I am comparing two strings, in Java, to see how many characters from the first string show up in the second string. The following is some expectations:

matchingChars("AC", "BA") ? 1  
matchingChars("ABBA", "B") ? 2  
matchingChars("B", "ABBA") ? 1  

My approach is as follows:

 public int matchingChars(String str1, String str2) {

    int count = 0;

     for (int a = 0; a < str1.length(); a++)
    {    
        for (int b = 0; b < str2.length(); b++)

          {  char str1Char = str1.charAt(a);
             char str2Char = str2.charAt(b);

                if (str1Char == str2Char)
                   {   count++;
                       str1 =  str1.replace(str1Char, '0');
                   }
          }
    }
     return count; 
    }  

I know my approach is not the best, but I think it should do it. However, for

   matchingChars("ABBA", "B") ? 2  

My code yields "1" instead of "2". Does anyone have any suggestion or advice? Thank you very much.