java 实现Comparable接口排序,升序、降序、倒叙

本人由于项目开发中需要对查询结果list进行排序，这里根据的是每一个对象中的创建时间降序排序。本人讲解不深，只实现目的，如需理解原理还需查阅更深的资料。

1.实现的效果

2.创建排序的对象

package com.practice.test.comparable;
import java.util.Date;


public class MySortBean implements Comparable {
  private String name;
  private int age;
  private Date createTime;


  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }

  public int getAge() {
    return age;
  }

  public void setAge(int age) {
    this.age = age;
  }

  public Date getCreateTime() {
    return createTime;
  }

  public void setCreateTime(Date createTime) {
    this.createTime = createTime;
  }

//  @Override
//  public int compareTo(MySortBean o) {
//    if (this.age>o.age){
//      return -1;
//    }else if (this.age==o.age){
//      return 0;
//    }
//    return 1;
//  }
    @Override
    public int compareTo(MySortBean o) {
      if (this.createTime.compareTo(o.getCreateTime())>0){
 return -1;
      }else if (this.createTime.compareTo(o.getCreateTime())==0){
 return 0;
      }
      return 1;
    }

  @Override
  public String toString() {
    return "MySortBean{" +
 "name='" + name + ''' +
 ", age=" + age +
 '}';
  }
}

3.编写test方法

package com.practice.test;
import com.practice.test.comparable.MySortBean;
import com.spring.testlist.util.DateUtil;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;

import java.util.ArrayList;
import java.util.Date;
import java.util.List;


@RunWith(SpringJUnit4ClassRunner.class)
public class TestCompare {

  @Test
  public void testComparable(){
    MySortBean m1 = new MySortBean();
    m1.setAge(1);
    m1.setCreateTime(DateUtil.parseDate("2019-01-21 16:13:18"));
    MySortBean m2 = new MySortBean();
    m2.setAge(2);
    m2.setCreateTime(DateUtil.parseDate("2019-01-23 16:13:18"));
    MySortBean m3 = new MySortBean();
    m3.setAge(3);
    m3.setCreateTime(DateUtil.parseDate("2019-01-22 16:13:18"));
    MySortBean m4 = new MySortBean();
    m4.setAge(4);
    m4.setCreateTime(DateUtil.parseDate("2019-01-24 16:13:18"));
    MySortBean m5 = new MySortBean();
    m5.setAge(5);
    m5.setCreateTime(DateUtil.parseDate("2019-01-25 16:13:18"));
    List l = new ArrayList<>(5);
    l.add(m1);
    l.add(m2);
    l.add(m4);
    l.add(m5);
    l.add(m3);

    System.out.println("排序前：");
    for (MySortBean i:l) {
      System.out.println(DateUtil.formatDate(i.getCreateTime(),"yyyy-MM-dd HH:mm:ss"));
    }
    

    l.sort(MySortBean::compareTo);
    System.out.println("--------");
    System.out.println("排序后：");
    for (MySortBean i:l) {
      System.out.println(DateUtil.formatDate(i.getCreateTime(),"yyyy-MM-dd HH:mm:ss"));
    }
  }
}

4.时间格式化工具类

package com.spring.testlist.util;
import org.apache.commons.lang3.time.DateFormatUtils;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;


public class DateUtil extends org.apache.commons.lang3.time.DateUtils{

    private static String[] parsePatterns = {
 "yyyy-MM-dd", "yyyy-MM-dd HH:mm:ss", "yyyy-MM-dd HH:mm", "yyyy-MM",
 "yyyy/MM/dd", "yyyy/MM/dd HH:mm:ss", "yyyy/MM/dd HH:mm", "yyyy/MM",
 "yyyy.MM.dd", "yyyy.MM.dd HH:mm:ss", "yyyy.MM.dd HH:mm", "yyyy.MM"};

    
    public static String getDate() {
      return getDate("yyyy-MM-dd");
    }
  
  public static String getSizeDate() {
    return getDate("yyyyMMdd");
  }

    
    public static String getDate(String pattern) {
      return DateFormatUtils.format(new Date(), pattern);
    }

    
    public static String formatDate(Date date, Object... pattern) {
      if (date == null) {
 return null;
      }
      String formatDate = null;
      if (pattern != null && pattern.length > 0) {
 formatDate = DateFormatUtils.format(date, pattern[0].toString());
      } else {
 formatDate = DateFormatUtils.format(date, "yyyy-MM-dd");
      }
      return formatDate;
    }

    
    public static String formatDateTime(Date date) {
      return formatDate(date, "yyyy-MM-dd HH:mm:ss");
    }

    
    public static String getTime() {
      return formatDate(new Date(), "HH:mm:ss");
    }

    
    public static String getDateTime() {
      return formatDate(new Date(), "yyyy-MM-dd HH:mm:ss");
    }

    
    public static String getYear() {
      return formatDate(new Date(), "yyyy");
    }

    
    public static String getMonth() {
      return formatDate(new Date(), "MM");
    }

    
    public static String getDay() {
      return formatDate(new Date(), "dd");
    }

    
    public static String getWeek() {
      return formatDate(new Date(), "E");
    }

    
    public static Date parseDate(Object str) {
      if (str == null) {
 return null;
      }
      try {
 return parseDate(str.toString(), parsePatterns);
      } catch (ParseException e) {
 return null;
      }
    }
    
