//这样有错
class DataIO{
    public static void writeFixedString(String name,int size,DataOutputStream out) 
            throws IOException{
        int i=0;
        char ch;
        while(i<size){
            ch=0;
            if(i<name.length()){
                ch=name.charAt(i);
            }
            out.writeChar(ch);
            i++;
        }
    }
    public static String readFixedString(int size,RandomAccessFile in) 
            throws IOException{
        int i=0;
        char ch;
        StringBuilder str=new StringBuilder();
        while(i<size){
            ch=in.readChar();
            if(ch==0)
                break;
            i++;
            str.append(ch);
        }
        in.skipBytes(2*(size-i));
        return str.toString();
    }
}
/*改成这样问题是
  if(ch=='0')
      break;
  从文本中读取'0'字符赋予ch，可ch不会等于'0'了，ch=='0'永远为假，这是为什么？
*/
class DataIO{
    public static void writeFixedString(String name,int size,DataOutputStream out) 
            throws IOException{
        int i=0;
        char ch;
        while(i<size){
            ch=0;
            if(i<name.length()){
                ch=name.charAt(i);
            }
            out.writeChar(ch);
            i++;
        }
    }
    public static String readFixedString(int size,RandomAccessFile in) 
            throws IOException{
        int i=0;
        char ch;
        StringBuilder str=new StringBuilder();
        while(i<size){
            ch=in.readChar();
            if(ch=='0')
                break;
            i++;
            str.append(ch);
        }
        in.skipBytes(2*(size-i));
        return str.toString();
    }
}

//换成这样也不对，到底是为什么，好纠结
class DataIO{
    public static void writeFixedString(String name,int size,DataOutputStream out) 
            throws IOException{
        int i=0;
        char ch;
        while(i<size){
            ch=' ';
            if(i<name.length()){
                ch=name.charAt(i);
            }
            out.writeChar(ch);
            i++;
        }
    }
    public static String readFixedString(int size,RandomAccessFile in) 
            throws IOException{
        int i=0;
        char ch;
        StringBuilder str=new StringBuilder();
        while(i<size){
            ch=in.readChar();
            if(ch==' ')
                break;
            i++;
            str.append(ch);
        }
        in.skipBytes(2*(size-i));
        return str.toString();
    }
}

class DataIO{
    public static void writeFixedString(String name,int size,DataOutputStream out) 
            throws IOException{
        int i=0;
        char ch;
        while(i<size){
            ch=0;
            if(i<name.length()){
                ch=name.charAt(i);
            }
            out.writeChar(ch);
            i++;
        }
    }
    public static String readFixedString(int size,RandomAccessFile in) 
            throws IOException{
        int i=0;
        char ch;
        StringBuilder str=new StringBuilder();
        while(i<size){
            ch=in.readChar();
            if(ch==0)
                break;
            i++;
            str.append(ch);
        }
        in.skipBytes(2*(size-i));
        return str.toString();
    }
}

------解决方案--------------------
改成0的Assic 码，试试