Comparator<T>实现自定义排序

迷南。 2022-05-20 06:42 253阅读 0赞

## 应用目的 ##

> **不在SQL中进行order by排序，而在Java代码中对某一字段进行排序，目的是为了缓解数据库的查询压力**

## 源码 ##

> *外比较器*  
> *java.util.Comparator*

public interface Comparator<T> { 
        /** * Compares its two arguments for order. * Returns a negative integer,zero, or a positive integer * as the first argument is less than, equal to, or greater than the second. **/
        int compare(T o1, T o2);
    }

## 项目例子 ##

### ComparatorDateTool源码 ###

public class ComparatorTool implements Comparator<CourierInfo> { 
        SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    
        @Override
        public int compare(CourierInfo t1, CourierInfo t2) { 
            if ((t1==null && t2==null) ||
                    (StringUtils.isBlank(t1.getSignTime()) && StringUtils.isBlank(t2.getSignTime()))){ 
                return 0;
            }
            if(t1==null || StringUtils.isBlank(t1.getSignTime())){ 
                return 1;
            }
            if(t2==null || StringUtils.isBlank(t2.getSignTime())){ 
                return -1;
            }
            Date d1, d2;
            try { 
                d1 = format.parse(t1.getSignTime());
                d2 = format.parse(t2.getSignTime());
            } catch (Throwable e) { 
                // 解析出错，则不进行排序
                return 0;
            }
            return d2.compareTo(d1);
        }
    
    }

### ConsignDTO源码 ###

package com.forezp;
    
    public class ConsignDTO { 
    
    	public String scantime; // 扫描时间
    
    	public String getScantime() { 
    		return scantime;
    	}
    	public void setScantime(String scantime) { 
    		this.scantime = scantime;
    	}
    	@Override
    	public String toString() { 
    		return "ConsignDTO [scantime=" + scantime + "]";
    	}
     
    }

### DateStringDescTest源码 ###

package com.forezp;
    
    import java.util.ArrayList;
    import java.util.Collections;
    import java.util.List;
    
    /** * 揽件扫描时间按时间降序排序，最近的日期显示在上面 */
    public class DateStringDescTest { 
    
    	public static void main(String[] args) { 
    		List<ConsignDTO> list = new ArrayList<ConsignDTO>();
    		
    		ConsignDTO teacher = new ConsignDTO();
    		teacher.setScantime("2018/6/8 10:00:00");
    		ConsignDTO teacher2 = new ConsignDTO();
    		teacher2.setScantime("2018/6/2 10:00:00");
    		ConsignDTO teacher3 = new ConsignDTO();
    		teacher3.setScantime("2018/7/10 12:10:00");
    		ConsignDTO teacher4 = new ConsignDTO();
    		teacher4.setScantime("2018/7/10 16:00:00");
    		
    		list.add(teacher);
    		list.add(teacher2);
    		list.add(teacher3);
    		list.add(teacher4);
    		// 排序前
    		System.out.println("=========排序前===========");
    		list.forEach(t -> System.out.println(t.toString()));
    		System.out.println("=========排序后===========");
    		ComparatorDateTool c = new ComparatorDateTool();
    		Collections.sort(list, c);
    		list.forEach(t -> System.out.println(t.toString()));
    		
    	}
    }

### 运行结果 ###

=========排序前===========
    ConsignDTO [scantime=2018/6/8 10:00:00]
    ConsignDTO [scantime=2018/6/2 10:00:00]
    ConsignDTO [scantime=2018/7/10 12:10:00]
    ConsignDTO [scantime=2018/7/10 16:00:00]
    =========排序后===========
    ConsignDTO [scantime=2018/7/10 16:00:00]
    ConsignDTO [scantime=2018/7/10 12:10:00]
    ConsignDTO [scantime=2018/6/8 10:00:00]
    ConsignDTO [scantime=2018/6/2 10:00:00]

## 简单写法 ##

List tracesT = new ArrayList();
    tracesT.add(baoTracesT);
    Collections.sort(tracesT, new Comparator<TaoBaoTraces>() { 
    	SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    
    	private Date getBirthday(TaoBaoTraces m) { 
    		if ((m == null) || (m.getTime() == null))
    			return null;
    		try { 
    			return this.sdf.parse(m.getTime());
    		} catch (ParseException e) { 
    		}
    		return null;
    	}
    
    	public int compare(TaoBaoTraces o1, TaoBaoTraces o2) { 
    		Date d1 = getBirthday(o1);
    		Date d2 = getBirthday(o2);
    		if ((d1 == null) && (d2 == null))
    			return 0;
    		if (d1 == null)
    			return -1;
    		if (d2 == null)
    			return 1;
    		return d1.compareTo(d2);
    	}
    });

## 常见报错 ##

### 报错信息 ###

java.lang.IllegalArgumentException: Comparison method violates its general contract!

### 解决方案 ###

*  选择使用老版本的排序方法，在代码前面加上这么一句

System.setProperty("java.util.Arrays.useLegacyMergeSort", "true");

*  判断排序两个相等返回0

> 可以用对象的compareTo方法，或者自己考虑相等的情况