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))){
// ...
}
What should I do inside the if block?
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());
}
});