Arithmetic Stacks

Question

My program distinguishes between the operators and operands and places them in separate stacks. If the character is an operator, I want compare its precedence to that of the operator on top of the operator stack. If the current operator has higher precedence than the one currently on top of the stack, or the stack is empty, it should be pushed onto the operator stack. If the current operator has the same or lower precedence, the operator on top of the operator stack must be evaluated next. This is done by popping that operator off the operator stack along with a pair of operands from the operand stack and writing a new line in the output table. help
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.Stack;
class Prog3<T> extends MyArrayList<T> {
// method compares operators to establish precedence
     private static int prec(char x) {
     if (x == '+' || x == '-')
          return 1;
     if (x == '*' || x == '/' || x == '%')
          return 2;
     return 0;
     public static void main (String args [])
          Stack operatorStack = new Stack();
          Stack operandStack = new Stack();
          BufferedReader keyboard = new BufferedReader (new InputStreamReader (System.in));
          String line = null;
          while (true){
                System.out.print("Enter an expression: ");
                try{
                      line = keyboard.readLine();
                }catch(IOException e){}
                for(int i=0; i < line.length(); i++){
                  if(line.charAt(i) == ' '){
                  }else if(line.charAt(i) == '/'){
                      operatorStack.push('/');
                  }else if(line.charAt(i) == '*'){
                      operatorStack.push('*');
                  }else if(line.charAt(i) == '+'){
                      operatorStack.push('+');
                  }else if(line.charAt(i) == '-'){
                      operatorStack.push('-');
                  }else if(line.charAt(i) >= 97 && line.charAt(i) <= 109){
                   operandStack.push(line.charAt(i));
                System.out.println(operatorStack);
                System.out.println(operandStack);
     }

Answer 1

My program distinguishes between the operators and operands and places them in separate stacks.Why?
If the character is an operator, I want compare its precedence to that of the operator on top of the operator stack.Why?
Why aren't you using recursive descent? or a parser generator?
Why are you implementing this technique? It's been obsolete for 40 years.