mouse click or a key press) and notifies all registered listeners • Each listener has a method for responding to the event • stimulus-response mechanism that allows multiple listeners to respond 23 of 88

  32. ActionListeners • Whenever a JButtonis clicked, it generates a java.awt.event.ActionEvent • We define class that listens for ActionEvents: must implement interface java.awt.event.ActionListener • ActionListenerinterface declares method: public void actionPerformed(ActionEvent e); • This method must be defined in our listener class • called by Java whenever an ActionEvent is fired • specifies response to event 24 of 88

  33. ActionListeners ActionListener ActionEvent e JButton Click! public void actionPerformed( ) { • // Respond here; may or may not use ActionEvent // (print something to the console, // tell a JLabel to change color, etc.) } ActionEvent e 25 of 88

  34. Our ActionListener: ColorListener • Our listener class is ColorListener • Its job is to listen for ActionEvents and respond to them by changing the color of a JLabel public class ColorListener implements ActionListener { • private JLabel _label; //associated label public ColorListener(JLabel label) { _label = label; } @Override public void actionPerformed(ActionEvent e) { int red = (int) (Math.random()*256); int green = (int) (Math.random()*256); int blue = (int) (Math.random()*256); Color random = new Color(red, green, blue); _label.setForeground(random); } } 26 of 88

  35. Our ActionListener: ColorListener • Associated with a JLabel, which it refers to as _label-- a ColorListenerwill be passed the JLabelwhose color it should change • When ActionEventis detected, will change _label’s color public class ColorListener implements ActionListener { private JLabel _label; public ColorListener(JLabel label) { _label = label; } @Override public void actionPerformed(ActionEvent e) { int red = (int) (Math.random()*256); int green = (int) (Math.random()*256); int blue = (int) (Math.random()*256); Color random = new Color(red, green, blue); _label.setForeground(random); } } 27 of 88

  36. Our ActionListener: ColorListener • actionPerformeddefines response to ActionEvent • In this case, we want it to generate a random color, and then set _labelto that color public class ColorListener implements ActionListener { private JLabel _label; public ColorListener(JLabel label) { _label = label; } @Override public void actionPerformed(ActionEvent e) { int red = (int) (Math.random()*256); int green = (int) (Math.random()*256); int blue = (int) (Math.random()*256); Color random = new Color(red, green, blue); _label.setForeground(random); } } 28 of 88

  37. Our ActionListener: ColorListener • Use Math.random()to generate three random ints 0-255 • Use them as RGB values for a new Color • Call JLabelmethod setForegroundon _labelto set its color to the random color we’ve created public class ColorListener implements ActionListener { private JLabel _label; public ColorListener(JLabel label) { _label = label; } @Override public void actionPerformed(ActionEvent e) { int red = (int) (Math.random()*256); int green = (int) (Math.random()*256); int blue = (int) (Math.random()*256); Color random = new Color(red, green, blue); _label.setForeground(random); } } 29 of 88

  38. Adding ActionListeners public class ColorTextPanel extends JPanel { public ColorTextPanel() { JLabel label = new JLabel("CS15 Rocks!"); JButton button = new JButton("Random Color"); ColorListener listener = new ColorListener(label); button.addActionListener(listener); Dimension panelSize = new Dimension(150, 75); this.setPreferredSize(panelSize); this.add(label); this.add(button); } } • First, instantiate a ColorListener • Pass in label as argument to listener’s constructor so it knows to change label’s color • Next, use method addActionListener to add our ColorListener to button • Typical pattern: listeners are added to the event-emitting component they listen to • Whenever button emits an ActionEvent, listener will hear and respond to it 30 of 88

  39. Process JFrame • Create a top-level App class that contains an instance of JFrame • Create a subclass of JFrame that contains an instance of JButton and an instance of JLabel • Add a JPanel to the JFrame • Set up an ActionListener that changes the label’s color each time the button is clicked JLabel JPanel JButton 31 of 88

