CS Programming I: File Input / Output

Transcription

1 CS Programming I: File Input / Output Marc Renault Department of Computer Sciences University of Wisconsin Madison Spring 2018 TopHat Sec 3 (AM) Join Code: TopHat Sec 4 (PM) Join Code:

2 Output Formatting

3 1/22 Output / String Formatting String.format(String format, Object... args) System.out.format(String format, Object... args) System.out.printf(String format, Object... args)

4 1/22 Output / String Formatting String.format(String format, Object... args) System.out.format(String format, Object... args) System.out.printf(String format, Object... args) String format The format string. A template string with placeholders (format specifiers) for the arguments.

5 1/22 Output / String Formatting String.format(String format, Object... args) System.out.format(String format, Object... args) System.out.printf(String format, Object... args) String format The format string. A template string with placeholders (format specifiers) for the arguments. Object... args Varargs Variadic or variable arity method. Allows any number of objects.

6 2/22 Format Specifiers Format Conversion Specification %[arg_idx$][flags][width][.precision]type

7 2/22 Format Specifiers Format Conversion Specification %[arg_idx$][flags][width][.precision]type arg_idx$ The position of the argument in the argument list (1-based).

8 2/22 Format Specifiers Format Conversion Specification %[arg_idx$][flags][width][.precision]type arg_idx$ The position of the argument in the argument list (1-based). flags - Left-align the output. + Prepend a + for positive numeric types. (space) prepend a space for positive numeric types. 0 Prepend with zeros (requires the width option). ( Negative numbers enclosed in parentheses., Locale-specific grouping separators.

9 2/22 Format Specifiers Format Conversion Specification %[arg_idx$][flags][width][.precision]type width Minimum number of characters.

10 2/22 Format Specifiers Format Conversion Specification %[arg_idx$][flags][width][.precision]type width Minimum number of characters..precision For floats: number of digits to the right of the decimal. For strings: maximum number of characters to output. Rest: Not applicable, causes exception.

11 2/22 Format Specifiers Format Conversion Specification %[arg_idx$][flags][width][.precision]type type b,b A boolean. s,s A string. c,c A char. d integer as a decimal integer. o integer as a octal integer. x,x integer as a hexadecimal integer. Note: Upper-case conversion converts argument to upper-case.

12 2/22 Format Specifiers Format Conversion Specification %[arg_idx$][flags][width][.precision]type type f float in decimal notation: [-]ddd.ddd. e,e float in scientific notation: [-]d.ddde±dd. a,a float in hexadecimal notation. g,g float in f or e (resp. E), depending on magnitude. % Prints a literal %. n Platform specific line separator. Note: Upper-case conversion converts argument to upper-case.

13 3/22 TopHat Question 1 What is the output? System. out. printf ("%B %d 0x %08 x", true, 1, 15);

14 4/22 TopHat Question 2 What is the output? System. out. printf ("%4 $c %3 $C %2c %1 $C", a, b, c, d );

15 5/22 TopHat Question 3 What is the output? System. out. printf ("%4 $C %c %3 $C %c %2 $C %c %1 $C %c", a, b, c, d );

16 Streams

17 6/22 Standard Output Stream Buffer: System. out. print (" Hello \ nworld!"); Console: System.out Reference to PrintStream object. Default behaviour is to output to console. Buffered output: Print methods adds data to buffer. Buffered data is output periodically.

18 6/22 Standard Output Stream System. out. print (" Hello \ nworld!"); Buffer: H e l l o \n W o r l d! Console: System.out Reference to PrintStream object. Default behaviour is to output to console. Buffered output: Print methods adds data to buffer. Buffered data is output periodically.

19 6/22 Standard Output Stream System. out. print (" Hello \ nworld!"); Buffer: W o r l d! Console: Hello System.out Reference to PrintStream object. Default behaviour is to output to console. Buffered output: Print methods adds data to buffer. Buffered data is output periodically.

20 6/22 Standard Output Stream Buffer: System. out. print (" Hello \ nworld!"); Console: Hello World! System.out Reference to PrintStream object. Default behaviour is to output to console. Buffered output: Print methods adds data to buffer. Buffered data is output periodically.

