CMSC 132: Object-Oriented Programming II Java Constructs Department of Computer Science University of Maryland, College Park

Overview Autoboxing Enumerated Types Iterator Interface Enhanced for loop Scanner class Exceptions Streams

Autoboxing & Unboxing Automatically convert primitive data types Data value  Object (of matching class) Data types & classes converted Boolean, Byte, Double, Short, Integer, Long, Float Example See SortValues.java

Enumerated Types New type of variable with set of fixed values Establishes all possible values by listing them Supports values(), valueOf(), name(), compareTo()… Can add fields and methods to enums Example public enum Color { Black, White } // new enumeration Color myC = Color.Black; for (Color c : Color.values()) System.out.println(c); When to use enums Natural enumerated types – days of week, phases of the moon, seasons Sets where you know all possible values

Enumerated Types The following example is from the presentation "Taming the Tiger" by Joshua Bloch and Neal Gafter that took place at Sun's 2004 Worldwide Java Developer Conference. Example public class Card implements Serializable { public enum Rank {DEUCE, THREE, FOUR, FIVE, SIX, SEVEN, EIGHT, NINE, TEN, JACK, QUEEN, KING, ACE } public enum Suit {CLUBS, DIAMONDS, HEARTS, SPADES} private final Rank rank; private final Suit suit; private Card(Rank rank, Suit suit) { this.rank = rank; this.suit = suit; } public Rank rank() {return rank;} public Suit suit() {return suit;} public String toString() {return rank + " of " + suit;} }

Iterator Interface Iterator Common interface for all Collection classes Used to examine all elements in collection Properties Can remove current element during iteration Works for any collection

Iterator Interface Interface public interface Iterator { boolean hasNext(); Object next(); void remove(); // optional, called once per next() } Example usage Iterator i = myCollection.iterator(); while (i.hasNext()) { myCollectionElem x = (myCollectionElem) i.next(); }

Enhanced For Loop Works for arrays and any class that implements the Iterable interface. For loop handles Iterator automatically Test hasNext(), then get & cast next() Example 1 // Iterating over a String array String[] roster = {"John", "Mary", "Peter", "Jackie", "Mark"}; for (String student : roster) System.out.println(student);

Enhanced For Loop Example 2 ArrayList roster = new ArrayList (); roster.add("John"); roster.add("Mary"); Iterator it = roster.iterator(); // using an iterator while (it.hasNext()) System.out.println(it.next()); for (String student : roster) // using for loop System.out.println(student);

Standard Input/Output Standard I/O Provided in System class in java.lang System.in An instance of InputStream System.out An instance of PrintStream System.err An instance of PrintStream

Scanner class Scanner Allow us to read primitive type and strings from the standard input Example See ScannerExample.java In the example notice the use of printf

Exception Handling Performing action in response to exception Example actions Ignore exception Print error message Request new data Retry action Approaches 1. Exit program 2. Exit method returning error code 3. Throw exception

Representing Exceptions Java Exception class hierarchy Two types of exceptions  checked & unchecked

Object Error Throwable Exception LinkageError VirtualMachoneError ClassNotFoundException CloneNotSupportedException IOException AWTError … AWTException RuntimeException … ArithmeticException NullPointerException IndexOutOfBoundsException Unchecked Checked NoSuchElementException … Representing Exceptions Java Exception class hierarchy

Unchecked Exceptions Class Error & RunTimeException Serious errors not handled by typical program Usually indicate logic errors Example NullPointerException, IndexOutOfBoundsException Catching unchecked exceptions is optional Handled by Java Virtual Machine if not caught

Checked Exceptions Class Exception (except RunTimeException) Errors typical program should handle Used for operations prone to error Example IOException, ClassNotFoundException Compiler requires “catch or declare” Catch and handle exception in method, OR Declare method can throw exception, force calling function to catch or declare exception in turn Example void A( ) throws ExceptionType { … }

Exceptions – Java Primitives Java primitives Try Forms try block Encloses all statements that may throw exception Throw Actually throw exception Catch Catches exception matching type Code in catch block  exception handler Finally Forms finally block Always executed, follows try block & catch code

Exceptions - Syntax try {// try block encloses throws throw new eType1(); // throw jumps to catch } catch (eType1 e) { // catch block 1...action... // run if type match } catch (eType2 e) { // catch block 2...action... // run if type match } finally { // final block...action... // always executes }