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Java Input/Output

Java Input/Output. Goals. Understanding Java’s File Management i.e Handling files and directories through File class Understanding which streams to use for character-based or byte-based streams Character file input and output Formatting output Reading data from the console

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Java Input/Output

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  1. Java Input/Output

  2. Goals • Understanding Java’s File Management i.e Handling files and directories through Fileclass • Understanding which streams to use for character-based or byte-based streams • Character file input and output • Formatting output • Reading data from the console • Binary file input and output

  3. File Management • File system is one of the most basic services provided by the OS • Most of the file systems allow hierarchical directory structure • Files are identified by filenames limited in length (32-256) • Although the basic services are identical in all OSs, their exact implementations make them mutually incompatible

  4. Java’s File Management • Java classFiledefines platform-independent manipulation of file system (Files & directories) by providing whether the file • exists • is read protected • is write protected • is, in fact, a directory • Instances of File class are immutable

  5. File Class • A File object can refer to either a file or directory File file1 = new File(“data.txt”); File file2 = new File(“C:\Java”); • To obtain the path to the current working directory use System.getProperty(“user.dir”); • To obtain the file or path separator use System.getProperty (“file.separator”); System.getProperty (“path.separator”); Or File.separator File.pathSeparator

  6. Useful File Methods • isFile/isDirectory • canRead/canWrite • length • Length of the file in bytes (long) or 0 if nonexistent • list • If the file object is a directory, returns a String array of all the files and directories contained in the directory; otherwise, null • mkdir • Creates a new subdirectory • delete • Deletes the directory and returns true if successful • toURL • Converts the file path to URL object

  7. Directory Listing Example import java.io.*; public class DirListing { public static void main(String[] args) { File dir = new File(System.getProperty(“user.dir”)); if (dir.isDirectory()) { System.out.println(“Directory of ” + dir); String[] listing = dir.list(); for (int i=0; i < listing.length; i++) { System.out.println(“\t” + listing[i]); } } } }

  8. Directory Listing, Result > java DirListing Direcotry of c:\Java\ DirListing.class DirListing.java Test TryCatchExample.class TryCatchExample.java XslTransformer.class XslTransformer.java

  9. Java IO package provides over 60 input/output classes (streams) Streams are ordered sequence of data that have a source (input), or a destination (output) Input/Output Streams bytes bytes OutputStream InputStream 010011001 010011001

  10. Streams • An abstraction of the continuous one-way of data • Transfers a sequence of bytes to or from • Disk files • Network connections • Memory • Threads • Another Computer • Streams are either byte-oriented or character-oriented • An IOExaception may occur during any IO operation

  11. Reading open a stream while more information read information close the stream Writing open a stream while more information write information close the stream Basic IO Algorithm

  12. The Stream Zoo Character Streams Byte Streams

  13. Character Streams • Read and write 16-bit characters • Reader and Writer are the abstract classes • Use readers and writers to read and write textual information

  14. Byte Streams • Read and write 8-bit bytes • InputStream and OutputStream are abstract super classes • Typically used to read and write binary data such as images and sounds

  15. Data Streams • Grouping of classes based on their purpose is often more convenient rather their data type, so that streams can be cross-grouped by: • Whether they read from and write to data ”sinks” or • Process the information as it is being read or written Byte Streams Character Streams Data Sink Streams Processing Streams

  16. Reader int read(); int read(char cbuf[]); int read(char cbuf[], int offset, int length) Writer int write(int c); int write(char cbuf[]); int write(char cbuf[], int offset, int length) InputStream int read(); intread(byte buffer[]); int read(byte buffer[], int offset, int length) OutputStream int write(byte b); int write(byte buffer[]); int write(byte buffer[], int offset, int length) I/O Super Classes

  17. Streams • All of the streams are automatically opened when created • Any stream can be closed explicitly by calling its close() method

  18. Using Character File Streams import java.io.*; public class Copy { public static void main(String[] args) throws IOException { File in = new File(“src.txt"); File out = new File("out.txt"); FileReader in = new FileReader(in); FileWriter out = new FileWriter(out); int c; while ((c = in.read()) != -1) { out.write(c); } in.close(); out.close(); } } FileReader in FileWriter out

