1. COMP 103 Comparators and Comparable

2. RECAP  Iterator and Iterable TODAY  Comparator and Comparable  Exceptions 2 RECAP-TODAY

3. Working with Collections  Done: Declaring and Creating collections Using collections: adding, removing, getting, setting, putting,…. Iterating through collections [ Iterators, Iterable, and the "for each" loop ]  What next ?  Comparable and Comparator for Sorting Collections - today  Implementing Collection classes – next time 3

4. Sorting a collection  What kinds of collections could you sort ?  Set ?  Stack ?  Queue ?  List ?  Map ?  How can you sort them ?  List does not have a sort method.  But the Collections class does! Collections.sort(creatures); Collections.sort(myTaskList);  Collections is a class with lots of useful static methods that apply to collections. 4

5. Sorting in “Natural order”  But what order will it sort into ?  “natural order of the values”  Fine for Strings, Integer, Double  Strings ordered alphabetically, as in a phonebook  Integer, Double ordered by numerical value  But what’s the “natural order” of Faces in a crowd?  Answer: Whatever you defined it to be, if you defined it. There is no order if you didn’t define it.  How do you define the natural order? 5

6. “Natural Ordering” & Comparable  If a class implements the Comparable interface  Objects from that class have a “natural ordering”  Objects can be compared using the compareTo() method  Collections.sort() can sort Lists of those objects automatically  Comparable is an Interface: Requires an implementation of  compareTo(E ob) → int ob1.compareTo(ob2)  returns –ve if ob2 ordered after ob1  returns 0 if ob2 ordered with ob1  returns +ve if ob2 ordered before ob1 6

7. Eg: Making a “Face” class Comparable public class Face implements Comparable { : public int size(){ // just a helper method return (wd * ht); } /** Natural ordering is by size, small to large. */ public int compareTo(Face other){ } if (button.equals(“SmallToBig")) { Collections.sort(crowd); for (Face f : crowd) UI.print(f.toString()); } return ( this.size() – other.size() ); if ( this.size() other.size() ) return 1; else return 0; 7 Elsewhere, in the program, we might see this

8. Sorting with Comparators Suppose we need two different sorting orders at different times ? Collections.sort(…) has two forms: 1. Sort by the “natural order” Collections.sort(todoList)  the values in todoList must be Comparable  i.e. the element class supplies a "compareTo()" method 2. Sort according to a specified order: Collections.sort(crowd, faceByArea)  faceByArea is a Comparator object for comparing the values in crowd. Comparable interface for objects that can be compared. method: compareTo() Comparator An object that can compare other objects, using compare() 8

9. Sorting with Comparators  Collections.sort(crowd, faceByArea); List Data Comparator -1 / 0 / +1 val1val2 -ve ⇒ val1 less than val2 0 ⇒ val1 equals val2 +ve ⇒ val1 greater than val2 Sort 9

10. Comparators  Comparator is an Interface  Requires  public int compare(E ob1, E ob2); → –veif ob1 ordered before ob2 → 0 if ob1 equals ob2 → +ve if ob1 ordered after ob2 /** Compares faces by the position of their top edge */ private class TopToBotComparator implements Comparator { public int compare(Face f1, Face f2){ return (f1.getTop() - f2.getTop()); } A tiny class! It's really just a method that we can pass around... In java we can only pass around objects, not methods. 10

11. Using Multiple Comparators if (button.equals("SmallToBig")){ Collections.sort(crowd); // use the "natural ordering" on Faces. render(); } else if (button.equals("BigToSmall")){ Collections.sort(crowd, new BigToSmallComparator()); render(); } else if (button.equals("LeftToRight")){ Collections.sort(crowd, new LfToRtComparator()); render(); } else if (button.equals("TopToBottom")){ Collections.sort(crowd, new TopToBotComparator()); render(); } 11 EXAMPLE

12. Exceptions This will be covered in detail in 2 nd year (SWEN221) What should a method do when something goes wrong? Throw an exception! (which is actually an object) Error of some kind, eg:  file doesn’t exist,  dividing by zero  calling a method on the null object  An exceptional situation, eg:  text in file doesn’t have the expected format  user enters an unexpected answer How do you deal with Exceptions?  handling (catching) exceptions  throwing exceptions eg. may be thrown by the JVM when executing code. eg. May be thrown by code from the java library eg. May be deliberately thrown by your code. 12

13. Catching exceptions exceptions thrown in a try... catch can be “caught”: try { … code that might throw an exception } catch ( 〈 ExceptionType1 〉 e1) { …actions to do in this case….} catch ( 〈 ExceptionType2 〉 e2) { …actions to do in this case….} catch ( 〈 ExceptionType3 〉 e3) { …actions to do in this case….} Meaning:  If an exception is thrown, jump to catch clauses,  Match exception type against each catch clause.  If caught, perform the actions, and jump to code after all the catch clauses. The actions may use information in the exception object. Exception object, containing information about the state UI.println(e3.getMessage()); e3.printStackTrace(); } Informative action for an error condition that the program can’t deal with itself 13

