java 把字符串中的小写字母换成大写字母，大写字母换成小写字母

public static String convert(String str){
        char[] cArray = str.toCharArray();
        StringBuffer sb = new StringBuffer();
        for(int i=0; i<cArray.length; i++) {
            if(cArray[i] >= 'a' && cArray[i] <='z'){
                sb.append((cArray[i]+"").toUpperCase());
            }else if(cArray[i] >= 'A' && cArray[i] <='Z'){
                sb.append((cArray[i]+"").toLowerCase());
            }else {
                sb.append(cArray[i]);
            }            
        }
        return sb.toString();
    }