How to Sort a List of Java Objects Based on Multiple Properties?

Sorting List of Objects Based on Property

You have a custom Java class ActiveAlarm that contains properties related to alarms, including timeStarted and timeEnded. You need to sort a List in ascending order by timeStarted and then by timeEnded.

Solution: Using Comparator

In Java, you can sort objects using a Comparator. A comparator is an interface that represents a function to compare two objects. You can create a custom comparator to compare ActiveAlarm objects based on the specified criteria.

Here's an example implementation using a comparator:

import java.util.Collections;
import java.util.Comparator;

class ActiveAlarm {
    public long timeStarted;
    public long timeEnded;
    // Other properties and accessor methods...
}

public class SortingAlarms {

    public static void main(String[] args) {
        List<ActiveAlarm> alarms = new ArrayList<>();
        
        // Add alarms to the list
        
        // Create a comparator to sort by timeStarted and then timeEnded
        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;
            }
        };

        Collections.sort(alarms, comparator);
        
        // Print the sorted list
        for (ActiveAlarm alarm : alarms) {
            System.out.println(alarm.timeStarted + " - " + alarm.timeEnded);
        }
    }
}

This comparator first compares the timeStarted values of the alarms, and if those are equal, it compares the timeEnded values. The result of the comparison (1, 0, or -1) is used to determine the sorting order.

Note: For Java 8 and later, you can use lambda expressions to simplify the implementation of the comparator:

Collections.sort(alarms, (a1, a2) -> Long.compare(a1.timeStarted, a2.timeStarted) != 0 ? Long.compare(a1.timeStarted, a2.timeStarted) : Long.compare(a1.timeEnded, a2.timeEnded));

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