Java输入输出流2.ppt

Java输入输出流2,伍淳华 北京邮电大学计算机学院,Java,Java输入输出流,例:用不同的方案实现将一个Employee记录数组写入文件,并将其读回内存(JAVA Core P602) 1)以文本格式保存相同类型(Employee)的数据; 2)以二进制格式保存相同类型的数据;,Java,Java输入输出流,采用分隔格式存储Employee记录的一个数组; -每条记录使用单独的一行存储; -不同的成员变量间由分隔符分开;以”|”作为分隔符; Harry Hacker|355500|1989|10|1 Carl Cracker|75000|1987|12|15 Tony Tester|38000|1990|3|15,Java,Java输入输出流,写入记录 -采用PrintWriter类; -成员变量之间用”|”分隔,一个记录占一行; -Employee类中增加一个方法:WriteData; public void writeData(PrintWriter out) throws IOException GregorianCalendar calendar = new GregorianCalendar(); calendar.setTime(hireDay); out.println(name + “|“ + salary + “|“ + calendar.get(Calendar.YEAR) + “|“ + (calendar.get(Calendar.MONTH) + 1) + “|“ + calendar.get(Calendar.DAY_OF_MONTH); ,Java,Java输入输出流,读出记录 -需要从一行中将各成员变量分离出来; -java.util.StringTokenizer 提供了很多实用的方法来分隔带分隔符的字符串; eg: StringTokenizer tokenizer = new StringTokenizer(line,”|”); StringTokenizer tokenizer = new StringTokenizer(line,”|,.”); 如果不指定分隔符集合,默认的就是” tnrf”;,Java,Java输入输出流,StringTokenizer(String str, String delim) StringTokeizer(String str) boolean hasMoreTokens() String nextToken() int countTokens() while(tokenizer.hasMoreTokens() String token = tokenizer.nextToken(); / process token ,Java,Java输入输出流,读出记录 -为Employee类增加一个readData方法 public void readData(BufferedReader in) throws IOException String s = in.readLine(); StringTokenizer t = new StringTokenizer(s, “|“); name = t.nextToken(); salary = Double.parseDouble(t.nextToken(); int y = Integer.parseInt(t.nextToken(); int m = Integer.parseInt(t.nextToken(); int d = Integer.parseInt(t.nextToken(); GregorianCalendar calendar = new GregorianCalendar(y, m - 1, d); hireDay = calendar.getTime(); ,Java,Java输入输出流,写入一个Employee数组 -主类中增加一个写数组的静态方法 static void writeData(Employee employees, PrintWriter out) throws IOException / write number of employees out.println(employees.length); for (Employee e : employees) e.writeData(out); ,Java,Java输入输出流,读出一个Employee数组 -主类中增加一个读数组的静态方法 static Employee readData(BufferedReader in) throws IOException / retrieve the array size int n = Integer.parseInt(in.readLine(); Employee employees = new Employeen; for (int i = 0; i n; i+) employeesi = new Employee(); employeesi.readData(in); return employees; ,Java,Java输入输出流,采用二进制的方式存储Employee数组; -当存储大量Employee对象时,采用文本方式保存每个对象的长度不等,因此很难实现随机存取; -采用二进制存储方式让每条记录具有相同的长度; -可以通过RandomAccessFile实现随机存取; -简单数据类型通过DataOutput接口的相关方法可以实现定长写入文件; -String类型的数据长度不定,需提供方法将字符串以固定长度写入文件; -自定类DataIO实现定长字符串的输入与输出;,Java,Java输入输出流,实现字符串定长写入 -方法以参数size指定字符串写入的长度,如果不够,以0补充; 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); ,Java,Java输入输出流,实现定长字符串输入 -方法从输入流中读取字符,直到读取了size个字符或者遇到一个0值的字符为止,并跳过输入域中其他的0值; public static String readFixedString(int size, DataInput in) throws IOException StringBuilder b = new StringBuilder(size); int i = 0; boolean more = true; while (more ,Java,Java输入输出流,写入定长的记录 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); ,Java,Java输入输出流,读出定长的记录 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(); ,Java,Java输入输出流,设定每个Employee记录的长度为100字节; -年、月、日三整型：12字节； -salary, double类型：8字节； -name:80字节； 随机存取 -把文件指针定位到第3条记录 long n=3; int RECORD_SIZE=100; in.seek(n-1)*RECORD_SIZE); -读取一条记录 Employee e=new Employee(); e.readData(in); -修改记录并存回相同的位置 in.seek(n-1)*RECORD_SIZE); e.writeData(out);,Java,Java输入输出流,设定每个Employee记录的长度为100字节; -年、月、日三整型：12字节； -salary, double类型：8字节； -name:80字节； 随机存取 -把文件指针定位到第3条记录 long n=3; int RECORD_SIZE=100; in.seek(n-1)*RECORD_SIZE); -读取一条记录 Employee e=new Employee(); e.readData(in); -修改记录并存回相同的位置 in.seek(n-1)*RECORD_SIZE); e.writeData(out);,