Special Exercise Unit: Introduction to Java

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1 Special Exercise Unit: Introduction to Java Introduction First Program / Applet : Hello World Java Virtual Machine Java Basics (types, operators, statements, etc.) Object Orientation Standard Libraries

2 Introduction 1 Java developed by Gosling et al. (Sun Microsystems 1995) originally for embedded systems new area of application: Internet => marketed as Java many things in common with C/C++ most important features: object-oriented (classes, instances/objects,...): abstraction, encapsulation, inheritance, polymorphism concurrent strongly typesafe, no direct memory access and manipulation as used in C / C++ => automatic garbage collection translated into bytecode (=> executed by interpreter) => independent of OS, one only needs the bytecode interpreter large standard libraries (Application Programmers Interface, API) drawback: sometimes poor runtime performance

3 Introduction 2 Online Documentation at Sun : or local: Tutorial by Sun: Lecture books: David Flanagan: Java in a nutshell (German/English) Bruce Eckel: Thinking in Java deutsche Lehrbücher: Christian Ullenboom: Java ist auch eine Insel, Online-Version unter Guido Krüger: Handbuch der Java-Programmierung, Download unter

4 Hello World 1 Using JDK, i.e. producing and executing Java- exec souce code in file hello.java: public class hello { public static void main(string args[]) { System.out.println("Hello World!"); execute "javac hello.java" javac (bytecode compiler) translates the program stored in the file hello.java into OS-independent bytecode and stores it in the file hello.class execute "java hello" java (bytecode interpreter/jvm) loads class hello und executes method main

5 Hello World 2 Programming Model source code translator libraries bytecode JVM: classloader, verifier, interpreter, scheduler, garbage collector operating system

6 Hello World 3 Executing a Program as an Applet (a) store program in file helloapplet.java: import java.applet.*; import java.awt.*; public class helloapplet extends Applet { public void paint(graphics g) { g.drawstring("hello world!", 50, 25); (b) translate: "javac helloapplet.java"

7 Hello World 4 (c) embed applet into.html-page: helloapplet.html : <HTML><HEAD> <TITLE>The Hello World Applet</TITLE> </HEAD> <BODY> <APPLET CODE="helloApplet.class" WIDTH=150 HEIGHT=25> </APPLET> </BODY> </HTML> (d) 1. execute: "appletviewer helloapplet.html" or 2. view html-page with browser

8 Hello World 5 Programming Model Server: src-code Client: translator library bytecode transmit bytecode library JVM JVM

9 Basic Java 1 basic data types type size remark byte 1 Byte natural number, [-128, 127] short 2 Bytes natural number, [ , ] int 4 Bytes nat. num, [ , ] long 8 Bytes nat. num, [ , ] float 4 Bytes floating point as defined in IEEE 754 double 8 Bytes floating point as defined IEEE 754 boolean 1 Bit true und false char 2 Bytes uni-code character, first 127 values are ASCII, remaining are int. fonts

10 Basic Java 2 reference type only other type references to objects (addresses) all complex data structures are realized as objects (see below) variables memory space for values of a basic data type or a reference type Example: int i; double x,y; comments traditional style (C): /*... */ line comments (C++): // comments for documentation: /**... */ (used by javadoc)

11 Basic Java 3 operators arithmetic: ++, --, +, -, *, /, % bitwise: ~, &, ^, logical:!, &&, shifting: <<, >>, >>> comparison: <, <=, >, >=, ==,!= condition:? : assignment: =, *=, /=, %=, +=, -=, <<=, >>=, >>>=, &=, ^=, = examples: int i,j,k=10; double x; i++; --j; x=i%j; k=(j<i)?-1>>>24:-1>>24;

12 Basic Java 4 statements block { empty assignment ; assignment x=i+j; branching (1) if (i<j) k=-1>>>24 else x=-1>>24; branching (2) switch (i) { case 1: out.print("eins");break; case 2: out.print("zwei");break; default: out.print("viele"); ; for-loop for (n=0,i=0;i<10;i++) n+=i; while-loop n=0;i=0; while (i<10) n+=i++; do-loop n=0;i=0; do n+=i++; while (i<10); others break; continue; return;

13 Basic Java 5 methods: control flow is steered by statements parts of objects (see below) example: recursive method for factorial public class Factorial { public static void main(string args[]){ long i,f; for (i=1;i<=20;i++){ f=factorial(i); System.out.println(i+" "+f); static long factorial(long k){ if (k==0) return 1; else return k*factorial(k-1);

14 Basic Java 6 exception handling handling of erroneous states, e.g.: try{ x=1/i; catch (java.lang.arithmeticexception e){ System.out.println(e); if no error occurs, try-block is executed if an error occurs execution of statement is aborted, exception is thrown respective handler (catch-clause) is searched for and executed try-block is terminated there can be several catch-clauses exceptions can be explicitly triggered by throw exceptions are objects

15 Object-oriented Programming 1 objects extension of the concept of abstract data type encapsulation of variables (data, state of object) and methods (access and manipulation of variables) class: pattern of an object instance: allocation of an object according to its pattern allocation/deallocation of memory space for objects (on heap) explicitly allocated by using new, deallocation is automatic (garbage collection) objects build a hierarchy, since they inherit variables and methods inheritance is a central concept of structuring programs

16 Object-oriented Programming 2 classes declaration / definition (complex numbers): public class ComplexNumber { // variables: protected double re, im; // constructors: ComplexNumber(){re=im=0.0; ComplexNumber(double re, double im){ this.re=re; this.im=im; // methods: public double Real(){return re; public double Imaginary(){return im; public double Magnitude(){return Math.sqrt(re*re+im*im); public double Angle(){return Math.atan(im/re); constructors are special methods called at allocation time of objects

17 Object-oriented Programming 3 instances (allocation of objects) declaration of variables by referencing to a class ComplexNumber (initial value: null): ComplexNumber c,d; allocation of objects (memory space), i.e. assigning a value to the reference by executing the constructors: c=new ComplexNumber(); d=new ComplexNumber(1.0,2.0); call a method System.out.println(d.Magnitude()+", "+d.angle()); assigning a reference (not an object itself): c=d; if there is no reference left to an object, it will be erased (garbage collection)

18 Object-oriented Programming 4 sub-classes declaration of a class of complex numbers, which enables us to use basic operations: public class ComplexNumberOp extends ComplexNumber { // constructors: ComplexNumberOp(){ ComplexNumberOp(double re, double im){super(re,im); // new methods: public void Add(ComplexNumber z){re+=z.re; im+=z.im; public void Subtract(ComplexNumber z){ re-=z.re; im-=z.im; public void Mult(ComplexNumber z){ re=re*z.re-im*z.im; im=re*z.im+z.re*im;

19 Object-oriented Programming 5 object creation and call of methods as usual: ComplexNumberOp e=new ComplexNumberOp(1.0,1.0); f=new ComplexNumberOp(2.0,2.0); e.add(f); e.subtract(f); e.mult(f); hierarchy of classes ComplexNumber is a sub-class of Object Object is a super-class of ComplexNumber ComplexNumberOP is a sub-class of ComplexNumber ComplexNumber is a super-class of ComplexNumberOp each class has at most one super class (no multiple inheritance!) classes without explicit super-class are sub-classes of class Object hierarchy of classes is a tree rooted with Object Object ComplexNumber ComplexNumberOP

20 Object-oriented Programming 6 inheritance variables and methods (not constructors) are handed down to the sub classes, e.g.: System.out.println(e.Real()+", "+e.imaginary()); methods of super-class can be redefined in sub-classes exact re-definition (name + arguments) leads to overloading re-definition of method name leads to polymorphism, non-determinism of method name is resolved by list of arguments constructors are not handed down

21 Object-oriented Programming 8 visibility visibility of classes if in the same package => standard access control use of keyword (modifier) public => visible if package is visibility of variables and methods: modifier same class in same package private visible in sub-class all over none visible visible protected visible visible visible public visible visible visible visible if class is

22 Object-oriented Programming 9 other modifiers: modifier for variables for methods for classes final symbolic constant write-protected in sub-class static one variable per can only access class, not instance class variables abstract only declaration, not definition, which is done in sub-class no sub-class can be derived contains abstract methods, no instantiation is possible (C++: virtual)

23 Object-orientated Programming 10 interfaces define a virtual class, where all methods are abstract : public interface Output { void Write(boolean b); void Write(char c);... public class StandardOutput implements Output { public void Write(boolean b){ System.out.print(b);... multiple inheritance possible, e.g.: public class X implements Y, Z {...

24 Object-oriented Programming 11 miscellaneous inner and anonymous classes fields simple objects, allocated dynamically super-class is an object examples: int[] v, w; //variables for referencing v={1,2,3,4; //creation of an initialized field w=new int[1000]; //creating an empty field v=w; //assigning a reference int i=v[0]; //accessing field at 1st pos. i=v.length; //accessing variable length multi-dimensional fields double[][] m={{1.0,0.0,{0.0,1.0;

25 Standard Libraries 1 pre-defined packages (libraries, included via import) java.lang basic classes, such as string, object, thread,... java.util data structures such as trees, hash tables,... java.text international formats such as date, time,... java.io Input / Output java.awt graphical user interfaces java.math arithmetics in arbitrary precision java.net network support, TCP/IP, DNS-access,... java.applet applets java.rmi remote method invocation java.security security such as signatures, authentification,... java.beans Java Beans (components that can be visualized) java.sql data base access via SQL interface

26 Standard Libraries 2 Input / Output console, files, pipes byte-/character-based buffer / un-buffered based on streams: uni-directional connections of sender and receiver large hierarchy of classes with multiple variants for In- / Output example System.out.println("Hello World!") ~ buffered character-based Output at stdout

27 Standard Libraries 3 Example: (echo of stdin) import java.io.*; public class Echo { public static void main(string args[]) throws IOException { BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out)); String s; for(;;) { s=br.readline(); bw.write(s); bw.newline(); bw.flush();

28 Standard Libraries 4 AWT: Abstract Windowing Toolkit components for all standard graphical user interfaces buttons, dialogues, menues, scroll bars,... events such as mouse-clicks, pressing of buttons etc., are recognized by AWT-threads component have their listeners (need to be logged in), which are called in case of respective events

29 Standard Libraries 5 Example: ( if button is pressed => counter++ => radius of circle increases) import java.applet.*; import java.awt.*; import java.awt.event.*; public class testawt extends Applet { private int z=0; private Label counter; private Button biggercircle; private ScrollPane scroller; private MyApplet circle; public void init() { add(new Label("Zaehlerstand:")); add(counter=new Label("0")); add(biggercircle=new Button("Groesser")); biggercircle.addactionlistener(new BiggerCircleButtonL()); add(scroller=new ScrollPane()); scroller.add(circle=new MyApplet());

30 Standard Libraries 6 class MyApplet extends Applet { public void paint(graphics g) { g.filloval(0,0,10+z*10,10+z*10); class BiggerCircleButtonL implements ActionListener { public void actionperformed(actionevent e) { z++; counter.settext(string.valueof(z)); circle.repaint();

Core JAVA Training Syllabus FEE: RS. 8000/-

Core JAVA Training Syllabus FEE: RS. 8000/- About JAVA Java is a high-level programming language, developed by James Gosling at Sun Microsystems as a core component of the Java platform. Java follows the "write once, run anywhere" concept, as it