try {
  FileOutputStream out = new FileOutputStream(FileDescriptor.out);
  out.write('A');
  out.close();
} catch (IOException e) {
}

System.out.print('A'); 

public final class FileDescriptor {
  private int fd;
  public static final FileDescriptor out = new FileDescriptor(1);
  private FileDescriptor(int fd) {
    this.fd = fd;
    useCount = new AtomicInteger();
  }
  ...
}

 import java.io.PrintStream;
 import java.io.FileDescriptor;
 import java.io.FileInputStream;
 import java.io.FileOutputStream;
 import java.io.IOException;
 /**
  * FileDescriptor 測試程序
  *
 * 
 */
 public class FileDescriptorTest {
   private static final String FileName = "file.txt";
   private static final String OutText = "Hi FileDescriptor";
   public static void main(String[] args) {
     testWrite();
     testRead();
     testStandFD() ;
     //System.out.println(OutText);
   }
   /**
   * FileDescriptor.out 的測試程序
   *
   * 該程序的效果 等價于 System.out.println(OutText);
   */
   private static void testStandFD() {
     // 創(chuàng)建FileDescriptor.out 對應(yīng)的PrintStream
     PrintStream out = new PrintStream(
         new FileOutputStream(FileDescriptor.out));
     // 在屏幕上輸出“Hi FileDescriptor”
     out.println(OutText);
     out.close();
   }
   /**
   * FileDescriptor寫入示例程序
   * 
   * () 為了說明，"通過文件名創(chuàng)建FileOutputStream"與“通過文件描述符創(chuàng)建FileOutputStream”對象是等效的
   * () 該程序會在“該源文件”所在目錄新建文件"file.txt"，并且文件內(nèi)容是"Aa"。
   */
   private static void testWrite() {
     try {
       // 新建文件“file.txt”對應(yīng)的FileOutputStream對象
       FileOutputStream out1 = new FileOutputStream(FileName);
       // 獲取文件“file.txt”對應(yīng)的“文件描述符”
       FileDescriptor fdout = out1.getFD();
       // 根據(jù)“文件描述符”創(chuàng)建“FileOutputStream”對象
       FileOutputStream out2 = new FileOutputStream(fdout);
      out1.write('A');  // 通過out1向“file.txt”中寫入'A'
       out2.write('a');  // 通過out2向“file.txt”中寫入'A'
       if (fdout!=null)
         System.out.printf("fdout(%s) is %s\n",fdout, fdout.valid());
       out.close();
       out.close();
     } catch(IOException e) {
       e.printStackTrace();
     }
   }
   /**
   * FileDescriptor讀取示例程序
   *
   * 為了說明，"通過文件名創(chuàng)建FileInputStream"與“通過文件描述符創(chuàng)建FileInputStream”對象是等效的
   */
   private static void testRead() {
     try {
       // 新建文件“file.txt”對應(yīng)的FileInputStream對象
       FileInputStream in = new FileInputStream(FileName);
       // 獲取文件“file.txt”對應(yīng)的“文件描述符”
       FileDescriptor fdin = in1.getFD();
       // 根據(jù)“文件描述符”創(chuàng)建“FileInputStream”對象
       FileInputStream in2 = new FileInputStream(fdin);
       System.out.println("in1.read():"+(char)in1.read());
       System.out.println("in2.read():"+(char)in2.read());
       if (fdin!=null)
         System.out.printf("fdin(%s) is %s\n", fdin, fdin.valid());
       in1.close();
       in2.close();
     } catch(IOException e) {
       e.printStackTrace();
     }
   }
 }

fdout(java.io.FileDescriptor@2b820dda) is true
in1.read():A
in2.read():a
fdin(java.io.FileDescriptor@675b7986) is true
Hi FileDescriptor