How to sort ArrayList Long in decreasing order

Question

How to sort an ArrayList lt Long gt  in Java in decreasing order

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Sort normally and use Collections reverse

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Using List sort   and Comparator comparingLong   numberList sort Comparator comparingLong x - gt  -x

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You can also sort an ArrayList with a TreeSet instead of a comparator  Here s an example from a question I had before for an integer array  I m using  numbers  as a placeholder name for the ArrayList          import java util            class MyClass          public static void main String   args           Scanner input   new Scanner System in           ArrayList lt Integer gt  numbers   new ArrayList lt Integer gt               TreeSet lt Integer gt  ts   new TreeSet lt Integer gt  numbers           numbers   new ArrayList lt Integer gt  ts           System out println   nThe numbers in ascending order are             for int i 0  i lt numbers size    i            System out print numbers get i  intValue                 System out println   nThe numbers in descending order are             for int i numbers size  -1  i gt  0  i--          System out print numbers get i  intValue

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Java 8  well doing this in java 8 is so much fun and easier   Collections sort variants  a b - gt a compareTo b    Collections reverse variants     Lambda expressions rock here     in case you needed a more than one line logic for comparing a and b you could write it like this  Collections sort variants  a b - gt       int result   a compareTo b       return result

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The following approach will sort the list in descending order and also handles the  null  values  just in case if you have any null values then Collections sort   will throw NullPointerException        Collections sort list  new Comparator lt Long gt                public int compare Long o1  Long o2                      return o1  null Integer MAX VALUE o2  null Integer MIN VALUE o2 compareTo o1

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Sort  then reverse

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You can use the following code which is given below   Collections sort list  Collections reverseOrder       or if you are going to use custom comparator you can use as it is given below  Collections sort list  Collections reverseOrder new CustomComparator       Where CustomComparator is a comparator class that compares the object which is present in the list

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A more general approach to implement our own Comparator as below  Collections sort lst new Comparator lt Long gt                     public int compare Long o1  Long o2                        return o2 compareTo o1

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So  There is something I would like to bring up which I think is important and I think that you should consider   runtime and memory   Say you have a list and want to sort it  well you can  there is a built in sort or you could develop your own   Then you say  want to reverse the list   That is the answer which is listed above   If you are creating that list though  it might be good to use a different datastructure to store it and then just dump it into an array   Heaps do just this   You filter in data  and it will handle everything  then you can pop everything off of the object and it would be sorted   Another option would be to understand how maps work   A lot of times  a Map or HashMap as something things are called  have an underlying concept behind it   For example     you feed in a bunch of key-value pairs where the key is the long  and when you add all the elements  you can do    keys and it would return to you a sorted list automatically   It depends on how you process the data prior as to how i think you should continue with your sorting and subsequent reverses

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For lamdas where your long value is somewhere in an object I recommend using    sorted  o1  o2  - gt  Long compare o1 getLong    o2 getLong       or even better     sorted Comparator comparingLong MyObject  getLong

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Comparator s comparing method can be used to compare the objects and then method reversed   can be applied to reverse the order -   list stream   sorted Comparator comparing Employee  getName  reversed    collect toList

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Here s one way for your list   list sort null   Collections reverse list     Or you could implement your own Comparator to sort on and eliminate the reverse step   list sort  o1  o2  - gt  o2 compareTo o1      Or even more simply use Collections reverseOrder   since you re only reversing   list sort Collections reverseOrder

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Comparator lt Long gt  comparator   Collections reverseOrder    Collections sort arrayList  comparator

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By using Collections sort   with a comparator that provides the decreasing order  See Javadoc for Collections sort

[java] How to sort ArrayList<Long> in decreasing order?

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