按日期排序ArrayList中的对象？ [英] Sort objects in ArrayList by date?

问题描述

有人可以帮我吗？我发现的每个例子都是按字母顺序排列，而我需要按日期排序的元素。

我的ArrayList包含其中一个数据组件是一个DateTime对象的对象。在DateTime我可以调用函数：

  lt（）// less-than 
 lteq（）// less -than-or-equal-to 
  

所以要比较我可以做一些像：

  if（myList.get（i）.lt（myList.get（j）））{
 // ... 
} 
  

我不知道在if块中该怎么做。任何想法？

解决方案

您可以使对象相当：

  public static class MyObject implements Comparable< MyObject> {
 
 private Date dateTime; 
 
 public Date getDateTime（）{
 return dateTime; 
} 
 
 public void setDateTime（Date datetime）{
 this.dateTime = datetime; 
} 
 
 @Override 
 public int compareTo（MyObject o）{
 return getDateTime（）。compareTo（o.getDateTime（））; 
} 
} 
  

然后通过调用排序：

  Collections.sort（myList）; 
  

但有时您不想更改模型，就像当您要对几种不同的排序属性。在这种情况下，您可以即时创建比较器：

  Collections.sort（myList，new Comparator< MyObject>（） 
 public int compare（MyObject o1，MyObject o2）{
 return o1.getDateTime（）。compareTo（o2.getDateTime（））; 
} 
}）; 
  

但是，只有当您确定dateTime在比较时不为null 。处理null也是明智的，以避免NullPointerExceptions：

  public static class MyObject implements Comparable< MyObject> {
 
 private Date dateTime; 
 
 public Date getDateTime（）{
 return dateTime; 
} 
 
 public void setDateTime（Date datetime）{
 this.dateTime = datetime; 
} 
 
 @Override 
 public int compareTo（MyObject o）{
 if（getDateTime（）== null || o.getDateTime（）== null） 
 return 0; 
 return getDateTime（）。compareTo（o.getDateTime（））; 
} 
} 
  

或者在第二个例子中：

  Collections.sort（myList，new Comparator< MyObject>（）{
 public int compare（MyObject o1，MyObject o2）{
 if（o1.getDateTime（）== null || o2.getDateTime（）== null）
 return 0; 
 return o1.getDateTime（）。compareTo（o2.getDateTime（））; 
} 
}）; 
  

Can someone help me with this? Every example I find is about doing this alphabetically, while I need my elements sorted by date.

My ArrayList contains objects on which one of the datamembers is a DateTime object. On DateTime I can call the functions:

lt() // less-than
lteq() // less-than-or-equal-to

So to compare I could do something like:

if(myList.get(i).lt(myList.get(j))){
    // ...
}

I don't really know what to do inside the if block. Any ideas?

解决方案

You can make your object comparable:

public static class MyObject implements Comparable<MyObject> {

  private Date dateTime;

  public Date getDateTime() {
    return dateTime;
  }

  public void setDateTime(Date datetime) {
    this.dateTime = datetime;
  }

  @Override
  public int compareTo(MyObject o) {
    return getDateTime().compareTo(o.getDateTime());
  }
}

And then you sort it by calling:

Collections.sort(myList);

However sometimes you don't want to change your model, like when you want to sort on several different properties. In that case, you can create comparator on the fly:

Collections.sort(myList, new Comparator<MyObject>() {
  public int compare(MyObject o1, MyObject o2) {
      return o1.getDateTime().compareTo(o2.getDateTime());
  }
});

However, the above works only if you're certain that dateTime is not null at the time of comparison. It's wise to handle null as well to avoid NullPointerExceptions:

public static class MyObject implements Comparable<MyObject> {

  private Date dateTime;

  public Date getDateTime() {
    return dateTime;
  }

  public void setDateTime(Date datetime) {
    this.dateTime = datetime;
  }

  @Override
  public int compareTo(MyObject o) {
    if (getDateTime() == null || o.getDateTime() == null)
      return 0;
    return getDateTime().compareTo(o.getDateTime());
  }
}

Or in the second example:

Collections.sort(myList, new Comparator<MyObject>() {
  public int compare(MyObject o1, MyObject o2) {
      if (o1.getDateTime() == null || o2.getDateTime() == null)
        return 0;
      return o1.getDateTime().compareTo(o2.getDateTime());
  }
});

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