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Thinking in Java
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Converting data

If you look back at GetChannel.java, you’ll notice that, to print the information in the file, we are pulling the data out one byte at a time and casting each byte to a char. This seems a bit primitive—if you look at the java.nio.CharBuffer class, you’ll see that it has a toString( ) method that says: “Returns a string containing the characters in this buffer.” Since a ByteBuffer can be viewed as a CharBuffer with the asCharBuffer( ) method, why not use that? As you can see from the first line in the expect( ) statement below, this doesn’t work out:

//: c12:BufferToText.java
// Converting text to and from ByteBuffers
// {Clean: data2.txt}
import java.io.*;
import java.nio.*;
import java.nio.channels.*;
import java.nio.charset.*;
import com.bruceeckel.simpletest.*;

public class BufferToText {
  private static Test monitor = new Test();
  private static final int BSIZE = 1024;
  public static void main(String[] args) throws Exception {
    FileChannel fc =
      new FileOutputStream("data2.txt").getChannel();
    fc.write(ByteBuffer.wrap("Some text".getBytes()));
    fc.close();
    fc = new FileInputStream("data2.txt").getChannel();
    ByteBuffer buff = ByteBuffer.allocate(BSIZE);
    fc.read(buff);
    buff.flip();
    // Doesn't work:
    System.out.println(buff.asCharBuffer());
    // Decode using this system's default Charset:
    buff.rewind();
    String encoding = System.getProperty("file.encoding");
    System.out.println("Decoded using " + encoding + ": " 
      + Charset.forName(encoding).decode(buff));
    // Or, we could encode with something that will print:
    fc = new FileOutputStream("data2.txt").getChannel();
    fc.write(ByteBuffer.wrap(
      "Some text".getBytes("UTF-16BE")));
    fc.close();
    // Now try reading again:
    fc = new FileInputStream("data2.txt").getChannel();
    buff.clear();
    fc.read(buff);
    buff.flip();
    System.out.println(buff.asCharBuffer());
    // Use a CharBuffer to write through:
    fc = new FileOutputStream("data2.txt").getChannel();
    buff = ByteBuffer.allocate(24); // More than needed
    buff.asCharBuffer().put("Some text");
    fc.write(buff);
    fc.close();
    // Read and display:
    fc = new FileInputStream("data2.txt").getChannel();
    buff.clear();
    fc.read(buff);
    buff.flip();
    System.out.println(buff.asCharBuffer());
    monitor.expect(new String[] {
      "????",
      "%% Decoded using [A-Za-z0-9_\\-]+: Some text",
      "Some text",
      "Some text\0\0\0"
    });
  }
} ///:~


The buffer contains plain bytes, and to turn these into characters we must either encode them as we put them in (so that they will be meaningful when they come out) or decode them as they come out of the buffer. This can be accomplished using the java.nio.charset.Charset class, which provides tools for encoding into many different types of character sets:

//: c12:AvailableCharSets.java
// Displays Charsets and aliases
import java.nio.charset.*;
import java.util.*;
import com.bruceeckel.simpletest.*;