  19. Using String Streams import java.io.*; class Capitalize { public static void main(String[] args) { Reader in = new StringReader (“advance programming”); Writer out = new StringWriter(10); int nextChar = 0; while ((nextChar = in.read()) != -1) { out.write(Character.toUpperCase ((char)nextChar)); } System.out.println(out.toString()); in.close(); out.close(); } } ADVANCE PRGRAMMING

  20. Buffer Streams • Buffer data while reading or writing, thereby reducing the number of accesses required on the original data source. • More efficient than similar non-buffered streams and are often used with other streams • The buffer size may be specified, or default size may be accepted BufferedReader BufferedWriter BufferedInputStream BufferedOutputStream Byte Streams Character Streams

  21. Using Buffer Streams import java.io.*; public class Copy { public static void main(String[] args) throws IOException { // opening the streams FileReader in = new FileReader (“infile.txt"); BufferedReader br = new BufferedReader(in); FileWriter out = new FileWriter ("outfile.txt"); BufferedWriter bw = new BufferedWriter(out); // processing the streams String aLine = null; while ((aLine = br.readLine()) != null) { bw.write(aLine, 0, aLine.length()); } // closing the streams in.close(); out.close(); } }

  22. Console Input • To read input from the console, a stream must be associated with the standard input, System.in import java.io.*; public class IOInput { public static void main(String[] args) { BufferedReader keyboard; String aLine; try { System.out.print(“Enter value: “); System.flush(); keyboard = new BufferedReader( new InputStreamReader(System.in)); aLine = keyboard.readLine(); } catch (IOExcerption ex) { System.out.println(“Error reading input!”); } } }

  23. Using String Streams • StringReader character stream whose source is string • StringWriter character stream that collects its output in string buffer

  24. Example: String Streams class Read { public static void main(String[] args) { Reader in = new StringReader (“abcdefghij”); System.out.println(in.read() + “ raw read, ascii value”); System.out.println((char)in.read() + “ cast read to char”); } } $ java Read 97 raw read, ascii value b cast read to char

  25. Example: Using String Streams classCapitalize { public static void main(String[] args) { Reader in = new StringReader (“advance programming”); Writer out = new StringWriter(10); int nextChar = 0; while ((nextChar = in.read()) != -1) { out.write(Character.toUpperCase ((char)nextChar)); } System.out.println(out.toString()); in.close(); out.close(); } } $ java Capitalize ADVANCE PRGRAMMING

  26. Binary File I/O • Handle byte base I/O using a DataInputStream and DataOutputStream

  27. Binary File Output File file = new File(“filename”); FileOutputStream fout = new FileOutputStream(file); OR FileOutputStream fout = new FileOutputStream(“filename”); • Methods • write(byte b) • write(byte[] buffer);

  28. Binary File Output File file = new File(“filename”); FileOutputStream fout = new FileOutputStream(file); DataOutputStream dout = new DataOutputStream(fout); OR DataOutputStream dout = new DataOutputStream( new FileOutputStream( new File(“filename”))); • Methods: writeByte(byte); writeInt(int); writeLong(long); writeDouble(double); ...

  29. Binary File Output File file = new File(“filename”); BufferedOutputStream bout = new BufferedOutputStream(file); DataOutputStream dout = new DataOutputStream(bout); • Methods: flush(); write(byte) write(byte[] buffer, int off, int len)

  30. Example: BinaryFileOutput class BinaryFileOutput { public static void main(String[] args) { int[] primes={1,2,3,5,11,13, 17,19}; DataOutputStream dout = null; try { dout = new DataOutputStream( new FileOutputStream(“primes.bin”)); for (int i=0; i < primes.length; i++) { dout.writeInt(primes[n]); } dout.close(); } catch (IOException ex) { ex.printStackTrace(); } } }

  31. Binary File Input File file = new File(“filename”); FileInputStream fin = new FileInputStream(file); Or FileInputStream fin = new FileInputStream(“filename”); • Methods • read() • read(byte[] buffer);

