Chapter 10. IO Streams

Transcription

1 Chapter 10 IO Streams

2 Java I/O The Basics Java I/O is based around the concept of a stream Ordered sequence of information (bytes) coming from a source, or going to a sink Simplest stream reads/writes only a single byte, or an array of bytes at a time Designed to be platform-independent The stream concept is very generic Can be applied to many different types of I/O Files, Network, Memory, Processes, etc

3 Java I/O The Basics The java.io package contains all of the I/O classes. Many classes specialised for particular kinds of stream operations, e.g. file I/O Reading/writing single bytes is quite limited So, it includes classes which provide extra functionality e.g. buffering, reading numbers and Strings (not bytes), etc. Results in large inheritance hierarchy, with separate trees for input and output stream classes

4 Binary Java I/O Streams Object InputStream OutputStream FileInputStream FilterInputStream ObjectInputStream FileOutputStream FilterOutputStream ObjectOutputStream DataInputStream BufferedInputStream BufferedOutputStream DataOutputStream PrintStream

5 Java I/O Using InputStreams Basic pattern for I/O programming is as follows: Open a stream While there s data to read Process the data Close the stream I/O in Java: InputStream in = new FileInputStream( c:\\temp\\myfile.txt ); int b = in.read(); //EOF is signalled by read() returning -1 while (b!= -1) { b = in.read(); } in.close();

6 Java I/O Using InputStreams But using buffering is more efficient, therefore we always nest our streams because A BufferedInputStream adds functionality to another input stream-namely, the ability to buffer the input and to support the mark and reset methods. When the BufferedInputStream is created, an internal buffer array is created. InputStream in1 = new FileInputStream( c:\\temp\\myfile.txt ); InputStream in = new BufferedInputStream(in1); int b = in.read(); //EOF is signalled by read() returning -1 while (b!= -1) { //do something with the data b = in.read();} in.close();

7 public int read() Reads the next byte of data from this input stream. The value byte is returned as an int in the range 0 to 255. If no byte is available because the end of the stream has been reached, the value -1 is returned.

8 Example: let myfile1.txt contains import java.io.*; import java.util.*; public class Filelist { public static void main(string[] args) { try{inputstream in1 = new FileInputStream("c:\\myfile1.txt"); InputStream in = new BufferedInputStream(in1); int b = in.read(); //EOF is signalled by read() returning -1 while (b!= -1) { System.out.println((char)b); b = in.read(); } in.close(); } catch(ioexception e) { System.out.println(e);} } } Output: If the file not found, then java.io.filenotfoundexception: c:\myfile2.txt (The system cannot find the file specified)

9 Java I/O Using InputStreams We ve omitted exception handling in the previous examples Almost all methods on the I/O classes (including constructors) can throw an IOException or a subclass. Always wrap I/O code in try catch blocks to handle errors.

10 Example 2: myfile.txt contains(abcvxv) import java.io.*; import java.util.*; public class Filelist { public static void main(string[] args) { byte buf1[]=new byte[100]; try{inputstream in1 = new FileInputStream("c:\\myfile.txt"); InputStream in = new BufferedInputStream(in1); int b=in.available(); // b=6 System.out.println(b); in.read(buf1,0,b); for (int i=0;i<b; i++) System.out.println((char)buf1[i]); in.close();} catch(ioexception e){//e.printstacktrace(); System.out.println(e); }}}

11 OutputStream The value is a byte as an int type. java.io.outputstream +write(int b): void +write(b: byte[]): void +write(b: byte[], off: int, len: int): void +close(): void +flush(): void Writes the specified byte to this output stream. The parameter b is an int value. (byte)b is written to the output stream. Writes all the bytes in array b to the output stream. Writes b[off], b[off+1],, b[off+len-1] into the output stream. Closes this input stream and releases any system resources associated with the stream. Flushes this output stream and forces any buffered output bytes to be written out. 11

12 Java I/O Using InputStreams Basic pattern for output is as follows: Open a stream While there s data to write Write the data Close the stream Output in Java: OutputStream out = new FileOutputStream( c:\\temp\\myfile.txt ); while ( ) { out.write( ); } out.close();

