以转小写为例

方法一

    public static String firstToLowerCase1(String s){
        if(Character.isLowerCase(s.charAt(0)))
            return s;
        else
            return (new StringBuilder()).append(Character.toLowerCase(s.charAt(0))).append(s.substring(1)).toString();
    }

方法二

    public static String firstToLowerCase2(String s){
        Character c = s.charAt(0);
        if (Character.isLowerCase(c)){
            return s;
        }else {
            char[] chars = s.toCharArray();
            chars[0] = Character.toLowerCase(c);
            return String.valueOf(chars);
        }
    }

对比

public class Main {
    public static void main(String[] args) {
        /**
         * 各运行10000000次
         */

        long time1 = getTaskRunTime(() -> {
            for (int i = 0; i < 10000000; i++) {
                firstToLowerCase1("AbCCC");
            }
        });

        long time2 = getTaskRunTime(() -> {
            for (int i = 0; i < 10000000; i++) {
                firstToLowerCase2("AbCCC");
            }
        });
        
        System.out.println("time1:" + time1);  // time1:434
        System.out.println("time2:" + time2);  // time2:190
    }

    private static long getTaskRunTime(Runnable runnable){
        long start = System.currentTimeMillis();
        runnable.run();
        return System.currentTimeMillis() - start;
    }
}