How to Sort a List of Objects by Multiple Properties in Java?

Sorting a List of Objects by Multiple Properties in Java

In Java, sorting a list of objects by properties requires the use of comparators. Consider the following example:

public class ActiveAlarm {
    public long timeStarted;
    public long timeEnded;
    // ... other fields
}

To sort a list of ActiveAlarm objects by both timeStarted and timeEnded in ascending order, you can use a nested comparator:

List<ActiveAlarm> alarms = ...;

Comparator<ActiveAlarm> comparator =
    Comparator.comparing(alarm -> alarm.timeStarted)
              .thenComparing(alarm -> alarm.timeEnded);

Collections.sort(alarms, comparator);

Using Java 8 Lambda Expressions

Java 8 introduced lambda expressions, which provide a concise way to represent comparators:

Comparator<ActiveAlarm> comparator =
    (alarm1, alarm2) -> {
        int result = Long.compare(alarm1.timeStarted, alarm2.timeStarted);
        if (result == 0) {
            result = Long.compare(alarm1.timeEnded, alarm2.timeEnded);
        }
        return result;
    };

Alternative Sorting Method

Alternatively, you can use a custom Comparator implementation that directly accesses the properties:

Comparator<ActiveAlarm> comparator = new Comparator<ActiveAlarm>() {
    @Override
    public int compare(ActiveAlarm o1, ActiveAlarm o2) {
        int result = Long.compare(o1.timeStarted, o2.timeStarted);
        if (result == 0) {
            result = Long.compare(o1.timeEnded, o2.timeEnded);
        }
        return result;
    }
};

By using comparators, you can easily sort a list of objects by any combination of properties and in any order.

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