ExpressionParser.java

import java.util.ArrayList;
import java.util.List;

public class ExpressionParser {

    public static void main(String[] arg) {
        // Assume the entered expression has only the expected characters otherwise they are ignored
        String input = "(400+8) * (6-5)/((311-2) *(2+2)) xxx";

        // Parse input
        List<String> infix = parse(input.toCharArray());

        // Print the output
        for(String element : infix) {
            System.out.print(element + " ");
        }
    }

    private static List<String> parse(char[] input) {
        List<String> parsed = new ArrayList<String>();
        for (int i = 0; i < input.length; ++i) {
            char c = input[i];
            if (Character.isDigit(c)) {
                String number = input[i] + "";
                for (int j = i + 1; j < input.length; ++j) {
                    if (Character.isDigit(input[j])) {
                        number += input[j];
                        i = j;
                    } else {
                        break;
                    }
                }
                parsed.add(number);
            } else if (c == '*' || c == '/' || 
                       c == '+' || c == '^' || 
                       c == '-' || c == '(' || c == ')') {
                parsed.add(c + "");
            }
            // else ignore other characters
        }
        return parsed;
    }

}