21 7/22 Standard Input Stream Buffer: Scanner sc = new Scanner ( System. in ); String s = sc. next (); Console: System.in Reference to InputStream object. Default behaviour is to input from the console. Buffered input: Input enters a buffer. Buffered data is read periodically.

22 7/22 Standard Input Stream Buffer: Scanner sc = new Scanner ( System. in ); String s = sc. next (); H e l l o W o r l d! Console: Hello World! System.in Reference to InputStream object. Default behaviour is to input from the console. Buffered input: Input enters a buffer. Buffered data is read periodically.

23 7/22 Standard Input Stream Buffer: Scanner sc = new Scanner ( System. in ); String s = sc. next (); W o r l d! Console: Hello World! System.in Reference to InputStream object. Default behaviour is to input from the console. Buffered input: Input enters a buffer. Buffered data is read periodically.

24 Paths

25 8/22 Navigating the File System / home renault workspace

26 8/22 Navigating the File System /home/renault/workspace

27 Navigating the File System /home/renault/workspace CS200Ex bin Data In Out src Palindrome bin src OrdinalEx bin src CmdLine bin src 8/22

28 Navigating the File System /home/renault/workspace CS200Ex bin Data In Out src Palindrome bin src OrdinalEx bin src CmdLine bin src Current Working Directory (CWD) Current location in the file system. Denoted by:. The directory from which the jvm was launched. 8/22

29 Navigating the File System /home/renault/workspace CS200Ex bin Data In Out src Palindrome bin src OrdinalEx bin src CmdLine bin src Current Working Directory (CWD) Current location in the file system. Denoted by:. The directory from which the jvm was launched. Relative Path A path relative to the CWD. To move up:.. 8/22

30 Navigating the File System /home/renault/workspace CS200Ex bin Data In Out src Palindrome bin src OrdinalEx bin src CmdLine bin src Current Working Directory (CWD) Current location in the file system. Denoted by:. The directory from which the jvm was launched. Relative Path A path relative to the CWD. To move up:.. Absolute Path A path that starts at the root of the file system. Root: / 8/22

31 Navigating the File System /home/renault/workspace CS200Ex bin Data In Out src Palindrome bin src OrdinalEx bin src CmdLine bin src TopHat Question 4 Is./Palindrome/src an absolute or a relative path? 8/22

32 Navigating the File System /home/renault/workspace CS200Ex bin Data In Out src Palindrome bin src OrdinalEx bin src CmdLine bin src TopHat Question 4 Is./Palindrome/src an absolute or a relative path? TopHat Question 5 Is /home/renault/workspace/cmdline an absolute or a relative path? 8/22

33 Navigating the File System /home/renault/workspace CS200Ex bin Data In Out src Palindrome bin src OrdinalEx bin src CmdLine bin src TopHat Question 4 Is./Palindrome/src an absolute or a relative path? TopHat Question 5 Is /home/renault/workspace/cmdline an absolute or a relative path? TopHat Question 6 If the CWD is /home/renault/workspace/cs200ex/bin, what is the (shortest) relative path to In? 8/22

34 File I/O

35 9/22 File Class java.io.file Constructor File f = new File(pathToFile) Creates a File object, but not an actual file on the disc. pathtofile Path to file. All the instance methods are base on this path. Permissions boolean canread() Can read the contents? boolean canwrite() Can change the contents? boolean canexecute() Can execute the path? There are also methods to set the permissions.

36 9/22 File Class java.io.file Constructor File f = new File(pathToFile) Creates a File object, but not an actual file on the disc. pathtofile Path to file. All the instance methods are base on this path. File Tests boolean exists() Does the file exist? boolean isfile() Is it a normal file? boolean isdirectory() Is it a directory? boolean ishidden() Is it a directory?

37 9/22 File Class java.io.file Constructor File f = new File(pathToFile) Creates a File object, but not an actual file on the disc. pathtofile Path to file. All the instance methods are base on this path. Other File Properties long lastmodified() Date last modified. long length() Number of bytes. String getpath() Returns the path. String getabsolutepath() Returns the absolute path. String getparent() Returns the parent from the path.

