如何从带有空格的字符串中删除最后一个字符

Question

I am creating a calculator app with a backspace button, and I need to remove the characters that are not spaces.

Say I have a string such as:

String s = "1 + 2 - 3";

I want to be able to click the button and have it remove 'o' first. If I click the button again, instead of removing the space after the '-', I want to remove the '-' itself, etc.

I initially tried something like:

s = s.replaceAll(" ", "");
s = s.substring(0,field.length()-1);
s = s.replaceAll("", " ").trim();

The only reason I need the whitespace is that there is a class that evaluates the expression and requires the white space to evaluate it correctly.

With what I have right now, if I were to click backspace and delete '3', then click another button like 2, what I get is:

"1 + 2 -2"

However, what I want is:

"1 + 2 - 2" 

Answer 1

The trick would be to use .trim() when you remove a char to also remove the space :

static String removeAChar(String s) { //1st trim() to be sure, 2nd trim() to remove extra space
    return s.trim().substring(0, s.length() - 1).trim();
}

---

And this would give :

public static void main(String[] args) {
    String s = "h e l l o";
    System.out.println(s);
    for (int i = 0; i < 4; i++) {
        s = removeAChar(s);
        System.out.println(s);
    }
}
/*
h e l l o
h e l l
h e l
h e
h

Answer 2

Trim method will remove all the leading whitespaces also. So if you have " hello" it will remove the first space before "h". If you don't want that, here is a solution

public static String RemoveLastNotSpaceChar(String str){
      if(str==null)
          return null;
      str = removeTrailingSpaces(str);
      if(str.length()>0)
          return str.substring(0, str.length() - 1);
      return "";
}

public static String removeTrailingSpaces(String param) 
    {
        if (param == null)
            return null;
        int len = param.length();
        for (; len > 0; len--) {
            if (!Character.isWhitespace(param.charAt(len - 1)))
                break;
        }
        return param.substring(0, len);
    }