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