38 9/22 File Class java.io.file Constructor File f = new File(pathToFile) Creates a File object, but not an actual file on the disc. pathtofile Path to file. All the instance methods are base on this path. Basic Operations boolean createnewfile() Create an empty file. boolean renameto(file dest) Move to dest. boolean delete() Delete the path. void deleteonexit() Delete the path when the JVM terminates.

39 9/22 File Class java.io.file Constructor File f = new File(pathToFile) Creates a File object, but not an actual file on the disc. pathtofile Path to file. All the instance methods are base on this path. Directory Operations boolean mkdir() Create director based on path. boolean mkdirs() Create all the directories in the path. String[] list() List files contained in directory.

40 Eclipse CWD TopHat Question 7 What is the CWD in this case? This code is in an Eclipse project called CS200Example: import java.io. File ; public class FileInfo { public static void main ( String [] arg ){ File f = new File ("."); System. out. println (f. getpath ()); System. out. println (f. getabsolutepath ()); When run, the output is:. /home/renault/workspace/cs200examples/. 10/22

41 Creating Files TopHat Question 8 What is the output, assuming proper permissions and that the files don t yet exist? import java.io. File ; import java.io. IOException ; public class CreateFile { public static void main ( String [] arg ){ File f = new File (" tmp. txt "); File f2 = new File ("/home / renault /tmp2.txt "); File f3 = new File (File. separatorchar + "home " + File. separatorchar +" renault " + File. separatorchar +"tmp2.txt "); try { if (!f. createnewfile ()){ System.out. print ("The file tmp.txt already exists."); f2. createnewfile (); if (! f3. createnewfile ()){ System.out. print ("The file tmp2.txt already exists."); catch ( IOException e){ e. printstacktrace (); System.out. print (" Exception when creating the files."); 11/22

42 12/22 Deleting Files TopHat Question 9 What is the output, assuming proper permissions and that the files don t yet exist? import java.io. File ; public class DeleteFile { public static void main ( String [] args ){ File f = new File (" tmp. txt "); File f2 = new File ("t. txt "); try { f. createnewfile (); if(f. delete ()){ System.out. println ("File tmp.txt deleted "); if(f2. delete ()){ System.out. println ("File t.txt deleted "); catch ( Exception e){ e. printstacktrace (); System.out. println (" Exception creating file ");

43 13/22 Listing Files TopHat Question 10 How would you run the following code from the command line so that it prints out the contents of the current directory? import java.io. File ; public class FileListing { public static void main ( String [] args ) { File f = new File ( args [0]); for ( String s: f. list ()) { System. out. println (s);

44 14/22 File Input In Java, there are many ways to read a file. You can...

45 14/22 File Input In Java, there are many ways to read a file. You can... Start with... java.io.file java.io.filereader java.io.fileinputstream

46 14/22 File Input In Java, there are many ways to read a file. You can... Start with... java.io.file java.io.filereader java.io.fileinputstream Wrap it in... java.util.scanner java.io.reader wrapped in a java.io.bufferedreader

47 14/22 File Input In Java, there are many ways to read a file. You can... Start with... java.io.file java.io.filereader java.io.fileinputstream Wrap it in... java.util.scanner java.io.reader wrapped in a java.io.bufferedreader Exercise: Write a simple cat. Write a program that prints out every line from the filename passed in on the command line.

