import java.io.FileOutputStream;
import java.io.IOException;


public class FileOutput {

  /**
   * @param args
   * @throws IOException 
   */
  public static void main(String[] args) throws IOException {
      FileOutputStream f = new FileOutputStream("test.txt");
      f.write("Hello world!\n".getBytes());
      f.close();
      
      FileOutputStream f2 = new FileOutputStream("test.txt", true);
      f2.write("Hello world again!".getBytes());
      f2.close();
      

  }

}

import java.io.FileOutputStream;
import java.io.FileInputStream;
import java.io.File;
import java.io.IOException;

public class FileOutput {

  /**
   * @param args
   * @throws IOException 
   */
  public static void main(String[] args) throws IOException {
    try {
      FileOutputStream f = new FileOutputStream("test.txt");
      f.write("Hello world!\n".getBytes());
      f.close();
      
      FileOutputStream f2 = new FileOutputStream("test.txt", true);
      f2.write("Hello world again!".getBytes());
      f2.close();
      
      FileInputStream in = new FileInputStream("test.txt");
      byte[] buffer = new byte[256];
      int c;
      int offset = 0;
      while ((c = in.read(buffer, offset, buffer.length - offset)) != -1) {
        offset += c;
      }

      if (! "Hello world!\nHello world again!".equals
          (new String(buffer, 0, offset)))
      {
        throw new RuntimeException();
      }
    } finally {
      new File("test.txt").delete();
    }
  }

}

verify file contents in FileOutput test