personsaver.java

JAVA 2入门经典 练习答案

// Chapter 12 Exercise 1

/*
  This is is not easy. A major problem with the ObjectOutputStream class is that
  is does not provide for appending objects to an existing file. If you do so, then
  reading back thge appended objects will cause a SteamCorruptedException to be
  thrown. This is because the ObjectOutputStream constructor calls the protected
  writeStreamHeader() method, even when the file is to be appended to. Thus your
  file will have two stream headers - one at the beginning and one at the end
  before any objects have been appended.
  
*/

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;

public class PersonSaver {
  private String directory = "C:/Beg Java Stuff";
  private String filename = "Persons.bin";                    // Stores Person objects


  // Buffered keyboard input stream
  private BufferedReader kb = new BufferedReader(new InputStreamReader(System.in));

  // File paths
  private File file = null;                                   // Person file
  
  private ObjectOutputStream objectsOut = null;

  public PersonSaver(){
    File dir = new File(directory);                            // Directory File object
    if(!dir.isDirectory())                                     // If there's no directory
      dir.mkdir();                                             // create one

    file = new File(dir, filename);                            // Person file
  }
  
  public static void main(String[] args) {
    PersonSaver saver = new PersonSaver();          // Create a saver object
    saver.operate();                                // and execute it
  }

  // Runs the Person saver
  private void operate(){
    // Get operation selection from the keyboard
    while(true) {  
      switch(selectOperation()) {

        case 'N': case 'n':  // Read and add a new person
          addPerson();
          break;

        case 'Q': case 'q':  // Quit the program, but first close all streams
          try {
            if(objectsOut != null)
              objectsOut.close();
          }catch(IOException e){
            e.printStackTrace();
          }
          System.exit(0);
          break;

        default:
          System.out.println("Invalid Input. Try again.");
          break;
      }
    }
  }
    
  // Method to read a character entered on the keyboard
  private char selectOperation() {
    System.out.println("\nEnter a letter to select an operation:"
                      +"\n Q or q to quit the program."
                      +"\n N or n to enter a new name and address.");
                            
    String s = null;
    try {
      s = kb.readLine();
    } catch (IOException e) {
        e.printStackTrace(System.err);
        System.exit(1);
    }
    
    return s.trim().charAt(0);  // Return the first non-blank letter
  }

  // Method to add a new Person object to the file  
  public void addPerson() {
    Person person = null;
    String secondname = null;
    try {
      System.out.println("Enter first name:");
      String firstname = kb.readLine().trim();
      System.out.println("Enter second name:");
      secondname = kb.readLine().trim();
      Name name = new Name(firstname, secondname);

      StringBuffer sb = new StringBuffer();  // To store address
      
      // We will add each address line to sb terminated by '\n'
      System.out.println("Enter address one line at a time. Enter ! to end:");
      String s = null; 
      while((s = kb.readLine().trim()).charAt(0) != '!')
       sb.append(s).append('\n');
      person = new Person(new Name(firstname, secondname), new Address(sb.toString()));
    } catch(IOException e) {
      System.err.println("Error reading from keyboard\n");
      e.printStackTrace();
      System.exit(1); 
    }   

    // Write the new Person object to the file
    try {
      if(objectsOut == null) {
      
        boolean personFileCheck = file.exists() && file.length()>0;
        FileOutputStream ostream = new FileOutputStream(file, true);
        if(personFileCheck) {
          // The file exists so use the class type that inhibits writing the header
          objectsOut = new ReuseObjectOutputStream(ostream);
        } else {
          // Since the file does not exist, creating the output stream 
          // this will create a file to which a header will be written.
          objectsOut = new ObjectOutputStream(ostream);  // To write Person objects
        }
      }
      objectsOut.writeObject(person);
      objectsOut.flush();
    } catch(IOException e){
      e.printStackTrace();
      System.exit(1); 
    }   
  }
}