J2EE学习总结

/**
  * 时间转换成时间戳,参数和返回值都是字符串
  * @param  s
  * @return res
  * @throws ParseException
 */
public static String dateToStamp(String s) throws ParseException {
    String res;
    //设置时间模版
    SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    Date date = simpleDateFormat.parse(s);
    long ts = date.getTime();
    res = String.valueOf(ts);
    return res;
}

 String date=dateToStamp("2021-09-07 09:09:39");
 System.out.println(date);
//结果：1630976979000

/**
  * 日期转换成时间戳,参数可以是日期,返回值是字符串
 */
public static void Date2TimeStamp() {
   SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
   String format = sdf.format(new Date());
   try {
       String valueOf = String.valueOf(sdf.parse(format).getTime() / 1000);
       System.err.println(valueOf);
   } catch (ParseException e) {
       e.printStackTrace();
   }
}

/**
  * 将时间戳转换为时间,参数和返回值都是字符串
  * @param s
  * @return res
 */
public static String stampToDate(String s){
    String res;
    SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    long lt = new Long(s);
    Date date = new Date(lt);
    res = simpleDateFormat.format(date);
    return res;
    }

 String s="1630339200000";
 String date=stampToDate(s);
 System.out.println(date);
 //2021-08-31 00:00:00

/**
  * 时间戳先转换成日期字符串再转换成日期类型,参数是字符串,返回值是字符串也可是日期
 */
public static void TimeStamp2Date() {
   String timestampString = "1631008034";
   String formats = "yyyy-MM-dd HH:mm:ss";
   Long timestamp = Long.parseLong(timestampString) * 1000;
   //日期格式字符串
   String dateStr = new SimpleDateFormat(formats, Locale.CHINA).format(new Date(timestamp));
   System.err.println(dateStr);
   Date date = null;
   SimpleDateFormat formater = new SimpleDateFormat();
   formater.applyPattern("yyyy-MM-dd HH:mm:ss");
   try {
       date = formater.parse(dateStr);
       System.err.println(date);
   } catch (ParseException e) {
       e.printStackTrace();
   }
}

@Target({ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Autowired {
    boolean required() default true;
}

public enum RetentionPolicy {
    SOURCE,
    CLASS,
    RUNTIME
}

BigDecimal scale = new BigDecimal(totalScore).setScale(2, BigDecimal.ROUND_DOWN);
totalScore = scale.doubleValue();

**补充：**
*   如果传入的对象中包含空指针，会直接抛出空指针异常。Joiner 提供了两个方法，让我们能够优雅的处理待拼接集合中的空指针。
*   如果我们希望忽略空指针，那么可以调用 `skipNulls`方法，得到一个会跳过空指针的 Joiner 实例。如果希望将空指针变为某个指定的值，那么可以调用 `useForNull` 方法，指定用来替换空指针的字符串。
    ```
    Joiner.on(",").skipNulls().join(spuIdList);
    Joiner.on(",").useForNull("#").join(spuIdList);
    ```
*   Joiner 不仅可以返回string ，还有方法能够处理StringBuilder类：
[](https://blog.csdn.net/LXYDSF/article/details/128011305?ops_request_misc=%257B%2522request%255Fid%2522%253A%2522167056121516800182186127%2522%252C%2522scm%2522%253A%252220140713.130102334..%2522%257D&request_id=167056121516800182186127&biz_id=0&utm_medium=distribute.pc_search_result.none-task-blog-2~all~sobaiduend~default-1-128011305-null-null.142^v68^control,201^v4^add_ask,213^v2^t3_esquery_v3&utm_term=%20Joiner.on&spm=1018.2226.3001.4187)二、Joiner.MapJoiner
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
MapJoiner 是 Joiner 的内部静态类，用于帮助将 Map 对象拼接成字符串。

// 将可迭代的参数中所有子元素连接。
String join(Iterable<?> parts)
// 为static对象设置分隔符
static Joiner on(char separator)

@Test
public void testJoiner(){
    Joiner joiner = Joiner.on(',');
    joiner.skipNulls(); // 不起作用!
    String str = joiner.join("wrong", null, "wrong");
    System.out.println(str);
}
// 会抛出 NullPointerException

Joiner skipNulls()
Joiner useForNull(String nullText)
Joiner.MapJoiner withKeyValueSeparator(String keyValueSeparator)