求两个字符串中的最大公共字串

public static String getCommonSub(String str1, String str2) { 
        String bigStr = null; 
        String smallStr = null; 
        if (str1.length() - str2.length() > 0) { 
            bigStr = str1; 
            smallStr = str2; 
        } else { 
            bigStr = str2; 
           smallStr = str1; 
       } 
 
       for (int i = smallStr.length(); i > 0; i--) { 
           int begain = 0; 
           int end = i; 
           for (int j = smallStr.length() - i; j >= 0; j--) { 
               String result = smallStr.substring(begain, end); 
               if (bigStr.contains(result)) { 
 
                   return result; 
 
               } 
 
               begain++; 
               end++; 
           } 
       } 
       return null; 
 
   }