JAVA 如何将String进行大小写转换

/**格式化字段**/
    private String convertString(String str, Boolean beginUp){
        char[] ch = str.toCharArray();
		StringBuffer sbf = new StringBuffer();
        for(int i=0; i< ch.length; i++){
            if(i == 0 && beginUp){//如果首字母需大写
                sbf.append(charToUpperCase(ch[i]));
            }else{
                sbf.append(charToLowerCase(ch[i]));
            }
        }
        return sbf.toString();
    }
    
    /**转大写**/
    private char charToUpperCase(char ch){
        if(ch <= 122 && ch >= 97){
            ch -= 32;
        }
        return ch;
    }
    /***转小写**/
    private char charToLowerCase(char ch){
        if(ch <= 90 && ch >= 65){
            ch += 32;
        }
        return ch;
    }/**格式化字段**/
    private String convertString(St