How to sort List of objects by some property

Question

I have simple class   public class ActiveAlarm       public long timeStarted      public long timeEnded      private String name           private String description           private String event      private boolean live   false      and List lt ActiveAlarm gt  con  How to sort in ascending order by timeStarted  then by timeEnded  Can anybody help  I know in C   with generic algorithm and overload operator  lt   but I am new to Java

User · Answer

Either make ActiveAlarm implement Comparable lt ActiveAlarm gt  or implement Comparator lt ActiveAlarm gt  in a separate class  Then call   Collections sort list     or  Collections sort list  comparator     In general  it s a good idea to implement Comparable lt T gt  if there s a single  natural  sort order    otherwise  if you happen to want to sort in a particular order  but might equally easily want a different one  it s better to implement Comparator lt T gt   This particular situation could go either way  to be honest    but I d probably stick with the more flexible Comparator lt T gt  option    EDIT  Sample implementation   public class AlarmByTimesComparer implements Comparator lt ActiveAlarm gt       Override   public int compare ActiveAlarm x  ActiveAlarm y           TODO  Handle null x or y values     int startComparison   compare x timeStarted  y timeStarted       return startComparison    0   startComparison                                   compare x timeEnded  y timeEnded             I don t know why this isn t in Long      private static int compare long a  long b        return a  lt  b   -1            a  gt  b   1            0

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public class ActiveAlarm implements Comparable lt ActiveAlarm gt        public long timeStarted      public long timeEnded      private String name           private String description           private String event      private boolean live   false       public int compareTo ActiveAlarm a            if   this timeStarted  gt  a timeStarted               return 1          else if   this timeStarted  lt  a timeStarted               return -1          else                if   this timeEnded  gt  a timeEnded                    return 1               else                  return -1                 That should give you a rough idea  Once that s done  you can call Collections sort   on the list

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Java-8 solution using Stream API  A  When timeStarted and timeEnded are public  as mentioned in the requirement  and therefore do not  need to  have public getter methods  List lt ActiveAlarm gt  sorted        list stream            sorted Comparator comparingLong  ActiveAlarm alarm  - gt  alarm timeStarted                           thenComparingLong  ActiveAlarm alarm  - gt  alarm timeEnded            collect Collectors toList      B  When timeStarted and timeEnded have public getter methods  List lt ActiveAlarm gt  sorted        list stream            sorted Comparator comparingLong ActiveAlarm  getTimeStarted                           thenComparingLong ActiveAlarm  getTimeEnded            collect Collectors toList      If you want to sort the original list itself  A  When timeStarted and timeEnded are public  as mentioned in the requirement  and therefore do not  need to  have public getter methods  list sort Comparator comparingLong  ActiveAlarm alarm  - gt  alarm timeStarted                       thenComparingLong  ActiveAlarm alarm  - gt  alarm timeEnded     B  When timeStarted and timeEnded have public getter methods  list sort Comparator comparingLong ActiveAlarm  getTimeStarted                       thenComparingLong ActiveAlarm  getTimeEnded

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JAVA 8 and Above Answer  Using Lambda Expressions   In Java 8  Lambda expressions were introduced to make this even easier   Instead of creating a Comparator   object with all of it s scaffolding  you can simplify it as follows   Using your object as an example   Collections sort list   ActiveAlarm a1  ActiveAlarm a2  - gt  a1 timeStarted-a2 timeStarted     or even shorter   Collections sort list  Comparator comparingInt ActiveAlarm   getterMethod      That one statement is equivalent to the following   Collections sort list  new Comparator lt ActiveAlarm gt           Override     public int compare ActiveAlarm a1  ActiveAlarm a2            return a1 timeStarted - a2 timeStarted              Think of Lambda expressions as only requiring you to put in the relevant parts of the code   the method signature and what gets returned   Another part of your question was how to compare against multiple fields   To do that with Lambda expressions  you can use the  thenComparing   function to effectively combine two comparisons into one   Collections sort list   ActiveAlarm a1  ActiveAlarm a2  - gt  a1 timeStarted-a2 timeStarted                      thenComparing   ActiveAlarm a1  ActiveAlarm a2  - gt  a1 timeEnded-a2 timeEnded       The above code will sort the list first by timeStarted  and then by timeEnded  for those records that have the same timeStarted    One last note  It is easy to compare  long  or  int  primitives  you can just subtract one from the other   If you are comparing objects   Long  or  String    I suggest you use their built-in comparison   Example   Collections sort list   ActiveAlarm a1  ActiveAlarm a2  - gt  a1 name compareTo a2 name       EDIT   Thanks to Lukas Eder for pointing me to  thenComparing   function

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Using Comparator  For Example   class Score        private String name      private List lt Integer gt  scores          accessor methods           Collections sort scores  new Comparator lt Score gt               public int compare Score o1  Score o2                   compare two instance of  Score  and return  int  as result              return o2 getScores   get 0  compareTo o1 getScores   get 0                        With Java 8 onwards  you can simply use lambda expression to represent Comparator instance   Collections sort scores   s1  s2  - gt       compute and return int

