Java OOP Programming language (Part 4) - Collection

Java OOP Programming language (Part 4) - Collection

• 1. Java Programming - Collection Oum Saokosal Master’s Degree in information systems,Jeonju University,South Korea 012 252 752 / 070 252 752 oumsaokosal@gmail.com
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• 3. 3 Agenda •Building arrays •Vectors and Hashtables •Data structures introduced in Java 2 •Using wrappers to convert primitive data types to objects •Handling exceptions
• 4. 4 Arrays • Accessingarrays • Accessarraysby supplying the indexin square bracketsafter the variable name,variableName[index] • The first indexis0, not 1 • Example • Here, the argument to main is an array of Strings called args public class Test { public static void main(String[] args) { System.out.println("First argument: " + args[0]); } } > javac Test.java > java Test Hello There First argument is Hello
• 5. 5 The Array length Field • Arrays have a built-in field called length that storesthe size of the array • The length is one bigger than the biggest index, due to the fact that the index starts at 0 • Example public class Test2 { public static void main(String[] args) { System.out.println("Number of args is " + args.length); } } > javac Test2.java > java Test2 Number of args is 0 > java Test2 Hello There Number of args is 2
• 6. 6 Building Arrays • Arrays can be built in a one-step or two- step process 1. The one-step process is of the following form: type[] var = { val1, val2, ... , valN }; • For example: int[] values = { 10, 100, 1000 }; Point[] points = { new Point(0, 0), new Point(1, 2), ... };
• 7. 7 Building Arrays, cont. 2. With the two-step process, first allocate an array of references: type[] var = new type[size]; • For example: int[] values = new int[7]; Point[] points = new Point[length]; • Second, populate the array points[0] = new Point(...); points[1] = new Point(...); ...
• 8. 8 Multidimensional Arrays •Multidimensional arrays are implemented as an arrayof arrays int[][] twoD = new int[64][32]; String[][] cats = { { "Caesar", "blue-point" }, { "Heather", "seal-point" }, { "Ted" , "red-point" } };
• 9. 9 Data Structures • Java 1.0 introduced two synchronized data structures in the java.util package • Vector • A stretchable(resizable) array of Objects • Time to accessan element is constant regardlessof position • Time to insert element is proportionalto the size of the vector • Hashtable • Stores key-value pairs as Objects • Neither thekeysor values can be null • Time to access/insert is proportionaltothe size of the hashtable
• 10. 10 UsefulVector Methods • addElement / insertElementAt / setElementAt • Add elementsto the vector • removeElement / removeElementAt • Removesan element from the vector • firstElement / lastElement • Returnsa reference to the first and last element,respectively (without removing) • elementAt • Returnsthe element at the specified index • indexOf • Returnsthe indexof an element that equals the object specified • contains • Determinesif the vector containsan object
• 11. 11 UsefulVector Methods (cont) • elements • Returns an Enumerationof objects in the vector Enumeration elements = vector.elements(); while(elements.hasMoreElements()) { System.out.println(elements.nextElement()); } • size • The number of elements in the vector • capacity • The number of elements the vector can hold before becoming resized
• 12. 12 Useful Hashtable Methods • put / get • Stores or retrievesa value in the hashtable • remove / clear • Removesa particular entry or all entriesfrom the hashtable • containsKey / contains • Determinesif the hashtable containsa particular key or element • keys / elements • Returnsan enumeration ofall keysor elements,respectively • size • Returnsthe number of elementsin the hashtable • rehash • Increasesthe capacity ofthe hashtable and reorganizesit
• 13. 13 Collections Framework •Additional data structures Collection Set SortedSet List ArrayList LinkedList Vector* HashSet TreeSet Map SortedMap HashMap Hashtable* TreeMap Interface Concrete class *Synchronized Access
• 14. 14 Collection Interfaces • Collection • Abstract classfor holding groupsofobjects • Set • Group of objectscontainingno duplicates • SortedSet • Set of objects(no duplicates)stored in ascending order • Order is determined by a Comparator • List • Physically (versus logically)ordered sequence ofobjects • Map • Stores objects(unordered)identified by unique keys • SortedMap • Objectsstored in ascending order based on their key value • Neither duplicate or null keysare permitted
• 15. 15 Collections Class • Use to create synchronized data structures List list = Collection.synchronizedList(new ArrayList()); Map map = Collections.synchronizedMap(new HashMap()); • Provides useful (static) utility methods • sort • Sorts (ascending) theelementsin thelist • max, min • Returns the maximum or minimum element in the collection • reverse • Reverses the order of theelements in the list • shuffle • Randomly permutes theorder of the elements
• 16. 16 WrapperClasses •Each primitive data type has a corresponding object (wrapperclass) Primitive Corresponding Data Type Object Class byte Byte short Short int Integer long Long float Float double Double char Character boolean Boolean
• 17. 17 Wrapper Uses • Defines useful constants for each data type • For example, Integer.MAX_VALUE Float.NEGATIVE_INFINITY • Convert between data types • Use parseXxx method to convert a String to the correspondingprimitivedata type try { String value = "3.14e6"; double d = Double.parseDouble(value); } catch (NumberFormatException nfe) { System.out.println("Can't convert: " + value); }
• 18. 18 Exception Hierarchy •Simplified Diagram of Exception Hierarchy Throwable Error IOException RuntimeException Exception …
• 19. 19 ThrowableTypes • Error • A non-recoverableproblem that should not be caught (OutOfMemoryError, StackOverflowError, …) • Exception • An abnormalcondition that should be caught and handled by the programmer • RuntimeException • Special case; does not have to be caught • Usually the result of a poorly written program (integer division by zero, array out-of-bounds,etc.) • A RuntimeException is considereda bug
• 20. 20 Multiple CatchClauses • A single try can have more that one catch clause • If multiple catch clauses are used, order them from the most specific to the most general • If no appropriatecatch is found,the exception is handedto any outer try blocks • If no catch clause is found within the method, then the exception is thrown by the method try { ... } catch (ExceptionType1 var1) { // Do something } catch (ExceptionType2 var2) { // Do something else }
• 21. 21 Try-Catch, Example ... BufferedReader in = null; String lineIn; try { in = new BufferedReader(new FileReader("book.txt")); while((lineIn = in.readLine()) != null) { System.out.println(lineIn); } in.close(); } catch (FileNotFoundException fnfe ) { System.out.println("File not found."); } catch (EOFException eofe) { System.out.println("Unexpected End of File."); } catch (IOException ioe) { System.out.println("IOError reading input: " + ioe); ioe.printStackTrace(); // Show stack dump }
• 22. 22 The finally Clause •After the final catch clause, an optional finally clause may be defined •The finally clause is always executed, even if the try or catch blocks are exited through a break, continue, or return try { ... } catch (SomeException someVar) { // Do something } finally { // Always executed }
• 23. 23 Thrown Exceptions • If a potential exception is not handled in the method, then the method must declare that the exception can be thrown public SomeType someMethod(...) throws SomeException { // Unhandled potential exception ... } • Note: Multiple exception types (comma separated) can be declared in the throws clause • Explicitly generating an exception throw new IOException("Blocked by firewall."); throw new MalformedURLException("Invalid protocol");