Converting an Infix Expression to Postfix

I wrote the following method to convert an infix expression to a postfix expression. If the commented code is removed, the program will return a valid expression given single digit integers; 11 + 2 will result in 1 1 2 + rather than 11 2 +, but 1 + 2 - 3 returns 1 2 + 3 -. In order to include n-digit numbers, I implemented an if-statement which concatenate a space to postfix string if the adjacent character is NOT a number. However, during execution the following error is produced:

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 3

Code:

public static String postfixConversion(String input) {

            int i;
            String postfix = "";

            Stack<Character> stack = new Stack<Character>();

            for (i = 0; i < input.length(); i++) {
                    while (input.charAt(i) == ' ') {
                            ++i;
                    }

                    if (Character.isDigit(input.charAt(i))) { 
                                    postfix += input.charAt(i);

                                    if (!Character.isDigit(input.charAt(i+1))) { //EXCEPTION OCCURS
                                           postfix += ' ';
                                    } 
                    }

                    else if (precedenceLevel(input.charAt(i)) != 0) {
                            while ((!stack.isEmpty()) && (precedenceLevel(stack.peek()) >= precedenceLevel(input.charAt(i))) && (stack.peek() != '(')) {
                                    postfix += stack.peek();
                                    postfix += ' ';
                                    stack.pop();
                            }

                            stack.push(input.charAt(i));
                    }

                    else if (input.charAt(i) == '(') {
                            stack.push(input.charAt(i));
                    }

                    else if (input.charAt(i) == ')') {
                            while (!stack.isEmpty() && stack.peek() != '(') {
                                    postfix += stack.peek();
                                    stack.pop();
                            }

                            stack.pop();
                    }
            }

            while (!stack.isEmpty()) {
                    postfix += stack.peek();
                    postfix += ' ';
            }

            return postfix;

    }

Any assistance would be appreciated :)