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Since Java8 this can be done even cleaner using a combination of Comparator and Lambda expressions  For Example   class Student       private String name      private List lt Score gt  scores           accessor methods    class Score        private int grade          accessor methods           Collections sort student getScores    Comparator comparing Score  getGrade

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In java you need to use the static Collections sort method  Here is an example for a list of CompanyRole objects  sorted first by begin and then by end  You can easily adapt for your own object    private static void order List lt TextComponent gt  roles         Collections sort roles  new Comparator              Override         public int compare Object o1  Object o2                int x1     CompanyRole  o1  getBegin                int x2     CompanyRole  o2  getBegin                 if  x1    x2                    return x1 - x2                else                   int y1     CompanyRole  o1  getEnd                    int y2     CompanyRole  o2  getEnd                    return y2 - y1

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We can use the Comparator comparing   method to sort a list based on an object s property  class SortTest      public static void main String   args            ArrayList lt ActiveAlarm gt  activeAlarms   new ArrayList lt  gt                  add new ActiveAlarm  quot Alarm 1 quot   5  10                add new ActiveAlarm  quot Alarm 2 quot   2  12                add new ActiveAlarm  quot Alarm 3 quot   0  8                            I sort the arraylist here using the getter methods            activeAlarms sort Comparator comparing ActiveAlarm  getTimeStarted                   thenComparing ActiveAlarm  getTimeEnded             System out println activeAlarms            Note that before doing it  you ll have to define at least the getter methods of the properties you want to base your sort on  public class ActiveAlarm       public long timeStarted      public long timeEnded      private String name    quot  quot       private String description    quot  quot       private String event      private boolean live   false       public ActiveAlarm String name  long timeStarted  long timeEnded            this name   name          this timeStarted   timeStarted          this timeEnded   timeEnded             public long getTimeStarted             return timeStarted             public long getTimeEnded             return timeEnded              Override     public String toString             return name           Output   Alarm 3  Alarm 2  Alarm 1

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We can sort the list in one of two ways   1  Using Comparator   When required to use the sort logic in multiple places If you want to use the sorting logic in a single place  then you can write an anonymous inner class as follows  or else extract the comparator and use it in multiple places    Collections sort arrayList  new Comparator lt ActiveAlarm gt              public int compare ActiveAlarm o1  ActiveAlarm o2                  Sorts by  TimeStarted  property             return o1 getTimeStarted   lt o2 getTimeStarted   -1 o1 getTimeStarted   gt o2 getTimeStarted   1 doSecodaryOrderSort o1 o2                        If  TimeStarted  property is equal sorts by  TimeEnded  property         public int doSecodaryOrderSort ActiveAlarm o1 ActiveAlarm o2                return o1 getTimeEnded   lt o2 getTimeEnded   -1 o1 getTimeEnded   gt o2 getTimeEnded   1 0                      We can have null check for the properties  if we could have used  Long  instead of  long    2  Using Comparable natural ordering   If sort algorithm always stick to one property  write a class that implements  Comparable  and override  compareTo  method as defined below  class ActiveAlarm implements Comparable lt ActiveAlarm gt    public long timeStarted  public long timeEnded  private String name       private String description       private String event  private boolean live   false   public ActiveAlarm long timeStarted long timeEnded        this timeStarted timeStarted      this timeEnded timeEnded     public long getTimeStarted         return timeStarted     public long getTimeEnded         return timeEnded     public int compareTo ActiveAlarm o        return timeStarted lt o getTimeStarted   -1 timeStarted gt o getTimeStarted   1 doSecodaryOrderSort o      public int doSecodaryOrderSort ActiveAlarm o        return timeEnded lt o getTimeEnded   -1 timeEnded gt o getTimeEnded   1 0         call sort method to sort based on natural ordering  Collections sort list

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Guava s ComparisonChain   Collections sort list  new Comparator lt ActiveAlarm gt                  Override             public int compare ActiveAlarm a1  ActiveAlarm a2                     return ComparisonChain start                           compare a1 timestarted  a2 timestarted                                                       compare a1 timeEnded  a1 timeEnded  result

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As mentioned you can sort by    Making your object implement Comparable  Or pass a Comparator to Collections sort   If you do both  the Comparable will be ignored and Comparator will be used  This helps that the value objects has their own  logical  Comparable which is most reasonable sort for your value object  while each individual use case has its own implementation

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You can use Collections sort and pass your own Comparator lt ActiveAlarm gt

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In java8  this can be written in single line as follows  collectionObjec sort comparator lamda  or comparator comparing CollectionType  getterOfProperty  code  ListOfActiveAlarmObj sort  a b- gt a getTimeStarted   compareTo b getTimeStarted         or ListOfActiveAlarmObj sort Comparator comparing ActiveAlarm  getTimeStarted

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You can call Collections sort   and pass in a Comparator which you need to write to compare different properties of the object

[java] How to sort List of objects by some property

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