DataInput&DataOutput&RandomAccessFile

public class DataInputStream extends FilterInputStream implements DataInput
public class DataOutputStream extends FilterOutputStream
implements DataOutput

可以看出此2类的设计还是用来对付二进制的，这两个类通常来说是配对使用的，这两类中定义的方法也是很配的。主要是用来对付java中的primitive data的，因为它们出现的频率过多了。
不同的primitive data有不同的方法。

RandomAccessFile extends Object implements DataInput DataOutput
但是它牛逼的地方在于可以随机访问，看他的方法
length()
seek()
skipByte()...
而且是很独立的一个类

还有就是GregorianCalendar 和 Date的使用，再研究!

package xml;
import java.util.*;
import java.io.*;

public class RandomFileTest {
	public static void main(String[] args){
		Employee[] employees=new Employee[3];
		employees[0]=new Employee("a",2500,1989,1,1);
		employees[1]=new Employee("b",2500,1989,1,1);
		employees[2]=new Employee("c",2500,1989,1,1);

		try{
			DataOutputStream out=new DataOutputStream(new FileOutputStream("randomFile.txt"));

			for(Employee e : employees){
				e.writeData(out);
			}
			//关闭
			out.close();

			//以只读方式打开文件
			RandomAccessFile in=new RandomAccessFile("randomFile.txt","r");
			//有多少个记录，in.length()是个double
			int n=(int)in.length()/Employee.RECORD_SIZE;
			Employee[] newStaff=new Employee[n];
			for(int i=0;i<n;i++){
				newStaff[i]=new Employee();
				//读下一条记录
				in.seek(i*Employee.RECORD_SIZE);
				newStaff[i].readData(in);
			}
			in.close();

			//我可定义了toString()方法哦！
			for(Employee e : newStaff)
				System.out.println(e);
		}catch(IOException e){}
	}
}
class Employee{
	private String name;
	private double salary;
	private Date hireDay;
	//40代表40个char
	public static final int NAME_SIZE=40;
	//每个记录占多杀个字节
	public static final int RECORD_SIZE=NAME_SIZE*2+8+3*4;
	public Employee(){}
	public Employee(String n,double s,int y,int m,int d){
		this.name=n;
		this.salary=s;
		GregorianCalendar calendar=new GregorianCalendar(y,m-1,d);
		this.hireDay=calendar.getTime();
	}
	public String getName(){
		return this.name;
	}
	public double getSalary(){
		return this.salary;
	}
	public Date getHireDay(){
		return this.hireDay;
	}
	public String toString(){
		return getClass().getName()+"[name="+name+",salary="+this.salary+",hireday="+this.hireDay+"]";
	}

	//parameter is interface DataOutput
	public void writeData(DataOutput out)throws IOException{
		DataIO.writeFixString(this.name,this.NAME_SIZE,out);
		out.writeDouble(this.salary);

		GregorianCalendar calendar=new GregorianCalendar();
		calendar.setTime(this.hireDay);

		out.writeInt(calendar.get(Calendar.YEAR));
		out.writeInt(calendar.get(Calendar.MONTH)+1);
		out.writeInt(calendar.get(Calendar.DAY_OF_MONTH));		
	}

	public void readData(DataInput in)throws IOException{
		this.name=DataIO.readFixString(this.NAME_SIZE, in);
		this.salary=in.readDouble();
		int y=in.readInt();
		int m=in.readInt();
		int d=in.readInt();
		GregorianCalendar calendar=new GregorianCalendar(y,m-1,d);
		this.hireDay=calendar.getTime();
	}
}

//因为DataOutput DataInput在处理字符串的时候，在文本中占用的byte难以控制，所以写了这个静态工具类
class DataIO{

	public static void writeFixString(String name,int size,DataOutput out)throws IOException{
		for(int i=0;i<size;i++){
			char c=0;
			if(i<name.length())
				c=name.charAt(i);
			out.writeChar(c);
		}
	}

	public static String readFixString(int size,DataInput in)throws IOException{
		StringBuilder sb=new StringBuilder(size);
		boolean more=true;
		int i=0;
		while(i<size&&more){
			char c=in.readChar();
			if(c==0)
				more=false;
			else
				sb.append(c);
			i++;
		}
		in.skipBytes(2*(size-i));
		return sb.toString();
	}

}