我写了一个简单的词法分析器，我不知道它有什么问题？

Question

用Java编写一个简单的词法扫描程序，识别以下词汇：

• 数字（即数字序列）
• 符号算术运算：+，*
• 左右括号
• 标识符，定义为不以数字开头的字母和数字序列

Lexemes被空格分隔（空格，制表符，换行符）。任何不匹配的输入都是错误。

运行程序的示例如下：

% echo " 1* (27+x) " | java LexerTest
NUMBER [1]
TIMES [*]
LPAREN [(]
NUMBER [27]
PLUS [+]
ID [x]
RPAREN [)]
% echo "((1:  + 877x)+var1*x)" | java LexerTest
LPAREN [(]
LPAREN [(]
NUMBER [1]
ERROR [Unknown char :]
PLUS [+]
NUMBER [877]
ID [x]
RPAREN [)]
PLUS [+]   
ID [var1]
TIMES [*]
ID [x]
RPAREN [)]

这是我的代码......有人帮助我！

    import java.io.BufferedReader;
    import java.io.IOException;
    import java.io.InputStream;
    import java.io.InputStreamReader;


public class Lexer {
private Token object;
private char[] a= new char[100];
private int i=0;
private int j=0;

/**
 *
 * @param is
 * @throws IOException
 */
public Lexer(InputStream is) throws IOException
{
     try (BufferedReader input = new BufferedReader(new InputStreamReader(System.in)))                    {
    int x;
    char[] b= new char[1000];

        while((x = input.read()) != -1){

                b[j]=(char)x;
                j++;
        }
    }
    //a = char[j];
}

private Lexer() {
    throw new UnsupportedOperationException("Not supported yet."); 
}

 public Token getToken()    {
    if(j== 0)
    {return null;}
    else if(Character.isLetter(a[i]))
            { object.id = a[i]+object.id;
              i++;
              while(Character.isLetter(a[i])||Character.isDigit(a[i]))
              {
                  object.id = a[i]+object.id;
                  i++;
              }
              object.format=2;
              return object;
         }   
    else if(Character.isDigit(a[i]))
    {object.id = a[i]+object.id;
              i++;
              while(Character.isDigit(a[i]))
              {
                  object.id = a[i]+object.id;
                  i++;
              }
              object.format=1;
              return object;
    }
    else{
         switch (a[i]) {
case '(':
          object.id = a[i]+object.id;
          i++;
          object.format=4;
          return object;

case ')':
    object.id = a[i]+object.id;
          i++;
          object.format=5;
          return object;

case '+':
    object.id = a[i]+object.id;
          i++;
          object.format=6;
          return object;


case '*':
    object.id = a[i]+object.id;
          i++;
          object.format=3;
          return object;

case '\n':
   object.id = a[i]+object.id;
          i++;
          object.format=8;
          return object;

default:
    object.id = a[i]+object.id;
          i++;
          object.format=7;
          return object; }        
    }

}           
public class Token{
    private Token(String id,int format)
      {
        this.id = id;   
        this.format = format;  
      }      
    public  String id;
    public int format;

    public String toString(){

         switch (object.format) {

case '1':
    return("NUMBER [" + id + "]");
case '2':
    return("ID [" + id + "]");

case '4':
    return("LPAREN [" + id + "]");

case '5':
    return("RPAREN [" + id + "]");

case '6':
    return("PLUS [" +  id + "]");


case '3':
    return("TIMES [" + id + "]");

case '8':
    return null;

case '7'://default:
    return("ERROR [Unknown char" + id + "]"); }

 return null;

    }
}
public static void main(String[] args) {
     // Get tokens and display each on a line
    Lexer lexer = new Lexer();
    Lexer.Token token;
    while ((token = lexer.getToken()) != null)
        System.out.println(token.toString());
 }
 }