    public static long pastDays(Date date) {

      long t = System.currentTimeMillis()- date.getTime();
      return t / (24 * 60 * 60 * 1000);
    }
    
    public static long pastHour(Date date) {
      long t =System.currentTimeMillis() - date.getTime();
      return t / (60 * 60 * 1000);
    }
    
    public static long pastMinutes(Date date) {
      long t = System.currentTimeMillis() - date.getTime();
      return t / (60 * 1000);
    }
    
    public static String formatDateTime(long timeMillis) {
      long day = timeMillis / (24 * 60 * 60 * 1000);
      long hour = (timeMillis / (60 * 60 * 1000) - day * 24);
      long min = ((timeMillis / (60 * 1000)) - day * 24 * 60 - hour * 60);
      long s = (timeMillis / 1000 - day * 24 * 60 * 60 - hour * 60 * 60 - min * 60);
      long sss = (timeMillis - day * 24 * 60 * 60 * 1000 - hour * 60 * 60 * 1000 - min * 60 * 1000 - s * 1000);
      return (day > 0 ? day + "," : "") + hour + ":" + min + ":" + s + "." + sss;
    }
    
    public static double getDistanceOfTwoDate(Date before, Date after) {
      long beforeTime = before.getTime();
      long afterTime = after.getTime();
      return (afterTime - beforeTime) / (1000 * 60 * 60 * 24);
    }
    public static String getFirstDayOfMonth() {
      SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd");
      //获取当前月第一天：
      Calendar c = Calendar.getInstance();
      c.add(Calendar.MONTH, 0);
      c.set(Calendar.DAY_OF_MONTH, 1);//设置为1号,当前日期既为本月第一天
      String first = format.format(c.getTime());
      return first;
    }
    
    public static void main(String[] args) throws ParseException {
// System.out.println(formatDate(parseDate("2010/3/6")));
// System.out.println(getDate("yyyy年MM月dd日 E"));
// long time = new Date().getTime()-parseDate("2012-11-19").getTime();
// System.out.println(time/(24*60*60*1000));
      String sizeDate = formatDate(new Date(),"yyyyMMdd");
      System.out.println(sizeDate);
    }
}

到此就结束了，具体的要实现什么排序，根据示例自己脑补扩展就好了，例子中备注也已经标注。

在使用排序注意两个坑

1.要排序的字段为空的异常处理

2.要排序的字段相等的异常处理

补充知识：Java中的自然排序和比较器排序

写在前面的话：刚开始学习着两者排序时我也是一头雾水，虽然能写出来但是稀里糊涂，几时该用哪个排序一点想法都没有，后来经过研究这两者的作用点不同，自然排序作用在实体类上，而比较器排序作用在装实体类的集合上。

1、自然排序：java.lang.Comparable

Comparable 接口中只提供了一个方法： compareTo(Object obj) ，该方法的返回值是 int 。如果返回值为正数，则表示当前对象(调用该方法的对象)比 obj 对象“大”；反之“小”；如果为零的话，则表示两对象相等。

总结为一句话：实现Comparable，重写 compareTo方法

案列：以TreeMap为例，默认的升序，可以重写自然排序的方法改变原有排序

public static void testComparable(){
    TreeMap tmp = new TreeMap();
    tmp.put(new Car(4), "肆");
    tmp.put(new Car(1), "壹");
    tmp.put(new Car(5), "伍");
    tmp.put(new Car(3), "三");
    tmp.put(new Car(2), "贰");
    System.out.println(tmp);
    //结果：//{Car [price=5.0]=伍, Car [price=4.0]=肆, Car [price=3.0]=三, Car [price=2.0]=贰, Car [price=1.0]=壹}
  }
//自定义TreeMap排序方法  自然排序  

class Car implements Comparable{
  private double price;
  
  public double getPrice() {
    return price;
  }
 
  public void setPrice(double price) {
    this.price = price;
  }
 
  public Car(int price) {
    super();
    this.price = price;
  }
 
  @Override
  public int compareTo(Car o) {
    // TODO Auto-generated method stub
    if(this.price>o.getPrice()){
      return -1;//大的往前排
    }else if(this.price

2、比较器排序：java.util.Comparator 
总结为一句话：实现Comparator 接口，重写compare方法

public static void testComparator(){
    //HashMap hm = new HashMap();
    TreeMap tmp = new TreeMap(new MyComparatorBigtoSmall());
    tmp.put(4, "肆");
    tmp.put(1, "壹");
    tmp.put(5, "伍");
    tmp.put(3, "三");
    tmp.put(2, "贰");
    //System.out.println(tmp);//默认排序结果：{1=壹, 2=贰, 3=三, 4=肆, 5=伍}
    System.out.println(tmp);//修改为比较器排序(升序){5=伍, 4=肆, 3=三, 2=贰, 1=壹}
  }
 
//自定义TreeMap排序方法  比较器排序  
  class MyComparatorBigtoSmall implements Comparator{
 
    @Override
    public int compare(Integer o1, Integer o2) {
      // TODO Auto-generated method stub
      return o2-o1;
    }
  }

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