What is the correct way to sort a list using a Java lambda comparator, and how can I fix the type mismatch error?

General Discussions
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1

I’m trying to sort a List<Message> using a lambda expression in Java 8 like this:

List<Message> messagesByDeviceType = new ArrayList<>();
messagesByDeviceType.sort((Message o1, Message o2) -> o1.getTime() - o2.getTime());

However, I’m getting compilation errors related to a type mismatch: it says it cannot convert from long to int.

I understand this has something to do with how Comparator expects the return type, but I’m not sure how to properly use a Java lambda comparator when the comparison involves long values like timestamps. What’s the right way to resolve this?

2

The safest and most readable way is to use the Long.compare() method, which handles long values properly without manual casting or subtraction.

messagesByDeviceType.sort((o1, o2) -> Long.compare(o1.getTime(), o2.getTime()));

Long.compare() takes two long values and returns an int result, which is exactly what Comparator needs. It avoids issues with overflow or type mismatch.

3

Java 8 provides utility methods that make sorting by a primitive field super clean.

If you’re sorting by a long-returning getter like getTime(), this approach is elegant:

messagesByDeviceType.sort(Comparator.comparingLong(Message::getTime));

4

If you’re absolutely sure the values returned by getTime() fit safely within an int range (which is rare with timestamps), you could cast them.

But this is not recommended for general use:

messagesByDeviceType.sort((o1, o2) -> (int)(o1.getTime() - o2.getTime()));