//FileRandomRW.java

import java.io.*;
import java.util.*;

public class FileRandomRW
{
//需要輸入的person數目。
	public static int NUMBER = 3;
	public static void main(String[] args)
	{
		Person[] people = new Person[NUMBER];
		people[0] = new Person("張三", 25, 3000);
		people[1] = new Person("李四", 35, 10000, "Y");
		people[2] = new Person("王大五", 30, 4000, "F");

		try
		{
			DataOutputStream out = new DataOutputStream(new
				FileOutputStream("peoplerandom.dat"));
//將人員數據保存至“peoplerandom.dat”二進制文件中。
			writeData(people, out);
//關閉流。
			out.close();

//從二進制文件“peoplerandom.dat”中逆序讀取數據。
			RandomAccessFile inOut = new RandomAccessFile("peoplerandom.dat", "rw");
			Person[] inPeople = readDataReverse(inOut);
//輸出讀入的數據。
			System.out.println("原始數據：");
			for (int i = 0; i < inPeople.length; i++)
				System.out.println(inPeople[i]);
//修改文件的第三條記錄。
			inPeople[2].setSalary(4500);
//將修改結果寫入文件。
			inPeople[2].writeData(inOut, 3);
//關閉流。
			inOut.close();

//從文件中讀入的第三條記錄，并輸出，以驗證修改結果。
			RandomAccessFile in = new RandomAccessFile("peoplerandom.dat", "r");
			Person in3People = new Person();
//隨機讀第三條記錄。
			in3People.readData(in, 3);
//關閉流。
			in.close();
			System.out.println("修改后的記錄");
			System.out.println(in3People);
		}
		catch(IOException exception)
		{
			System.err.println("IOException");
		}
	}

//將數據寫入輸出流。
	static void writeData(Person[] p, DataOutputStream out) 
		throws IOException
	{
		for (int i = 0; i < p.length; i++)
			p[i].writeData(out);
	}

//將數據從輸入流中逆序讀出。
	static Person[] readDataReverse(RandomAccessFile in)
		throws IOException
	{
//獲得記錄數目。
		int record_num = (int)(in.length()/Person.RECORD_LENGTH);

		Person[] p = new Person[record_num];
//逆序讀取。
		for (int i = record_num-1 ; i >= 0 ; i--)
		{
			p[i] = new Person();
//文件定位。
			in.seek(i * Person.RECORD_LENGTH);
			p[i].readData(in, i+1);
		}
		return p;
	}
}

class Person
{
	private String name;
	private int age;        //4個字節 
	private double salary;  //8個字節
	private String married;

	public static final int NAME_LENGTH = 20;  //姓名長度
	public static final int MARRIED_LENGTH = 2;//婚否長度
	public static final int RECORD_LENGTH = NAME_LENGTH * 2 + 4 + 8 + MARRIED_LENGTH * 2;

	public Person()
	{
	}

	public Person(String n, int a, double s)
	{
		name = n;
		age = a;
		salary = s;
		married = "F";
	}

	public Person(String n, int a, double s, String m)
	{
		name = n;
		age = a;
		salary = s;
		married = m;
	}

	public String getName()
	{
		return name;
	}

	public int getAge()
	{
		return age;
	}

	public double getSalary()
	{
		return salary;
	}

	public String getMarried()
	{
		return married;
	}

	public String setName(String n)
	{
		name = n;
		return name;
	}

	public int setAge(int a)
	{
		age = a;
		return age;
	}

	public double setSalary(double s)
	{
		salary = s;
		return salary;
	}

	public String setMarried(String m)
	{
		married = m;
		return married;
	}

//設置輸出格式。
	public String toString()
	{
		return getClass().getName() + "[name=" + name
			+ ",age=" + age
			+ ",salary=" + salary
			+ ",married=" + married
			+ "]";
	}

//寫入一條固定長度的記錄，即一個人的數據到輸出流。
	public void writeData(DataOutput out) throws IOException
	{
		FixStringIO.writeFixString(name, NAME_LENGTH, out);
		out.writeInt(age);
		out.writeDouble(salary);
		FixStringIO.writeFixString(married, MARRIED_LENGTH, out);
	}

//寫入一條固定長度的記錄到隨機讀取文件中。
	private void writeData(RandomAccessFile out) throws IOException
	{
		FixStringIO.writeFixString(name, NAME_LENGTH, out);
		out.writeInt(age);
		out.writeDouble(salary);
		FixStringIO.writeFixString(married, MARRIED_LENGTH, out);
	}

//隨機寫入一條固定長度的記錄到輸出流的指定位置。
	public void writeData(RandomAccessFile out, int n) throws IOException
	{
		out.seek((n - 1) * RECORD_LENGTH);
		writeData(out);
	}

//從輸入流隨機讀入一條記錄，即一個人的數據。
	private void readData(RandomAccessFile in) throws IOException
	{
		name = FixStringIO.readFixString(NAME_LENGTH, in);
		age = in.readInt();
		salary = in.readDouble();
		married = FixStringIO.readFixString(MARRIED_LENGTH, in);
	}

//從輸入流隨機讀入指定位置的記錄。
	public void readData(RandomAccessFile in, int n) throws IOException
	{
		in.seek((n - 1) * RECORD_LENGTH);
		readData(in);
	}
}

//對固定長度字符串從文件讀出、寫入文件
class FixStringIO
{
//讀取固定長度的Unicode字符串。
	public static String readFixString(int size, DataInput in)
		throws IOException
	{
		StringBuffer b = new StringBuffer(size);
		int i = 0;
		boolean more = true;
		
		while (more && i < size)
		{
			char ch = in.readChar();
			i++;
			if (ch == 0) more = false;
			else b.append(ch);
		}
//跳過剩余的字節。
		in.skipBytes(2 * (size - i));
		return b.toString();
	}

//寫入固定長度的Unicode字符串。
	public static void writeFixString(String s, int size, DataOutput out) 
		throws IOException
	{
		int i;
		for (i = 0; i < size; i++)
		{
			char ch = 0;
			if (i < s.length()) ch = s.charAt(i);
			out.writeChar(ch);
		}
	}
}