Programmierpraktikum

Transcription

1 Programmierpraktikum Claudius Gros, SS2012 Institut für theoretische Physik Goethe-University Frankfurt a.m. 1 of 21 05/07/ :31 AM

2 Input / Output Streams 2 of 21 05/07/ :31 AM

3 Java I/O streams stream from/to everywhere screen file internet connection... transmit all kind of data byte, char, int,... string, objects,... 3 of 21 05/07/ :31 AM

4 java.io IO - communiation with the world input / output a class for every type of stream modern programming know what exists look it up exception handeling mandatory for IO input files may not be present possibly no writing permissions on output... 4 of 21 05/07/ :31 AM

5 byte streams IO byte per byte input/output byte by byte is low level, inefficient read() returns integer (Unicode, table) : -1 at end null: object not yet been instantiated import java.io.fileinputstream; import java.io.fileoutputstream; import java.io.ioexception; // normally just use import java.io.*; public class CopyBytes public static void main(string[] args) throws IOException // what happens if you would just use // public static void main(string[] args) FileInputStream in = null; FileOutputStream out = null; try in = new FileInputStream("input.txt"); out = new FileOutputStream("output.txt"); int c; // *** start loop over all bytes in input file while ((c = in.read())!= -1) out.write(c); // *** all reading has been done 5 of 21 05/07/ :31 AM

6 finally // finally: always close streams if (in!= null) in.close(); if (out!= null) out.close(); appending to output file with true catch exception import java.io.*; public class CopyBytes public static void main(string[] args) // no exception thrown try FileInputStream instream = new FileInputStream("input.txt"); FileOutputStream outstream = new FileOutputStream("output.txt",true); // are equivalent new FileOutputStream("output.txt",false); // new FileOutputStream("output.txt"); // new FileOutputStream("output.txt",(1==2)); int c; while ((c = instream.read())!= -1) outstream.write(c); catch(ioexception e) System.out.println(e.toString()); // end of CopyBytes.main() // end of CopyBytes 6 of 21 05/07/ :31 AM

7 formatted output PrintWriter: has member function.print().printf() formatted output is alway sent to a buffer, and not directly to the output file use member function.flush() to sent bufffer to file import java.io.*; public class FormattedOutput public static void main(string[] args) try PrintWriter myoutput = new PrintWriter("output.txt"); for (int i=0;i<8;i++) myoutput.printf("%2d modulo 3 is %2d\n",i,i%3); myoutput.flush(); // sent buffer to file catch(ioexception e) System.out.println(e.toString()); // end of FormattedOutput.main() // end of FormattedOutput 7 of 21 05/07/ :31 AM

8 buffered input buffering every read/write request to disk is slow (try to read/write a 100 GByte file without buffering) buffering: systems reads/writes large chunks of data to file at once BufferedReader: wrapper around a Reader has member function.readline() 8 of 21 05/07/ :31 AM

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10 buffered input with formatted output FileInputStream: byte Reader: 8 bits FileReader: char Reader: 16 bits reading String data line by line import java.io.filereader; // character reader import java.io.filewriter; // character writer import java.io.bufferedreader; // buffering input import java.io.printwriter; // formatting output import java.io.ioexception; public class CopyLines public static void main(string[] args) try BufferedReader inputstream = new BufferedReader(new FileReader("input.txt")); PrintWriter outputstream = new PrintWriter(new FileWriter("output.txt")); // or new PrintWriter("output.txt"); String s; while ((s = inputstream.readline())!= null) outputstream.println(s); outputstream.flush(); catch(ioexception e) System.out.println(e.toString()); // end of CopyLines.main() // end of CopyLines 10 of 21 05/07/ :31 AM

11 buffered and formatted ouput a must: buffering input/output for large data sizes import java.io.*; public class FormattedOutput public static void main(string[] args) try PrintWriter myoutput = new PrintWriter( // for formatting new BufferedWriter( // for buffering new FileWriter("output.txt"))); // ouput stream for (int i=0;i<8;i++) myoutput.printf("%2d modulo 5 is %2d\n",i,i%5); myoutput.flush(); // sent buffer to file catch(ioexception e) System.out.println(e.toString()); // end of FormattedOutput.main() // end of FormattedOutput 11 of 21 05/07/ :31 AM

12 scientific data files how to read lists of numbers? typical science data file # list of planet properties by E.T. Skywatcher # actually from # # Planet Mass (* ME) # Diameter (km) # density g/cm3 # oblateness [=(De-Dp)/De] # rotation [h or d] # distance (A.U.) # revolution [d or y] # eccentricity # inclination [deg] # axis tilt [deg] # # String double double double double double String double String double double double # ndata ### 9 Mercury d d Venus d d Earth h d Mars h d Jupiter h y Saturn h y Uranus h y Neptune h y Pluto d y always include data description in comment section 12 of 21 05/07/ :31 AM