public class AvailableCharSets {
  private static Test monitor = new Test();
  public static void main(String[] args) {
    Map charSets = Charset.availableCharsets();
    Iterator it = charSets.keySet().iterator();
    while(it.hasNext()) {
      String csName = (String)it.next();
      System.out.print(csName);
      Iterator aliases = ((Charset)charSets.get(csName))
        .aliases().iterator();
      if(aliases.hasNext())
        System.out.print(": ");
      while(aliases.hasNext()) {
        System.out.print(aliases.next());
        if(aliases.hasNext())
          System.out.print(", ");
      }
      System.out.println();
    }
    monitor.expect(new String[] {
      "Big5: csBig5",
      "Big5-HKSCS: big5-hkscs, Big5_HKSCS, big5hkscs",
      "EUC-CN",
      "EUC-JP: eucjis, x-eucjp, csEUCPkdFmtjapanese, " +
      "eucjp, Extended_UNIX_Code_Packed_Format_for" +
      "_Japanese, x-euc-jp, euc_jp",
      "euc-jp-linux: euc_jp_linux",
      "EUC-KR: ksc5601, 5601, ksc5601_1987, ksc_5601, " +
      "ksc5601-1987, euc_kr, ks_c_5601-1987, " +
      "euckr, csEUCKR",
      "EUC-TW: cns11643, euc_tw, euctw",
      "GB18030: gb18030-2000",
      "GBK: GBK",
      "ISCII91: iscii, ST_SEV_358-88, iso-ir-153, " +
      "csISO153GOST1976874",
      "ISO-2022-CN-CNS: ISO2022CN_CNS",
      "ISO-2022-CN-GB: ISO2022CN_GB",
      "ISO-2022-KR: ISO2022KR, csISO2022KR",
      "ISO-8859-1: iso-ir-100, 8859_1, ISO_8859-1, " +
      "ISO8859_1, 819, csISOLatin1, IBM-819, " +
      "ISO_8859-1:1987, latin1, cp819, ISO8859-1, " +
      "IBM819, ISO_8859_1, l1",
      "ISO-8859-13",
      "ISO-8859-15: 8859_15, csISOlatin9, IBM923, cp923," +
      " 923, L9, IBM-923, ISO8859-15, LATIN9, " +
      "ISO_8859-15, LATIN0, csISOlatin0, " +
      "ISO8859_15_FDIS, ISO-8859-15",
      "ISO-8859-2", "ISO-8859-3", "ISO-8859-4",
      "ISO-8859-5", "ISO-8859-6", "ISO-8859-7",
      "ISO-8859-8", "ISO-8859-9", 
      "JIS0201: X0201, JIS_X0201, csHalfWidthKatakana",
      "JIS0208: JIS_C6626-1983, csISO87JISX0208, x0208, " +
      "JIS_X0208-1983, iso-ir-87",
      "JIS0212: jis_x0212-1990, x0212, iso-ir-159, " +
      "csISO159JISC02121990",
      "Johab: ms1361, ksc5601_1992, ksc5601-1992",
      "KOI8-R",
      "Shift_JIS: shift-jis, x-sjis, ms_kanji, " +
      "shift_jis, csShiftJIS, sjis, pck",
      "TIS-620",
      "US-ASCII: IBM367, ISO646-US, ANSI_X3.4-1986, " +
      "cp367, ASCII, iso_646.irv:1983, 646, us, iso-ir-6,"+
      " csASCII, ANSI_X3.4-1968, ISO_646.irv:1991",
      "UTF-16: UTF_16",
      "UTF-16BE: X-UTF-16BE, UTF_16BE, ISO-10646-UCS-2",
      "UTF-16LE: UTF_16LE, X-UTF-16LE",
      "UTF-8: UTF8", "windows-1250", "windows-1251",
      "windows-1252: cp1252",
      "windows-1253", "windows-1254", "windows-1255",
      "windows-1256", "windows-1257", "windows-1258",
      "windows-936: ms936, ms_936",
      "windows-949: ms_949, ms949", "windows-950: ms950",
    });
  }
} ///:~


So, returning to BufferToText.java, if you rewind( ) the buffer (to go back to the beginning of the data) and then use that platform’s default character set to decode( ) the data, the resulting CharBuffer will print to the console just fine. To discover the default character set, use System.getProperty("file.encoding"), which produces the string that names the character set. Passing this to Charset.forName( ) produces the Charset object that can be used to decode the string.

Another alternative is to encode( ) using a character set that will result in something printable when the file is read, as you see in the third part of BufferToText.java. Here, UTF-16BE is used to write the text into the file, and when it is read, all you have to do is convert it to a CharBuffer, and it produces the expected text.

Finally, you see what happens if you write to the ByteBuffer through a CharBuffer (you’ll learn more about this later). Note that 24 bytes are allocated for the ByteBuffer. Since each char requires two bytes, this is enough for 12 chars, but “Some text” only has 9. The remaining zero bytes still appear in the representation of the CharBuffer produced by its toString( ), as you can see in the output.
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire