programming languages

compiled

• Algol
• C
• C++
• Fortran
• (Latex)
• Lisp
• Java
• Pascal

development environments

• Eclipse

compilation and execution

javac – Java programming language compiler

• javac Program.java:
  compile the source code Program.java
  and create the executable Program.class
• the executable .class file is independent of the
  operating system (Linux, Windows, ...)

java – the Java application launcher

• java Program: launch (execute) the Java executable Program.class
• java -jar file.jar: launches the Java program in the package file.jar
• abort with Ctrl + C

javadoc – creating html documentation

• javadoc Program: scans the Java code Program.java for comments
  formatted in javadoc style and creates html-doc pages
• all official Java docs are creates this way

a first program

trivial Java source example

• create file HelloWorld.java
• copy/paste complete source code
• compile source code using javac HelloWorld.java
• run program using java HelloWorld

/** Class HelloWorld prints out a standard string.
 * This is a comment in javadoc format.
 * @date 11.11.2011
 * @author Hebert Fasching
 * @version 11.11
 */
public class HelloWorld {
  public static void main(String[] args){
// --- this is another comment, explaining the command
// --- print the line (println) 'Hello World!' 
// --- the standard (default) output, the console
    System.out.println("Hello World!");
  }
}

entry point and scope

• class: the fundamental building block;
  nothing exists outside a class (object)
• main(): entry point; here does the execution starts,
  stopping automatically at the end of main(){}
• main() is a member function of the class HelloWorld;
  there is a strict naming convention
• { ... }: scope
  
  • defines begining and end of a class, function, control loop, ...
  • everything defined within a given scope is not known outside
  • if available use keyboards with english layout
• a program is a sequence of commands, any command ends with a semicolon ;

class HelloWorld {
  public static void main(String[] args){
    System.out.println("Hello World!");
  }
}

example: calculation

• example programs, systematics of commands and definitions later
• the format specfiers like %3d used for
  formated printing with prinf will be explained later

/** Evaluates sum of integers.
  */
public class ExampleCalc {
  public static void main(String[] args)
    {

// --- define an integer variable and set its value to zero
    int sum = 0;                                 

// --- loop example: do a calculation many times
// --- increase 'index' by one as long as 'index' is smaller than 10
    for (int index=0; index<10; index=index+1)
      {
      sum = sum + index;
      System.out.printf("index and sum: %2d %3d\n",index, sum);
      }  // the loop over 'index' ends here
    }  // the main() routine of the class ExampleCalc ends here
} // the class ExampleCalc ends here

example: functions

• . : dot operator for access to member functions and variables
• return the result of a function
• predefined mathematical functions in class Math

/** Evaluates the area of a circle.
  */
public class ExampleFunction {

/** Caclulates the area of a circle with of a given input radius.
  * Both input and return value are real (double precison) numbers.
  */
  static public double area(double radius)
    {
// --- the predefined (real) constant 'PI' of the predefined 
// --- class Math is accessed via the dot operator
    return Math.PI*radius*radius;
    }

  public static void main(String[] args)
    {

// --- define real(double) precision variables
    double result;
    double inputRadius = 10.0;
   
// --- the member function area() of the class ExampleFunction
// --- is accessed via the dot operator
    result = ExampleFunction.area(inputRadius);

// --- a command end with a semicolon, can wrap an
// --- arbitrary number of lines
    System.out.printf("radius and area: %10.4f %10.4f\n",
                      inputRadius, result);

// --- another time
    inputRadius = Math.sqrt(10.0);
    result = ExampleFunction.area(inputRadius);
    System.out.printf("radius and area: %10.4f %10.4f\n",
                      inputRadius, result);
  
    }
}

example: input and output

• Scanner: a predefined Java class to access console input
• information about Java classes available on the web,   →   search
• with new a new object is created (instantiated), more details lateron

import java.util.Scanner;
 
public class ExampleIO {

/** A simple subroutine function multiplying two integer numbers
  * and returning the integer (int) result.
  */
  public static int area(int length, int width) {
    int result = length * width;                           // make calculation
    return result;                                         // return result
  }
 
  public static void main(String[] args) {

// --- creating an object of type Scanner named scanner
// --- we will learn lateron what this does exactly means
    Scanner scanner = new Scanner(System.in).useDelimiter("\n");

    System.out.print("Enter length: ");                    // input
    int a = scanner.nextInt();                             // length
    System.out.print("Enter width: ");
    int b = scanner.nextInt();                             // width

// -- call the (static) function area() of the class ExampleIO
    int calculatedValue = ExampleIO.area(a, b);   
    System.out.println("calculation complete");
    System.out.printf("area with length %d and width %d is %d\n", 
                      a, b, calculatedValue);              // formatted output
  }   // end of ExampleIO.main()
}   // end of class ExampleIO

output basics

• formated ouput is easier to read and
  cuts down debugging time substantially
• printf() takes exactly the same formating options as in C
  • %b: boolean
  • %c: single char
  • %d: integer of arbitrary length
  • %s: string of arbitrary length
  • %10s: allocat 10 spaces for the string
  • %12.4f: real number with 4 decimals
  • \n: end of line (linebreak)

public class OutputBasics 
{
public static void main(String[] args) 
  {           // non-standard bracket positioning
  final int nArgs = 1;
  if (args.length != nArgs)
    {         // non-standard bracket positioning
    return;
    }

  String[] sentence = {"This","is","a","sentence"};
  System.out.println("  ");
  for (int ss=0; ss<sentence.length; ss++)
    System.out.printf("%s ",sentence[ss]);
  System.out.printf("\n");

  double number = Double.parseDouble(args[0]);
  System.out.println("  ");
  System.out.printf("floating-point %10.3f number\n",number);
  System.out.printf("   exponential %10.3e number\n",number);
  System.out.printf("   fixed-width %10s string  \n","short");
  System.out.format("   fixed-width %10d integer \n",11);
             // printf (from C) and format are equivalent
  }
}

user@pc:~$ java OutputBasics 13.445
 
This is a sentence 
 
floating-point     13.445 number
   exponential  1.345e+01 number
   fixed-width      short string 
   fixed-width         11 integer 

language and locale

• the language setting is always via the locale
  browser, desktop, Java, ...
• attention
  german 3,141, US 3.141

import java.util.*;      // for Locale, Scanner

public class TestLocale {

public static void main(String[] args) {

// --- in- and ouput with German numbers "3 Komma 141"
  Locale.setDefault(Locale.GERMAN);              
  Scanner scanner = new Scanner(System.in).useDelimiter("\n");
  System.out.print("enter German floating point number: ");
  double rr = scanner.nextDouble();      
  System.out.printf("you have entered (German): %8.3f\n\n",rr);

// --- in- and ouput with US numbers "3 dot 141"
  Locale.setDefault(Locale.US);
  scanner = new Scanner(System.in).useDelimiter("\n");
  System.out.print("enter US floating point number: ");
  rr = scanner.nextDouble();      
  System.out.printf("you have entered (US): %8.3f\n\n",rr);

  }  // end of TestLocale.main()
}  // end of TestLocale

user@pc:~$ java TestLocale 
enter German floating point number: 3,345
you have entered (German):    3,345

enter US floating point number: 7.145
you have entered (US):    7.145