  40. public class ColorTextPanel extends JPanel { public ColorTextPanel() { JLabel label = new JLabel("CS15 Rocks!"); JButton button = new JButton("Random Color"); ColorListener listener = new ColorListener(label); button.addActionListener(listener); Dimension panelSize = new Dimension(150, 75); this.setPreferredSize(panelSize); this.add(label); this.add(button); } } The Whole App public class ColorTextApp { public ColorTextApp() { JFrame frame = new JFrame(); frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE); JPanel panel = new ColorTextPanel(); frame.add(panel); frame.pack(); frame.setVisible(true); } public static void main(String[] args) { new ColorTextApp(); } } public class ColorListener implements ActionListener { private JLabel _label; public ColorListener(JLabel label) { _label = label; } @Override public void actionPerformed(ActionEvent e) { int red = (int) (Math.random()*256); int green = (int) (Math.random()*256); int blue = (int) (Math.random()*256); Color random = new Color(red, green, blue); _label.setForeground(random); } } 32 of 88

  41. Putting It All Together ColorTextApp JFrame ColorTextPanel JButton JLabel ColorListener 33 of 88

  42. Logical vs. Graphical Containment JFrame ColorTextApp JLabel JFrame ColorTextPanel JPanel JButton • Graphically, the ColorTextPanelis contained within the JFrame, but logically, both are contained together in a top-level Appclass; App and ColorListener aren’t graphic! • Logical containment is based on where objects are created, while graphical is based on swing elements being added to other swing elements via the add(…) method • Note: The JButton also has a reference to ColorListener, but this reference is omitted from our diagram to avoid excessive complexity. JButton JLabel ColorListener 34 of 88

  43. Another Swing App: Clock • Specifications: App should display the current date and time, updating every second • Useful classes: JFrame, JPanel, JLabel, Timer, ActionListener JFrame JPanel JLabel 35 of 88

  44. DEMO: Clock 36 of 88

  45. Using Timers • The class javax.swing.Timercomes in handy when we need to perform a task repeatedly at regular intervals • When we instantiate a Timer, we pass it: • An int(number of milliseconds between Timerticks) • An ActionListenerthat should be notified when the Timeremits an ActionEvent(on every tick) • We’re going to use a Timerto update the time displayed on our JLabelat regular intervals 37 of 88

  46. Using Timers ActionListener ActionEvent e Tick! Timer ActionEvent e public void actionPerformed( ) { // Respond to ActionEvent here! // (print something to the console, tell // a JLabel to update its text, etc.) } 38 of 88

  47. Process: Clock • Create top-level class that contains a JFrame • Write a subclass of JPanelthat contains a JLabel. Add an instance of it to our JFrame • Write a class that implements ActionListener-- it should know about a JLabeland update its text on every ActionEvent • Instantiate a listener and a Timer, and start the Timer JFrame JPanel JLabel 39 of 88

  48. Top-level Class: Clock • As in the previous example, we create a top-level class that contains a JFrame • Specify closing behavior of JFramethe same way we did last time public class Clock { public Clock() { JFrame frame = new JFrame(); frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE); frame.pack(); frame.setVisible(true); } public static void main(String[] args) { new Clock(); } } 40 of 88

  49. Top-level Class: Clock • Call packon the JFrameso it will resize itself based on its contents (which we’ll be adding soon) • Last, make the frame show up by calling “setVisible(true); ” public class Clock { public Clock() { JFrame frame = new JFrame(); frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE); frame.pack(); frame.setVisible(true); } public static void main(String[] args) { new Clock(); } } 41 of 88

  50. Process: Clock • Create top-level class that contains a JFrame • Write a subclass of JPanel that contains a JLabel. Add an instance of it to our JFrame • Write a class that implements ActionListener -- it should know about a JLabeland update its text on every ActionEvent • Instantiate a listener and a Timer, and start the Timer JFrame JPanel JLabel 42 of 88

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