little tips

java-tips-1

        // 输出10位数字，左填充 0000000199
        int count = 199;
        // 字符串填充，10个'0'
        String s1 = "0000000000" + count;
        System.out.println(s1.substring(s1.length() - 10));

        // 19位以下，数字填充
        String s2 = String.valueOf(10_000_000_000L + count);
        System.out.println(s2.substring(1));

        // printf风格，性能差
        System.out.println(String.format("%1$010d", count));
        System.out.println(String.format("%010d", count));

        // 循环填充
        int len =10;
        StringBuilder sb = new StringBuilder(len);
        for (int i = len - (int) Math.log10(count) ; i > 1 ; i--) {
            sb.append('0');
        }
        sb.append(count);
        System.out.println(sb.toString());