Agenda

1. Agenda • interfaces and realization • type hierarchy • introduction to graphics and event handling

2. Interfaces and Realization • the form of an interface definition • the function of an interface • the realization (“implements”) relationship

3. Form of an interface • header + body • header • access control modifier • keyword ‘interface’ • name (generally an adjective, following class name conventions, but prefixed with an upper-case ‘I’) • body • method specifications (method headers followed by ‘;’, also called method declarations, as opposed to method definition)

4. Interfaces – no instantiation • While classes can be instantiated, interfaces cannot be instantiated. CSE 115/503 Introduction to Computer Science for Majors I

5. Examples • Goofy example, tying in with Dog example public interface ICollarable { public void addCollar(Collar collar); public Collar removeCollar(); } 2) Example from Java’s libraries (one detail omitted) public interface ActionListener { public void actionPerformed(ActionEvente); }

6. Implementation • A class can implement an interface: public class Dog implements ICollarable { ... } CSE 115/503 Introduction to Computer Science for Majors I

7. Implementation as contract • A class which implements an interface is obligated to provide full definitions of all the methods specified in the interface. CSE 115/503 Introduction to Computer Science for Majors I

8. ExampleProblem – methods of interface are not defined (incorrect name) public class Dog implements ICollarable { private Collar _collar; public Dog(Collar collar) { _collar = collar; } public void setCollar(Collar collar) { _collar = collar; } public Collar removeCollar() { Collar temp = _collar; _collar = null; return temp; } }

9. ExampleSolution – methods of interface defined (correct name) public class Dog implements ICollarable { private Collar _collar; public Dog(Collar collar) { _collar = collar; } public void addCollar(Collar collar) { _collar = collar; } public Collar removeCollar() { Collar temp = _collar; _collar = null; return temp; } }

10. Types • When you define a class, you are defining a type. • When you define an interface, you are also defining a type. • A class which implements an interface is a SUBTYPE of the interface type. CSE 115/503 Introduction to Computer Science for Majors I

11. Assignment • If a variable is declared to be of an interface type, it can be assigned an instance of a subtype class. CSE 115/503 Introduction to Computer Science for Majors I

12. Examples ICollarable x; x = new ICollarable(); NO! Cannot instantiate. x = new Dog(); OK if Dog implements ICollarable x = new Cat(); OK if Cat implements ICollarable

13. The point? • Subtypes of a common supertype can be treated uniformly when treated as members of the supertype: see next slide. CSE 115/503 Introduction to Computer Science for Majors I

14. Calling methods ICollarable x; x = new ICollarable(); NO! Cannot instantiate. x = new Dog(); OK if Dog implements ICollarable x = new Cat(); OK if Cat implements ICollarable x.addCollar(new Collar()); OK x.removeCollar(); OK x.walk(); NO! Not all ICollarable objects define this method!

15. Things to mention • public static void main(String [] args) • The starting point for a free-standing Java application (i.e. one not run from the DrJava interactions pane) • imports • Allow us to use, with unqualified names, things defined in other packages • comments – ignored by the compiler • block comments /* … */ • single-line comments // • javadoc comments and tags (?) /** … */ • String class and String literals (e.g. “Fred”) • true as a boolean literal (for setVisible method call) CSE 115/503 Introduction to Computer Science for Majors I

16. Using the Java graphics classes • In this slide set we will explain the basics of how to create graphical programs. • Some advanced issues will be glossed over (e.g. thread safety, graphics library design). • In CSE116 we will revisit these and other more advanced topics. CSE 115/503 Introduction to Computer Science for Majors I

17. Graphical elements • There are two basic types of graphical elements: • Containers • able to hold graphical objects, such as containers and components • Components • must be put into containers • able to generate events when manipulated CSE 115/503 Introduction to Computer Science for Majors I

18. Containers • Top-level containers • some containers are called “top-level” because they do not need to be place inside any other containers • javax.swing.JFrame is an example (JDialog and JApplet are others) • Other containers (not top-level) • most containers must be placed inside some other container • javax.swing.JPanel is an example CSE 115/503 Introduction to Computer Science for Majors I

19. Adding elements to a JFrame • Top-level containers have multiple panes • Content pane is the one which holds components • With javax.swing.JFrame, two ways: • call getContentPane() on frame to get frame’s content pane, then call add(…) on content pane to add a component • call add(…) directly on the JFrame object • Second approach is just a convenience method, does the same thing the first approach CSE 115/503 Introduction to Computer Science for Majors I

20. Example • Creating just a frame • new javax.swing.JFrame() • Creating a frame with a title • new javax.swing.JFrame(“My title”) • Making the frame visible • call setVisible(true) on the frame • Making application close when window is closed: • call setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE) on the frame • See code in: • the BasicExample class in the graphics package of the GraphicsExamples project in the repository, as well as, • the ex_1_JFrame_visible package in the FA10-CSE115-SwingExamples project in the repository. CSE 115/503 Introduction to Computer Science for Majors I

21. A simple component • A JLabel is a component that can display text or an image. • It can also have a “tooltip”, a note that appears when you hover the mouse pointer over it, without clicking. • Look at ex_2_JLabel_tooltip CSE 115/503 Introduction to Computer Science for Majors I

22. Layout managers • First, let’s consider what happens if we add another JLabelto the content pane of the JFrame. • Look at ex_3_JLabels_noLayout • This will show us the need to manage the layout of components added to a container. • See also code in GraphicsExamples project for this and the rest of the slides. CSE 115/503 Introduction to Computer Science for Majors I

23. Applying a layout management strategy • Look at ex_4_staticLayout • In this example we can apply one of four different layout management strategies to a set of seven JLabels that are added to the JFrame’s content pane. CSE 115/503 Introduction to Computer Science for Majors I

24. Another component: a JButton • A JButton is a component which is typically set up to react to clicks. • Look at ex_5_JButton_noEventHandler, this example just shows a JButton. CSE 115/503 Introduction to Computer Science for Majors I

25. Events • Clicks on buttons, mouse movements, etc. are all considered events. • A program can react to events by setting up event handlers. • An event handler defines what should happen when a particular event occurs. CSE 115/503 Introduction to Computer Science for Majors I

26. Event handling – 1 • The component which gives rise to an event is decoupled from the part of the code that handles the event. • This is called the observer pattern. • General form: • www.research.ibm.com/designpatterns/example.htm CSE 115/503 Introduction to Computer Science for Majors I

27. Event handling – 2 • Observer pattern in Java • An observer is called a listener in Java • Button clicks are “ActionEvents”. • Handlers for ActionEvents are ActionListeners. • An event-generator can have many listeners • Use “addActionListener” method to register a listener with a component CSE 115/503 Introduction to Computer Science for Majors I

28. Event handling for a JButton • Look at ex_6_JButton_eventHandler • In this example an event handler is defined and attached to the JButton. CSE 115/503 Introduction to Computer Science for Majors I

29. Putting it all together • Look at ex_7_dynamicLayoutEventHandling • In this example we put everything together: • different components are put into different containers • multiple components are put into each container • different layout managers can be applied dynamically • JButtons’ event handlers select and apply different layout managers to the container containing JLabels CSE 115/503 Introduction to Computer Science for Majors I