CSE 131 Computer Science 1 Module 5: Modular Design

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1 CSE 131 Computer Science 1 Module 5: Modular Design Modularity and the role of interfaces The list interface and classes Using interfaces for mouse input and to time program activities

2 Designing Programs as Modules Modern software apps can be very large and complex»many systems have >100,000 lines of code (LOC) and some have millions can take hundreds of developers years to complete»most systems are modified and extended over many years Organization is key to making large systems feasible»systems are broken into many small, mostly independent modules that interact through well-defined interfaces»modularity is key tool for reducing programming errors and is essential for large team development 2

3 Java Programs as Interacting Objects interface Robot Robot Controller Person Objects interact through interfaces defined by methods A method s header is its name, its list of formal parameters and the type of the value it returns»example: public void moveforward(int distance) The interface for an object is the set of method headers defined by its class (ignore data in some sense) In some languages interfaces are an implicit concept, used by developers but not supported by the language»java provides explicit support for interfaces 3

4 Example of interface: Lists of integers A list is an ordered, sequence of items What can we do to a list?»add an item, remove an item, retrieve an item,... Typically all items in a given list have the same type, but different lists may have different types»for example: list of Strings, list of integers, list of Books»note that the idea of a list is independent of the type of the items that the list contains»for now, we ll focus on lists of integers public interface IntList { void add(int pos, int item); // add item at pos int get(int pos); // return item at pos int size(); // # of items in list } 4

5 Different Ways to Represent a List Option 1: fixed grid of squares with 1 item per square»advantage: easy to find item at position i»disadvantage: adding an item requires moving others Option 2: circles linked by arrows, with 1 item per circle»advantage: easy to add items»disadvantage: need to follow arrows and count to find item i Each option may be better for some applications»and for others, choice may not matter Java interface mechanism can insulate programs from details of implementation 5

6 Implementing IntList Interface Using a fixed array of storage locations public class ArrayIntList implements IntList { } void add(int pos, int item){..}// add by shifting items int get(int pos){..} // direct from location pos int size(){..} // return stored size value... // instance variables, constructor and maybe more Using a linked set of nodes public class LinkedIntList implements IntList { void add(int pos, int item){..} // add by adjusting links int get(int pos){..} // follow links and count int size(){..} // ditto...} Using interface ArrayIntList x = new ArrayIntList(); IntList y = new LinkedIntList(); // not new IntList() IntList z = x; // can use only IntList methods on y, z 6

7 Why Use IntList Interface? Consider a general utility method that reverses a list»would like it to work for ArrayIntList, LinkedIntList and any other possible implementation of IntList void reverseintlist(intlist source, IntList target) { for (int i = 0; i < source.size(); i++) target.add(0,source.get(i)); }»works for all list implementations even with different implementations of source and target, but performance may depend on the underlying implementation Question to think about»consider ArrayIntList and LinkedIntList implementations for source and target (four combinations in all)»which is fastest? which is slowest?»how much faster is fastest? how much slower is slowest? 7

8 Beyond Lists of Integers The Java API defines a general list interface List<T>»where T indicates the type of the items on the list»for example List<String>, List<Book> This interface is implemented by the classes ArrayList<T> and LinkedList<T>»the <T> indicates that these are generic classes List<String> slist = new ArrayList<String>(); slist.add(0, abc ); slist.add(1, def );»the list item types are required to be object types, not primitive types»the Java API provides wrapped versions of each primitive type to allow us to construct lists of integers, doubles, etc Integer, Double, Character, etc. LinkedList<Integer> ilist = new LinkedList<Integer>(); ilist.add(0,25); int i = ilist.get(1).intvalue(); 8

9 Animating a Program With a Timer To produce motion effects in a graphics program, move screen objects by small amount, many times/second The Java API provides a timer class that can be used to call a method periodically» Timer(int millisecdelay, ActionListener timedobject);»uses the ActionListener interface public interface ActionListener { public void actionperformed(actionevent e); } Example: create an object for a class that implements the interface, then register it with a new Timer object class Bar implements ActionListener { void Bar(){... Timer t = new Timer(50, this); t.start(); } void actionperformed(actionevent e) { /* do some periodic action */ }..} 9

10 Animating a Program With a Timer Example: Bouncing Ball 10

11 Handling Mouse Input The Java API can inform an object in a program with a Graphical User Interface (GUI) when the mouse moves»uses the MouseMotionListener interface interface MouseMotionListener { void mousemoved(mouseevent me); // report when btn up void mousedragged(mouseevent me); // when btn down } where MouseEvent is an object that implements methods.getx() and.gety(), among others To use, create object for a class that implements the interface and register it as a MouseMotionListener class Foo implements MouseMotionListener { void Foo(){... panel.addmousemotionlistener(this ); } void mousemoved(mouseevent me) { int x = me.getx(); int y = me.gety();... }..} Demo 11