DataInputStream类和RandomAccessFile类的使用方法

// DataInputStream类实现了DataInput接口，要想从文件中读入二进制数据，
// 你需要将DataInputStream与某个字节源相结合，例如FileInputStream
// 与此同时，要想写出二进制数据，可以使用实现了DataOutput接口的DataOutputStream类
// RandomAccessFile类同时实现了DataInput和DataOutput接口。
// 以下程序将三条记录写到一个数据文件中，然后以逆序将它们从文件中读回。
// 为了高效地执行，这里需要使用随机访问，因为我们需要首先读入第三条记录
// 让我们来计算每条记录的大小：我们将使用40个字符来 表示姓名字符串，因此每条记录包含100个字节：
// 40字符 = 80字节，用于姓名
// 1 double = 8字节，用于薪水
// 3 int = 12字节，用于日期
package com.example.io;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.DataOutputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;

public class RandomFileTest {

public static void main(String[] args) {
Employee[] staff = new Employee[3];

staff[0] = new Employee("Carl Cracker", 75000, 1987, 12, 15);
staff[1] = new Employee("Harry Hacker", 50000, 1989, 10, 1);
staff[2] = new Employee("Tony Tester", 40000, 1990, 3, 15);

try {
// 保存所有雇员记录到文件当中
DataOutputStream out = new DataOutputStream(new FileOutputStream("employee.dat"));
for (Employee e : staff) {
e.writeData(out);
}
out.close();

// 读取所有记录到新数组中
RandomAccessFile in = new RandomAccessFile("employee.dat", "r");
// 计算数组的尺寸
int n = (int) (in.length() / Employee.RECORD_SIZE);
Employee[] newStaff = new Employee
;

//反序读入雇员记录
for (int i = n - 1, j = 0; i >= 0; i--) {
newStaff[j] = new Employee();
in.seek(i * Employee.RECORD_SIZE);
newStaff[j].readData(in);
j++;
}
in.close();

// 打印新读入数组记录内容
for (Employee e : newStaff) {
System.out.println(e);
}
} catch (Exception e) {
e.printStackTrace();
}

}

}

class Employee {

public Employee() {
}

public Employee(String n, double s, int year, int month, int day) {
name = n;
salary = s;
GregorianCalendar calendar = new GregorianCalendar(year, month - 1, day);
hireDay = calendar.getTime();
}

public String getName() {
return name;
}

public double getSalary() {
return salary;
}

public Date getHireDay() {
return hireDay;
}

public void raiseSalary(double byPercent) {
double raise = salary * byPercent / 100;
salary += raise;
}

@Override
public String toString() {
return "Employee{" + "name=" + name + ", salary=" + salary + ", hireDay=" + hireDay + '}';
}

public void writeData(DataOutput out) throws IOException {
DataIO.writeFixedString(name, NAME_SIZE, out);
out.writeDouble(salary);

GregorianCalendar calendar = new GregorianCalendar();
calendar.setTime(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 {
name = DataIO.readFixedString(NAME_SIZE, in);
salary = in.readDouble();
int y = in.readInt();
int m = in.readInt();
int d = in.readInt();
GregorianCalendar calendar = new GregorianCalendar(y, m - 1, d);
hireDay = calendar.getTime();
}

public static final int NAME_SIZE = 40;
public static final int RECORD_SIZE = 2 * NAME_SIZE + 8 + 4 + 4 + 4;
private String name;
private double salary;
private Date hireDay;
}

class DataIO {

public static String readFixedString(int size, DataInput in) throws IOException {
StringBuilder b = new StringBuilder(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);
}
}
in.skipBytes(2 * (size - i));
return b.toString();
}

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