©SoftMoore ConsultingSlide 1 Appendix G: Java Input/Output.

©SoftMoore Consulting Slide 1

Appendix G:Java Input/Output


The March of Progress(Cay Horstmann)

• 1980: Cprintf("%10.2f", x);

• 1988: C++cout << setw(10) << setprecision(2) << showpoint << x;

• 1996: Javajava.text.NumberFormat formatter = java.text.NumberFormat.getNumberInstance();formatter.setMinimumFractionDigits(2);formatter.setMaximumFractionDigits(2);String s = formatter.format(x);for (int i = s.length(); i < 10; i++) System.out.print(' ');System.out.print(s);

• 2004: JavaSystem.out.printf("%10.2f", x);

The printf() Method

• Java 5 (2004) added a printf() method to classes PrintStream and PrintWriter. The printf() method in Java is used in a manner similar to the printf() function in C.

• Each printf() method has at least two arguments– format string, which may contain fixed text and one or more

embedded format specifiers– a variable number of arguments whose values are printed

according to the format specifiers

• ExampleSystem.out.printf("The answer is %10.2f", x);



Simplified Input: Class Scanner

• In addition to the printf() method for output, Java 5 also provides the java.util.Scanner class to simplify input.

• Example: Before Java 5BufferedReader br = new BufferedReader (new InputStreamReader(System.in);String str = br.readline();int n = Integer.parseInt(str);

• Example: Using the Scanner classScanner s = new Scanner(System.in);int n = s.nextInt();


The File Class

• Abstract file name

• Not an actual file!

• Helps deal with platform differences– methods to manipulate name– rename/delete files– create directories– list directory contents

• ExampleFile a = new File("/usr/local/bin/foo");


File Names

Some information depends on VM

• Start of path

• Physical drives

• SlashesFile b = new File("bin/foo");

• Example (Windows file names)File c = new File("c:\\windows\\system\\foo.gif");File d = new File("system\\foo.gif");

(Note the double backslashes!)


Files Relative to Files

Can pass a File to the constructor as parent directory File g = new File("/windows/system"); File h = new File(g, "foo.gif"); File i = new File("/windows/system", "foo.gif");

(h and i refer to the same file)


Selected File Methods

boolean isDirectory() long lastModified() long length()

int compareTo(File) String getName() String getParent() File getParentFile() String getPath() URL toURL()

boolean createNewFile() boolean delete() boolean mkdir() boolean renameTo(File)

String[] list() File[] listFiles()


Unicode

• In Java, strings are two-byte Unicode characters

• Unicode - standard that represents many worldwide characters in two bytes

• Unicode chars 0-127 are ASCII– Other “pages” represent other character sets– Different languages– Dingbats– Currency symbols– Mathematical symbols– … and many others


Text Translation

• Text I/O classes translate native chars to/from Unicode

• I/O package provides "default mapping"

• Reading text: I/O classes translate from native to Unicode

• Writing text: I/O classes translate from Unicode to native

• Text I/O versus binary I/O– Reader and Writer classes handle text I/O (character

translation performed)– InputStream and OutputStream classes handle binary I/O

(bytes read/written as is)


Reading and Writing Files

• To read/write files on a disk:FileInputStreamFileOutputStreamFileReaderFileWriter

• Provided Constructorsclass(File)class(String)class(FileDescriptor)class(String, boolean)- [output only]


File Copy Example

FileInputStream inFile = new FileInputStream(args[0]);FileOutputStream outFile = new FileOutputStream(args[1]);

int byteRead;

while((byteRead = inFile.read()) != -1) outFile.write(byteRead);

inFile.close();outFile.close();

• Slow implementation!– reading/writing single character at a time– could be really bad over a network connection

• Can provide simple buffering using byte[] forms of read() and write()


Buffered File Copy

FileInputStream inFile = new FileInputStream(args[0]); FileOutputStream outFile = new FileOutputStream(args[1]); byte[] buffer = new byte[1024];int i; while((i = inFile.read(buffer)) > 0) outFile.write(buffer, 0, i); inFile.close();outFile.close();


Common I/O Filters

I/O package comes with several useful filters

• buffering to make I/O more efficient

• encoding primitive types

• writing the state of entire objects

• compressing I/O

• and many more!


Buffering

• Writing a single byte or character at a time is inefficient

• Better to write a block of bytes or chars in a single write()

• java.io includes: BufferedInputStream BufferedOutputStream BufferedReader BufferedWriter

• You should almost always use these!


Buffering example

BufferedInputStream bis = new BufferedInputStream(new FileInputStream("a.bin"));

int byte = bis.read(); // reads a block of bytes from // a.bin and returns onebyte = bis.read(); // returns next bytebyte = bis.read(); // returns next byte

bis.close(); // always close outermost!


Reading Lines of Text

• BufferedReader provides method readLine()– reads all characters up to a “newline”– returns String containing those characters without newline

• ExampleBufferedReader br = new BufferedReader(new FileReader(args[0]));

String line = null;

while((line = br.readLine()) != null) System.out.println(line);

br.close(); // always close outermost!


