Java Lecture 3

import java.io.*;

class LineNum {
  public static void main(String av[]) {
    try {
        // Class that allows reading of a line
        BufferedReader br;
        String line;
        int lineNo = 1;

        // Create instance tied to std input
        // (InputStreamReader is needed glue)
        br = new BufferedReader(new InputStreamReader(System.in));

        // readLine returns null at end of file
        while ((line = br.readLine()) != null)
        {
            System.out.println(lineNo + ": " + line);
            lineNo++;
        }
    }
    // Creating and using input streams/readers
    // may throw IO exceptions, so catch them
    catch (IOException e) {
        System.out.println("Error reading standard input");
    }
  }
}

General Design of Java's I/O System

• Consists of many (many) classes
• Classes live in java.io
• Instances of these are chained together
• For input:
  • usually one `source'
  • any number of `filters' stacked on top
    Example:

    InputStream	a `source' (e.g., System.in)
    InputStreamReader	filter: converts bytes to chars
    BufferedReader	filter: reads in large chunks, provides readLine()

• for output
  • usually one `destination'
  • any number of `filters' stacked on top
    Example:

    FileWriter	a `destionation' (opens file for writing)
    BufferedWriter	filter: writes in large chunks
    PrintWriter	filter: provides println(), etc.

Readers, Writers and Streams

• Readers, Writers: for unicode character I/O
  Examples:

  FileReader	source: reads from named file
  StringArrayReader	source: reads from String
  CharArrayReader	source: reads from char[]
  InputStreamReader	filter: converts from Stream to Reader
  BufferedReader	filter: reads in large chunks
  LineNumberReader	filter: counts line numbers
  PushbackReader	filter: allows `unreading'

• InputStreams, OutputStreams: for binary data
  Examples:

  FileInputStream	source: reads from named file
  ByteArrayInputStream	source: reads from char[]
  BufferedInputStream	filter: reads in large chunks
  PushbackInputStream	filter: allows `unreading'

• System.out is a PrintStream - backwards compatibility

import java.io.*;

class AddLineNum {
  public static void main(String args[]) {
    for (int i = 0; i < args.length; i++) {
        try {
            FileReader fr = new FileReader(args[i]);
            BufferedReader br = new BufferedReader(fr);

            FileWriter fw = new FileWriter(args[i] + ".xxx");
            BufferedWriter bw = new BufferedWriter(fw);
            PrintWriter pw = new PrintWriter(bw);

            String line;
            int lineNum = 1;

            while ((line = br.readLine()) != null)
            {
                pw.println(lineNum + ": " + line);
                lineNum++;
            }
            pw.close();
            br.close();
        } catch (FileNotFoundException e) {
            System.err.println("Can't open " + args[i]);
        } catch (IOException e) {
            System.err.println("IO error processing " + args[i] + ": "
                                + e.getMessage());
        }
    }
  }
}