48 15/22 Simple cat (one file) with Scanner import java.io. File ; import java.io. FileNotFoundException ; import java.util. Scanner ; public class CatScannerFile { public static void main ( String [] args ) { if(args. length >= 1) { Scanner s = null ; try { File f = new File ( args [0]); s = new Scanner (f); while (s. hasnextline ()) { System.out. println (s. nextline ()); catch ( FileNotFoundException e) { System.out. println ("File not found : " + args [0]); finally { if(s!= null ) s. close ();

49 16/22 Simple cat (one file) with BufferedReader import java.io. FileReader ; import java.io. FileNotFoundException ; import java.io. BufferedReader ; import java.io. IOException ; public class CatBuffReadFile { public static void main ( String [] args ) throws IOException { if(args. length >= 1) { BufferedReader br = null ; FileReader fr = null ; try { fr = new FileReader ( args [0]); br = new BufferedReader (fr ); while (br. ready ()) { System.out. print (br. readline ()); catch ( FileNotFoundException e) { System.out. println ("File not found : " + args [0]); finally { if(br!= null ) br. close ();

50 17/22 Basic try with Resources try ( SomeCloseable1 a = new SomeCloseable1 (...); SomeCloseable2 b = new SomeCloseable2 (...) ) { trystmtblock ; try-with-resources New in Java 7. try statement in which you declare the resources. The declared resources are closed automatically in the opposite order of their creation. The resources must implement AutoCloseable.

51 18/22 try-with-resources-catch-finally TopHat Question 11 If an exception is thrown in the try statement block what happens in what order? try ( SomeCloseable1 a = new SomeCloseable1 (...); SomeCloseable2 b = new SomeCloseable2 (...) ) { trystmtblock ; catch ( Exception e) { catchstmtblock ; finally { finallystmtblock ;

52 19/22 Simple cat (one file) with Scanner and try-with-resources import java.io. File ; import java. io. FileNotFoundException ; import java. util. Scanner ; public class CatScannerFileTryRes { public static void main ( String [] args ) { if( args. length >= 1) { try ( Scanner s = new Scanner ( new File ( args [0]))) { while (s. hasnextline ()) { System. out. println (s. nextline ()); catch ( FileNotFoundException e) { System. out. println (" File not found : " + args [0]);

53 20/22 File Output In Java, there are many ways to read a file. You can...

54 20/22 File Output In Java, there are many ways to read a file. You can... Start with... java.io.file java.io.filewriter java.io.fileoutputstream

55 20/22 File Output In Java, there are many ways to read a file. You can... Start with... java.io.file java.io.filewriter java.io.fileoutputstream Wrap it in... java.io.printwriter java.io.writer wrapped in a java.io.bufferedwriter

56 20/22 File Output In Java, there are many ways to read a file. You can... Start with... java.io.file java.io.filewriter java.io.fileoutputstream Wrap it in... java.io.printwriter java.io.writer wrapped in a java.io.bufferedwriter Some Examples zybooks has examples of FileOutputStream wrapped in a PrintWriter.

57 20/22 File Output In Java, there are many ways to read a file. You can... Start with... java.io.file java.io.filewriter java.io.fileoutputstream Wrap it in... java.io.printwriter java.io.writer wrapped in a java.io.bufferedwriter Some Examples zybooks has examples of FileOutputStream wrapped in a PrintWriter. Let s consider File wrapped in a PrintWriter.

58 Output Example TopHat Question 12 How many lines are written to test.txt when the code is executed with java FileOutput 10 test.txt 10000? import java.util. Random ; import java.io. File ; import java.io. PrintWriter ; import java.io. FileNotFoundException ; public class FileOutput { public static void main ( String [] args ) { if(args. length >= 3) { int numvals = Integer. parseint (args [0]); int modcols = numvals % 4; numvals = numvals + ( modcols!= 0? 4 - modcols : 0); try ( PrintWriter pout = new PrintWriter (new File (args [1]))) { Random rand = new Random (); for ( int i = 0; i < numvals ; i += 4) pout. printf ("%8d\t%8d\t%8d\t%8d\n", rand. nextint ( Integer. parseint (args [2])), rand. nextint ( Integer. parseint (args [2])), rand. nextint ( Integer. parseint (args [2])), rand. nextint ( Integer. parseint (args [2]))); catch ( FileNotFoundException e) { System.out. println (" Unable to open output file : " + args [1]); 21/22

59 22/22 Further Reading COMP SCI 200: Programming I zybooks.com, zybook code: WISCCOMPSCI200Spring2018 Chapter 11. File Input / Output

60 Appendix References Appendix

61 Appendix References References

62 Appendix References 23/22 Image Sources I