13 lines starting with a hashtag # are ignored by most plotting utilities 13 of 21 05/07/ :31 AM

14 scanning data input files scanning for data: double, int,... Scanner wraps a Reader new Scanner(System.in) for console data input has member functions Boolean.hasNext() : has next entry, seperated by blanks String.next() : next entry as a String object int.nextint() : next entry as a int primitive double.nextdouble() : next entry as a double primitive... note: use (string1.equals(string2)) for comparing two strings, (string1==string2) is not secure import java.io.*; import java.util.scanner; public class DataInput public static void main(string[] args) try Scanner myscanner = new Scanner( new FileInputStream("planetsData.list")); 14 of 21 05/07/ :31 AM

15 boolean readingcomments = true; while (readingcomments) // skipping initial comments if (myscanner.next().equals("###")) readingcomments = false; int ndata = myscanner.nextint(); System.out.printf("# ndata %7d \n",ndata); String name; double mass; int diameter; while (myscanner.hasnext()) name = myscanner.next(); mass = myscanner.nextdouble(); diameter = myscanner.nextint(); System.out.printf("# %10s %7.2f %8d \n",name, mass, diameter); for (int i=0;i<10;i++) myscanner.next(); // skipping rest of line catch(ioexception e) System.out.println(e.toString()); // end of DataInput.main() // end of DataInput 15 of 21 05/07/ :31 AM

16 output of objects object output streams all classes descendents of the Object class instantiated Serializable objects can be sent to output in one piece all classes can be made Serializable public class myclass implements Serializable Array objects are serializable per definition on input one needs to check the type of Object read using instanceof data files containing serializabled objects are binary and not human readable import java.io.*; public class ObjectsIo public static void main(string[] args) // *** // *** generate objects and write objects to file // *** String[] stringarrayobject = "This","is","an","array","of","strings"; int[] intarrayobject = 4444,333,22,1; try ObjectOutputStream myoutput = new ObjectOutputStream( new FileOutputStream("containsObject.txt")); 16 of 21 05/07/ :31 AM

17 myoutput.writeobject(stringarrayobject); myoutput.writeobject(intarrayobject); catch(ioexception e) System.out.println(e.toString()); // *** // *** read objects from file // *** try ObjectInputStream myinput = new ObjectInputStream( new FileInputStream("containsObject.txt")); Object obj = null; while ((obj = myinput.readobject())!= null) if (obj instanceof String[]) // string array String[] ss = (String[])obj; // casting Object to String[] for (int j=0;j<ss.length;j++) System.out.printf("%s ",ss[j]); System.out.printf("\n"); if (obj instanceof int[]) // int array int[] ii = (int[])obj; // casting Object to int[] for (int j=0;j<ii.length;j++) System.out.printf("%d ",ii[j]); System.out.printf("\n"); catch (EOFException ex) // EOF exception System.out.println("End of file reached."); catch(ioexception e) System.out.println(e.toString()); catch (ClassNotFoundException e) System.out.println(e.toString()); 17 of 21 05/07/ :31 AM

18 // end of ObjectsIo.main() // end of ObjectsIo 18 of 21 05/07/ :31 AM

19 exercise: IO and OOP Write a program for object oriented input/output. define a serializable class Planet storing all planet properties read the planet data file and create an instantiation of Planet for every planet write all instantiated Planet objects to file read the Planet objects from file, print data to console as a formatted table 19 of 21 05/07/ :31 AM

20 directory listings System.getProperty() for various properties of operating system.isdirectory() /.isfile() for File objects import java.util.*; import java.io.*; import java.math.*; class DirectoryHandling public static void main(string args[]) // *** // *** open file handler / directory listing // *** String inputdirpath = null; if (args.length==1) inputdirpath = args[0]; // directory path else if (args.length==0) inputdirpath = System.getProperty("user.dir"); // working directory else System.out.printf("number of arguments must be 0/1 and not %d\n", args.length); return; // exit main() File inputdir = new File(inputDirPath); // open file or directory String[] inputdirfiles = null; if (inputdir.isdirectory()) // got a directory? inputdirfiles = inputdir.list(); else System.out.println("input not a directory: " + inputdirpath); 20 of 21 05/07/ :31 AM

21 return; // exit main() // *** // *** (limited) output of directories/files found // *** String fileseparator = System.getProperty("file.separator"); // is "/" on UNIX and "\" on Windows // System.out.println(" "); System.out.println("files and directories found in: " + inputdirpath); System.out.println(" "); for (int i=0;i<math.min(10,inputdirfiles.length);i++) String fullpath = inputdirpath + fileseparator + inputdirfiles[i]; File listfile = new File(fullPath) ; // open file or directory if (listfile.isfile()) // got a file? System.out.println("file: " + fullpath); else System.out.println(" dir: " + fullpath); // end of DirectoryHandling.main() // end of DirectoryHandling 21 of 21 05/07/ :31 AM