java8 stream取出 最大值/最小值

注:轉載請注明出處!!!

這里直接用取出多個對象中某個值 最大/最小 來進行舉例

直接看代碼

/**
 * 時間測試類
 */
class TimeTest {
    private Date time;

    public Date getTime() {
        return time;
    }

    public void setTime(Date time) {
        this.time = time;
    }
}

工具類:

public class DateUtils extends org.apache.commons.lang.time.DateUtils{
    //自定義日期格式
    public static final String SIMPLE_DATEFORMAT = "yyyy-MM-dd HH:mm:ss";

    public static final String SIMPLE_DATEFORMAT_YMD = "yyyy-MM-dd";
    //日期比較方法
    public static int compareDate(Date first, Date second){
       return first.compareTo(second);
    }
    //日期轉string
   public static String formatDateYMDHMS(Date date){
        if(date==null){
            return null;
        }
        return format(date,SIMPLE_DATEFORMAT);
    }
    //通用格式化
    public static String format(Date date, String pattern) {
        if (date == null) {
            return null;
        }
        SimpleDateFormat format = new SimpleDateFormat(pattern);
        return format.format(date);
    }
}

 

運行:

public static void main(String[] args) {
        int i = 0;
        List<TimeTest> timeList = new ArrayList<>();
        while (++i <= 5) {
            TimeTest time = new TimeTest();
            time.setTime(DateUtils.addMonths(DateUtils.now(), i));
            timeList.add(time);
        }
        //插入一個空值
        TimeTest time = new TimeTest();
        timeList.add(time);
        System.out.println("---------------初始值------------------");
        timeList.forEach(o -> {
            System.out.println(DateUtils.formatDateYMDHMS(o.getTime()));
        });
        System.out.println("---------------------------------------");
        //下面有兩種寫法 推薦使用方法一
        //代碼解釋: 將timeList流首先過濾時間不為空的,將需要比較的值轉出map然后去重,最后取出最大值/最小值
        //方法一 min(DateUtils::compareDate)  這種寫法需要自己寫工具類,且compareDate必須是static修飾
        Date min = timeList.stream().filter(o -> o.getTime() != null).map(TimeTest::getTime).distinct().min(DateUtils::compareDate).get();
        //方法二 max((e1, e2) -> e1.compareTo(e2))這種寫法需要類里面含有比較方法
        Date max = timeList.stream().filter(o -> o.getTime() != null).map(TimeTest::getTime).distinct().max((e1, e2) -> e1.compareTo(e2)).get();
        System.out.println("min=" + DateUtils.formatDateYMDHMS(min));
        System.out.println("max=" + DateUtils.formatDateYMDHMS(max));
    }

輸出:

---------------初始值------------------
2019-09-09 14:35:53
2019-10-09 14:35:53
2019-11-09 14:35:53
2019-12-09 14:35:53
2020-01-09 14:35:53
null
---------------------------------------
min=2019-09-09 14:35:53
max=2020-01-09 14:35:53

Process finished with exit code 0