  32. Binary File Input File file = new File(“filename”); BufferedInputStream bin = new BufferedInputStream(file); DataInputStream din = new DataInputStream(bin); • Methods: read(); write(byte[] buffer, int off, int len) skip(long)

  33. Filter Streams • Abstract classes define the interface for filter streams, which filter data as it's being read or written • Attach a filter stream to another stream to filter data • java.io package contains the following filtered streams which are subclasses of either FilterInputStream or FilterOutputStream: • DataInputStream and DataOutputStrem • BufferedInputStream and BufferedOutputStream • LineNumberInputStream • PrintStream (output stream)

  34. PrintWriter and PrintStream • Easiest streams containing high level printing methods (print, println) to write • Can be created so as to flush automatically • PrintStream • Adds functionality to another output stream, namely the ability to print representations of various data values conveniently • Never throws an IOException • Flush automatically • All characters printed by a PrintStream are converted into bytes using the platform's default character encoding

  35. PrintWriter and PrintStream • PrintWriter • prints formatted representations of objects to text-output stream • Implements all of the print methods found in PrintStream • PrintWriter(Writer) creates a new PrintWriter • PrintWriter(Writer, boolean) creates a new PrintWriter with auto flushing

  36. Example Writer out = new FileWriter (“out.txt”); FileWriter (out)

  37. Example Writer out = new FileWriter (“out.txt”); PrintWriter easyOut = new PrintWriter(out); easyOut.println(“Hi Mom”); easyOut.println(5); easyOut.println(true); easyOut.close(); FileWriter (out) PrintWriter (easyOut)

  38. Conversion Streams • InputStreamReader • Reads bytes and translate them into character streams using the specified char set • Can be wrapped within a BufferedReader for efficiency BufferedReader in = new BufferedReader( new InputStreamReader(System.in)); System.in BufferedReader (in) InputStreamReader

  39. Conversion Streams • OutputStreamWriter • Writes character to an output stream translating into bytes • Can be wrapped within a BufferedWriter for efficiency OutputStreamWriter outWriter = new OutputStreamWriter(System.out); Bufferedwriter out = new Bufferedwriter(outWriter); BufferedWriter (out) System.out OutputStreamWriter

  40. Custom Filtered Streams • Steps writing your own filtered I/O streams: • Create a subclass of FilterInputStream and FilterOutputStream • Override the read and write methods, if needed • Provide any new methods • Make sure that the input and output streams work together

  41. Example import java.io.*; public class WhiteSpaceStripInputStream extendsFilterInpuStream { public WhiteSpaceStripInputStream(InputStream in) { super(in); } public int read() throws IOException { int c; do { c = in.read(); } while ((c ==‘ ‘ || c ==‘\t’|| c==‘\r’ || c ==‘\n’|| c ==‘\f’)&& c != -1 ); return c; }

  42. public int read(byte[] b) throws IOException { int c = 0, i = 0; for (i=0; i < b.length && (c=read())!= -1; i++){ b[i] = (byte) c; } } public int read(byte[] b, int offset, int len) throws IOException { int c = 0, i = 0; for (i=0; i < len && (c =read()) != -1; i++) { b[offset + i] = (byte) c; } return i; }

  43. public long skip(long n) throws IOException { int c = 0; for (int i = 0; i<n && (c=read()) != -1; i++) ; return i; } public int available() throws IOException { return 0; } }

  44. Random Access File • Permit nonsequential, or random, access to a file's contents • RandomAccessFile is used for both reading and writing files new RandomAccessFile ("farrago.txt", "r"); new RandomAccessFile("farrago.txt", "rw"); • It contains readXXX and writeXXX methods to perform I/O operations on the file

  45. Random Access File • Supports the notion of a file pointer which indicates the current location of in the file • Contains methods for explicitly manipulating the file pointer: • skipBytes(int): moves the file pointer forward • seek(long): position the file pointer • long getFilePointer(): returns the current byte location of the file pointer

  46. Question & Answer ?

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