13 Java I/O Using OutputStreams OutputStream out = null; try { OutputStream writer = new FileOutputStream( c:\\temp\\myfile.txt ); out = new BufferedOutputStream(writer); //write data to the file } catch (IOException e) { e.printstacktrace(); } finally { try { out.close(); } catch (Exception e) { } }

14 Example 2: public class Filelist { public static void main(string[] args) { OutputStream out = null; byte buf[]={ H', i'}; try { OutputStream writer = new FileOutputStream("c:\\myfile1.txt"); out = new BufferedOutputStream(writer); for (char i= a ; i< d ; i++){ out.write(i);} } //write data to the file out.write(buf); // write(byte[] b) out.write(buf,0,1); //write(byte[] b,offset,number of characters) catch (IOException e) { e.printstacktrace(); } finally { try { out.close(); } catch (Exception e) {} } try{inputstream inner = new FileInputStream("c:\\myfile2.txt"); InputStream in = new BufferedInputStream(inner); int b = in.read(); while (b!= -1) { System.out.println(b); b = in.read(); } in.close();} catch(ioexception e){ System.out.println(e);} } } output a b c H i H

15 But That s Not All! Input/OutputStream and sub-classes were part of Java 1.1. Java 1.2 adds more classes specialised for character based I/O The stream classes are for data I/O. Classes for character I/O are called Readers and Writers Why have specialised classes? To support foreign languages

16 Unicode Each character in the ASCII character set fits into a single byte but that s not enough for Chinese, and other complex alphabets Need more than a single byte A Java character (char) is 2 bytes Java handles text using Unicode International standard character set, containing characters for almost all known languages And a few imaginary ones! (Klingon, Elvish ) Inside the JVM all text is held as Unicode

17 Java Text I/O Because byte!= character for all languages, you have to turn bytes into chars using a Input/OutputStream Java provides Readers and Writers to save you this work. These classes deal with streams of characters Read/write single character or array of characters Again there are classes specialised for particular purposes

18 character Java I/O Reader

19 Java I/O Readers

20 Using Readers Reader in = null; try { Reader inner = new FileReader( c:\\temp\\myfile.txt ); in = new BufferedReader(inner); //process file } catch (IOException e) { e.printstacktrace(); } finally { try { in.close(); } catch (Exception e) {} }

21 Java I/O Writer

22 Java I/O Writers

23 Using Writers Writer out = null; try { Writer writer = new FileWriter( c:\\temp\\myfile.txt ); out = new BufferedWriter(writer); //write data to the file } catch (IOException e) { e.printstacktrace(); } finally { try { out.close(); } catch (Exception e) {} }

24 Example3: import java.io.reader; public class R1{ public static void main(string[] args) { Writer out = null; try { Writer writer = new FileWriter("c:\\rmyfile.txt"); out = new BufferedWriter(writer); out.write("hello how r u?\n"); out.write('6'); } catch (IOException e) {System.out.println("err"+e); e.printstacktrace(); } finally { try { out.close(); } catch (Exception e) {} } char[] C=new char[16]; for(int i=0; i<16 ;i++) C[i]=' '; int x; Reader in = null; try { Reader inner = new FileReader("c:\\rmyfile.txt"); in = new BufferedReader(inner); in.read(c); System.out.print(C); } catch (IOException e) { e.printstacktrace(); } finally { try { in.close(); } catch (Exception e) {} } }} hello how r u? 6

25 Example4: same result of Example 3 import java.io.*; import java.io.reader; public class R1{ public static void main(string[] args) { Writer out = null; try { Writer writer = new FileWriter("c:\\rmyfile.txt"); out = new BufferedWriter(writer); out.write("hello how r u?\n"); out.write('6'); } catch (IOException e) {System.out.println("err"+e); e.printstacktrace(); } finally { try { out.close(); } catch (Exception e) {} } int x; Reader in = null; Try { Reader inner = new FileReader("c:\\rmyfile.txt"); in = new BufferedReader(inner); x=in.read(); while (x!=-1) {System.out.print((char)x); x=in.read();} catch (IOException e) { e.printstacktrace(); } finally { try { in.close(); } catch (Exception e) {} } }}