1 Introduction To Java Professor Yihjia Tsai Tamkang University

2 Basic OOP Notions Java  Instance variables - describe the data in abstract data types (each object instance has its own copy of the data)  Class variables - describe the data in abstract data types all object instances share the value of the data (called static fields in Java)  Messages request for an operation  Methods implementation of a request

3 Data and Methods/Interface DATA Oper 1 Oper 3 Oper 4 Oper 2 Public Private

4 Introduction to Java Basic Ideas  All Java Programs, unlike C++ are built from classes, in this way Java is more strictly an OOP  A class is said to contain fields (members in C++) and methods (member functions in C++)

5 Fundamental Ideas  Java was designed with security and portability in mind  Supports standard length built in types  Java source gets translated into something called Java byte code that is run on something called the Java virtual machine.  Java bytecode is the machine language of the Java virtual machine  The virtual machine assigns each application its own runtime, which isolates applications from each other.

6 Java Infrastructure javac.java java bytecode.class file produced This is the JVM

7 A Code Example class HelloWorld { public static void main(String[] args) { System.out.println( “ Hello mom ” ); } Type this in, compile it, and add Java to your resume ;)

8 Similarities and differences from C++?  The Built In Types  Comments  Named Constants  Flow of Control

9 The Built In Types booleantrue or false char16 bit Unicode 1.1 character byte8 bits short16 bit integer signed int32 bit signed integer long64 bit signed integer float, doubleIEEE 754

10 Comments  Are identical to C++ except for javadoc  // Comment till end of line  /* Comments spanning a line */  /** */ javadoc comment  javadoc produces html documentation on methods, constructors and so on

11 Named Constants class CircleStuff { static final double  = ; } class Suit { public final static int CLUBS = 1; public final static int DIAMONDS = 2; public final static int HEARTS = 3; public final static int SPADES = 4; }

12 Classes and Objects  Java classes have fields and methods  Fields and Methods in Java can have associated visibility  Public  Private  Package  Protected

13 Methods class Point { //.. public void clear() { x = 0; y = 0; } public double distance(Point other) { double xdiff, ydiff; xdiff = x - other.x; ydiff = y - other.y; return Math.sqrt(xdiff * xdiff + ydiff * ydiff); }

14 Packages  Used to avoid name conflicts  Java has adopted a more formal notion of a package that has a set of types and subpackages as members.  Packages are named and can be imported - we'll look at a few examples.  Package names form a hierarchy with parts separated by dots.  When you use part of a package, either you use its fully qualified name or you import all or part of the package.

15 Examples class Date1 { public static void main(String[] args) { java.util.Date now = new java.util.Date(); System.out.println(now); } import java.util.Date; class Date2 { public static void main(String[] args) { Date now = new Date(); System.out.println(now); }

16 Self Reference  Java unlike C++ has no notion of a pointer  There is still a this construct however - an example will suffice class Point { //... public void clear () { this.x = 0; this.y = 0; } //... }

17 Creating Objects  Use new  Invokes constructor  No destructor  Super and this

18 Arrays class Deck { final int DECK_SIZE = 52; Card[ ] = new Card[DECK_SIZE]; //.... Init the array somewhere public void print() { for (int i = 0; i < cards.length; i++) System.out.println(cards[i]); } //... }

19 Strings are Built In Java class Yadayadayada { public static void main (String[ ] args ) { /// yadayadayada } // } There is also a StringBuffer class

20 Tokenizing Strings import java.util.*; String stg = “when the going gets weird the weird turn pro”; StringTokenizer tokens = new StringTokenizer (stg); while (tokens.hasMoreElements() ) { String next = tokens.nextToken(); System.out.println(next); }

21 Inheritance  One of major aspects of OOP is inheritance - lets look at an example in Java  You can inherit, override, or reuse behavior of parent class  Note that Pixel objects can be used by any code designed to work with Point objects.  If a method expects a parameter of type Point, you can hand it a Pixel object instance and it still works just fine.

22 Inheritance class Pixel extends Point { Color color; public void clear() { super.clear(); color = null; } Only public inheritance !

23 Java Doesn't support MI of Implementation interface Lookup { // Return the value associated w/ name or null Object find (String name); } class Foo implements Lookup extends Bar { // Foo must provide all the Lookup methods }

24 Using an Interface  Now lets look at some code that that uses the Lookup interface void processValues (String[] names, Lookup table) { for (int i = 0; i<names.length; i++) { Object value = table.find(names[i]); if (value != null) processValue(names[i], value); }

25 A Class that Implements the Lookup Interface  Now lets look at a class that implements the interface class SimpleLookup implements Lookup { private String[] Names; private Object[] Values; public Object find(String name) { for (int i = 0; i<Names.length; i++) { if (Names[i].equals(name)) return Values[i]; } } //.... }

26 Exceptions  Java uses checked exceptions to manage error handling. Exceptions force a programmer top deal with errors. If a checked exception is not handled, this noticed when the error happens, not latter when problems have potentially compounded.  A method that detects an unusual error condition throws an exception. Exceptions in turn may be caught by code further back on the calling stack - this prior code can handle the exception and continue processing.  Un-handled exceptions are handled by a default handler in the Java implementation which may report the exception and terminate the thread of execution.

27 More General Ideas About Exceptions in Java  Exceptions in Java are objects, with type, methods and fields of data. This representation is handy because an exception can include data or methods to report or recover from certain conditions.  Exceptions are generally extensions of the Exception class which provides a string field to describe the error.

28 class IllegalAverageException extends Exception {}; class MyUtilities { public double averageOf ( double[] vals, int i, int j) throws IllegalAverageException{ try { return (vals[i] + vals[j]) / 2; } catch (IndexOutOfBoundsException e) { throw new IllegalAverageException();} } An Example of Exceptions

29 Text Input  System.in is a java.io.InputStream reads raw bytes BufferedReader in = new BufferedReader( new InputStreamReader(System.in));  InputStreamReader coverts from bytes to Unicode  BufferedReader supports String readLine() returns null at EOF

30 File Text I/O PrintWriter out = new PrintWriter(new FileWriter(fileName)); BufferedReader in = new BufferedReader( new FileReader(fileName));

31 References  Flanagan, D., Java in a Nutshell, 2nd ed., O ’ Reilly, ISBN X.  Harold, E.R., Java Network Programming, 3rd ed., O ’ Reilly, ISBN  Niemeyer, P., J. Knudsen, Learning Java, 3rd ed., O ’ Reilly, ISBN  Meyer, J., T. Downing, Java Virtual Machine, O ’ Reilly, ISBN