Print Streams

• PrintStream and PrintWriter provide primitive/object value output

• System.out and System.err are two predefined instances of PrintStream

• print() and println() exist for each primitive type and for Object– print() converts its argument to a String, then writes it to its

delegate– println() does the same, but follows with the appropriate

newline


Flushing and PrintStreams

• Common use of PrintStream is communicating status to user– Flushing output becomes significant!– When you print output, you need to flush periodically

• ExamplePrintStream ps = new PrintStream(new FileOuputStream("foo.txt"));

ps.println("Hello!");ps.flush();

ps.println("How are you?");ps.flush();


Automatic Flushing

• Flushing can be easy to forget

• PrintStream and PrintWriter can “autoflush”– disabled by default.– enabled by passing true to PrintStream or PrintWriter

constructor

• ExamplePrintStream ps = new PrintStream(new FileOutputStream("foo.txt"), true);

ps.println("Hello!");ps.println("How are you?");


PrintStream and Text Output

• Note that PrintStream is commonly used for text output!– converts chars to appropriate binary on current platform– output is filtered to convert characters– allows backward compatibility with earlier JDK

• You should always use PrintWriter for non-System.out text output – it’s more efficient


IO Exceptions and PrintStream

• PrintStream and PrintWriter do not throw exceptions!

• System.out and System.err are very heavily used. Exception handling would be painful.

• PrintStream and PrintWriter catch their own exceptions and set an error flag.

• If you using these for real output, call checkError() to see if an error has occurred.


Primitive Data Input and Output

• You can store primitive types via PrintStream

• Reading these values requires parsing from String values! This is inefficient!

• Better way: write binary data to a file


Writing Objects

• Writing object values – object serialization

• Uses interfaces ObjectInput and ObjectOutput– ObjectInputStream and ObjectOutputStream implement

these interfaces

• Interface Serializable “marks” a class to be written

• Serialization covered in the Object Serialization course module – no further coverage here


Platform Specifics

• File systems vary greatly between operating systems– many “common concepts,” but implementations vary significantly – many “unique” features; e.g., UNIX's permission levels– Case-sensitive/insensitive– Characters allowed in file names– Many more

• I/O Package Helps, But Be Careful!– I/O is the easiest portability killer in applications.– Easy to create applications that work on your platform, but not

others.– Always think about portability issues when writing I/O code.


Characters in File Names

• Each platform defines– valid file name characters– case significant or not– file name length

• Examples– asterisk ('*')– space (' ') (some platforms allow, others don't)

• For maximum portability, restrict file name characters – letters (A-Z and a-z)– numbers (0-9)– start file names with a letter


File Name Case Sensitivity

• Unix is case sensitive,

• Windows is not.

• Other platforms differ as well.

• Do the following names refer to the same file?littleMissMuffet.txtLittleMissMuffet.txtlittleMissMuffet.TXT

• Recommendations– Use the same case always.– If possible, use all-lower case.– Never intentionally use same name with different cases.


New-Line Characters

• Where does one “line” of text end and the next begin?

• Three general patterns: – "\n" the “Unix way”– "\r" the “Macintosh way”– "\r\n" the “DOS/Windows way”

• This is the easiest trap to fall into with I/O


File Separator in Path Names

• Which character separates files/directories in path names?– Unix uses “/”– Windows uses “\” (which must be represented as “\\” in

strings.

• Recall that File “does the right thing” for slashes.

• Use system property file.separator for portability.


Path and Class Path Sequence Separator

c:;c:\programs [Windows]/usr/local/bin:/bin [UNIX]

• Note semicolon (';') use in Windows sequence, colon (':') in UNIX sequence

• These are “path separation characters”(differ between platforms)

• Use system property path.separator for portability.


Common I/O Exceptions

• IOException– General I/O exception– Superclass for all other I/O exceptions– Something undesirable happened while performing I/O

• EOFException– End of stream unexpectedly reached– Normally "end-of-file" is signaled by special value (e.g., 1)

• FileNotFoundException– File specified in constructor does not exist– Could also mean file being opened in wrong mode, trying to

open read-only file for write access


Closing Streams

Difficult I/O operation – several subtle problems

• Where to put close() call?

• How to deal with exceptions when opening filters?

• What if filter's close() throws exception?


Correct Approach for Simple File Closing

FileReader r = null;try { r = new FileReader("somefile.txt"); // NOTE: No close()! }catch(IOException e) { // message … }finally { if (r != null) try { r.close(); } catch(Exception ignore) {} // couldn't close -- inform user if important }


Using Finally to Copy Files Safely

public void copy(File f1, File f2) throws IOException { InputStream in = null; OutputStream out = null; try { in = new BufferedInputStream(new FileInputStream(f1)); out = new BufferedOutputStream(new FileOutputStream(f2)); int x;

while ((x = in.read()) != -1) out.write(x); }


Using Finally to Copy Files Safely(continued)

finally { if (in != null) try { in.close(); } catch (Exception ignore) {}

if (out != null) try { out.close(); } catch (Exception ignore) {} } }

New I/O

• In version 1.4 (2002), Java introduced several “new I/O” capabilities in package java.nio, including– Character set encoders and decoders– Nonblocking I/O– Memory-mapped files– File locking

• Java 7 contains additional enhancements, including– class Path (enhancement of class File)– class FileSystem– class WatchService (file change notification)– file attributes


©SoftMoore ConsultingSlide 1 Appendix G: Java Input/Output.

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