How to Sort an Arraylist of Objects by a Property

Sort ArrayList of custom Objects by property

Since Date implements Comparable, it has a compareTo method just like String does.

So your custom Comparator could look like this:

public class CustomComparator implements Comparator<MyObject> {
    @Override
    public int compare(MyObject o1, MyObject o2) {
        return o1.getStartDate().compareTo(o2.getStartDate());
    }
}

The compare() method must return an int, so you couldn't directly return a boolean like you were planning to anyway.

Your sorting code would be just about like you wrote:

Collections.sort(Database.arrayList, new CustomComparator());

A slightly shorter way to write all this, if you don't need to reuse your comparator, is to write it as an inline anonymous class:

Collections.sort(Database.arrayList, new Comparator<MyObject>() {
    @Override
    public int compare(MyObject o1, MyObject o2) {
        return o1.getStartDate().compareTo(o2.getStartDate());
    }
});

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Since java-8

You can now write the last example in a shorter form by using a lambda expression for the Comparator:

Collections.sort(Database.arrayList, 
                        (o1, o2) -> o1.getStartDate().compareTo(o2.getStartDate()));

And List has a sort(Comparator) method, so you can shorten this even further:

Database.arrayList.sort((o1, o2) -> o1.getStartDate().compareTo(o2.getStartDate()));

This is such a common idiom that there's a built-in method to generate a Comparator for a class with a Comparable key:

Database.arrayList.sort(Comparator.comparing(MyObject::getStartDate));

All of these are equivalent forms.

Sort arraylist of objects by one of their properties (double value)

Using Collections, you can do it like this

Collections.sort(bounties, new Comparator<BountyObject>() {
        @Override public int compare(BountyObject bo1, BountyObject bo2) {
            return (bo1.getAmount() >  bo2.getAmount() ? 1:-1); 
        }

Sorting an arraylist of object base on its property in custom order

Unless you actually try to sort a list containing the items listed in the parameters of Ordering.explicit, it doesn't know how to sort them. It's also not clear if you've overridden your WeekSales.equals (and hashCode), which would be required for that to work too.

You need to actually tell it the fields to compare:

class YourComparator implements Comparator<WeekSales> {
  private final Ordering<String> dayOrdering = Ordering.explicit(
    "Monday", "Tuesday", ... );

  @Override public int compare(WeekSales a, WeekSales b) {
    return dayOrdering.compare(a.getDay(), b.getDay());
  }
}

Sorting ArrayList of Objects by Object attribute

You need to write a Comparator<MyObject> and use Collections.sort(List<T>, Comparator<? super T> to sort your List.

Or else, your MyObject can also implements Comparable<MyObject>, defining a natural ordering that compares on your specific attribute, and then use Collections.sort(List<T> instead.

See also

• Java Tutorials/Object Ordering

Related questions

On sorting List on various criteria:

• Sorting an ArrayList of Contacts

On Comparator and Comparable

• When to use Comparable vs Comparator
• difference between compare() and compareTo()
• Comparable and Comparator contract with regards to null
• Why does the Java Collections Framework offer two different ways to sort?

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