Thread: convert infix to postfix

import java.io.IOException;
import java.io.*;
import java.util.Arrays;
 
public class datastruct {
  private Stack theStack;
 
  public static void main(String[] args) throws IOException {
 
  	BufferedReader A = new BufferedReader (new InputStreamReader(System.in));
  	String mathex;
    System.out.println("enter a mathematical expresion:" );
	  mathex = A.readLine();
 
    String input = null;
    String output;
    datastruct theTrans = new datastruct(mathex);
    output = theTrans.doTrans(); 
    System.out.println("postfix is" + output + '\n');
    System.out.println("infix is" + mathex + '\n');
 
 
  }
 
  private String input;
 
  private String output = "";
 
  public datastruct(String in) {
    input = in;
    int stackSize = input.length();
    theStack = new Stack(stackSize);
  }
 
  public String doTrans() {
    for (int j = 0; j < input.length(); j++) {
 
      char ch = input.charAt(j);
      switch (ch) {
      case '+': 
      case '-':
        gotOper(ch, 1); 
        break; 
      case '*': 
      case '/':
        gotOper(ch, 2);
        break; 
      case '(': 
        theStack.push(ch); 
        break;
      case ')':
        gotParen(ch); 
        break;
      default:
        output = output + ch; 
        break;
 
 
      }
    }
    while (!theStack.isEmpty()) {
      output = output + theStack.pop();
 
    }
 
    System.out.println(output);
    return output; 
  }
 
  public void gotOper(char opThis, int prec1) {
    while (!theStack.isEmpty()) {
      char opTop = theStack.pop();
      if (opTop == '(') {
        theStack.push(opTop);
        break;
      }
      else {
        int prec2;
        if (opTop == '+' || opTop == '-')
          prec2 = 1;
 
        else
          prec2 = 2;        if (prec2 < prec1) 
        { 
          theStack.push(opTop); 
          break;
        } else
 
          output = output + opTop;
 
      }
    }
    theStack.push(opThis);
  }
 
  public void gotParen(char ch){ 
    while (!theStack.isEmpty()) {
      char chx = theStack.pop();
      if (chx == '(') 
        break; 
      else
        output = output + chx; 
    }
  }
 
 
  class Stack {
    private int maxSize;
    private char[] stackArray;
    private int top;
    public Stack(int max) {
      maxSize = max;
      stackArray = new char[maxSize];
      top = -1;
    }
    public void push(char j) {
      stackArray[++top] = j;
    }
    public char pop() {
      return stackArray[top--];
    }
    public char peek() {
      return stackArray[top];
    }
    public boolean isEmpty() {
      return (top == -1);
 
    }
  }
 
}

hi, just want to ask how can I input the "System.out.println("invalid")"
it is when the user did not satisfy the condition it will display invalid... help me please?
I am stuck here...^^thankyou

help me..^^, thaks!!