14. Types of Exceptions  Lots of kinds of exceptions Exceptions AclNotFoundException, ActivationException, AlreadyBoundException, ApplicationException, AWTException, BackingStoreException, BadAttributeValueExpException, BadBinaryOpValueExpException, BadLocationException, BadStringOperationException, BrokenBarrierException, CertificateException, ClassNotFoundException, CloneNotSupportedException, DataFormatException, DatatypeConfigurationException, DestroyFailedException, ExecutionException, ExpandVetoException, FontFormatException, GeneralSecurityException, GSSException, IllegalAccessException, IllegalClassFormatException, InstantiationException, InterruptedException, IntrospectionException, InvalidApplicationException, InvalidMidiDataException, InvalidPreferencesFormatException, InvalidTargetObjectTypeException, InvocationTargetException, IOException, JMException, LastOwnerException, LineUnavailableException, MidiUnavailableException, MimeTypeParseException, NamingException, NoninvertibleTransformException, NoSuchFieldException, NoSuchMethodException, NotBoundException, NotOwnerException, ParseException, ParserConfigurationException, PrinterException, PrintException, PrivilegedActionException, PropertyVetoException, RefreshFailedException, RemarshalException, RuntimeException, SAXException, ServerNotActiveException, SQLException, TimeoutException, TooManyListenersException, TransformerException, UnmodifiableClassException, UnsupportedAudioFileException, UnsupportedCallbackException, UnsupportedFlavorException, UnsupportedLookAndFeelException, URISyntaxException, UserException, XAException, XMLParseException, XPathException RunTimeExceptions: AnnotationTypeMismatchException, ArithmeticException, ArrayStoreException, BufferOverflowException, BufferUnderflowException, CannotRedoException, CannotUndoException, ClassCastException, CMMException, ConcurrentModificationException, DOMException, EmptyStackException, EnumConstantNotPresentException, EventException, IllegalArgumentException, IllegalMonitorStateException, IllegalPathStateException, IllegalStateException, ImagingOpException, IncompleteAnnotationException, IndexOutOfBoundsException, JMRuntimeException, LSException, MalformedParameterizedTypeException, MissingResourceException, NegativeArraySizeException, NoSuchElementException, NullPointerException, ProfileDataException, ProviderException, RasterFormatException, RejectedExecutionException, SecurityException, SystemException, TypeNotPresentException, UndeclaredThrowableException, UnmodifiableSetException, UnsupportedOperationException 14

15. Throwable objects! Throwable ErrorsExceptions many types (classes)... subclasses... (eg: RuntimeException) 15

16. Types of Exceptions  There are lots of kinds of exceptions  RuntimeExceptions don’t have to be handled:  An uncaught RuntimeException will result in an error message  You can try-catch them if you want.  Other Exceptions must be handled:  Either explicitly caught, or the method must declare that it might throw that kind of exception.  Example: IOException (which is why we always have to use a try…catch when opening files).  An exception object contains information:  the state of the execution stack  any additional information specific to the exception 16

17. Throwing exceptions  The throw statement causes an exception.  eg from MiniDraw: suppose that a drawing file has a line with an unknown shape: if (name.equals("oval"))shapes.add(new Oval(file)); else if (name.equals("rectangle"))shapes.add(new Rectangle(file)); else if (name.equals("line"))shapes.add(new Line(file)); else if (name.equals("polyline"))shapes.add(new PolyLine(file)); else throw new RuntimeException(“Unknown shape in drawing file”); renderShape(); General RuntimeException object. Could use a more specific one Could define our own type of exception 17

18. Another example from earlier: iterator private class ArithSequenceIterator implements Iterator { private int nextNum; private ArithSequence source; public ArithSequenceIterator(ArithSequence seq) { source = seq; nextNum = seq.start; } public boolean hasNext() { return true; // there is always another one } public Integer next() { int ans = nextNum; nextNum += source.step; return ans; } public void remove() { throw new UnsupportedOperationException(); } Better to put in some informative message! 18

19. Implementing Collections  Part 1: using collections  Different types of collections  The Java collections library  Interfaces for List, Set, Map, Stack, Queue  Using Iterators and Iterable  Using Comparable and Comparators  Throwing exceptions  Part 2: implementing collections  What it means to implement a Collection Interface  Alternative data structures and algorithms  Cost and efficiency  Testing  Binary search algorithm  Sorting algorithms ✔ ✔ ✔ ✔ ✔ ✔ 19