Sort Java Collection

Question

I have a Java collection   Collection lt CustomObject gt  list   new ArrayList lt CustomObject gt       CustomObject has an id field now before display list I want to sort this collection by that id   Is there any way I could that do that

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You should implement the Comparator interface   example   public class CustomComparator implements Comparator lt CustomObject gt          Override     public int compare CustomObject o1  CustomObject o2            return o1 getId   compareTo o2 getId               Then you can use the Collections classes Collections sort   method   Collections sort list  new CustomComparator

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Use sort   You just have to do this      All elements in the list must implement the Comparable interface     Or use the version below it  as others already said

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Implement the Comparable interface on your customObject

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You can also use   Collections sort list  new Comparator lt CustomObject gt          public int compare CustomObject obj1  CustomObject obj2            return obj1 id - obj2 id            System out println list

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To be super clear  Collection sort list  compartor  does not return anything so something like this list   Collection sort list  compartor   will throw an error  void cannot be converted to  list type   and should instead be Collection sort list  compartor

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A slightly different example say if you have a class that doesn t implement Comparable but you still want to sort it on a field or method    Collections sort allMatching  new Comparator lt ClassOne gt         Override public int compare final ClassOne o1  final ClassOne o2        if  o1 getMethodToSort    gt  o2 getMethodToSort            return 1        else if  o1 getMethodToSort    lt  o2 getMethodToSort            return -1              return 0

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A lot of correct answers  but I haven t found this one  Collections cannot be sorted  you can only iterate through them   Now you can iterate over them and create a new sorted something  Follow the answers here for that

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As of Java 8 you now can do it with a stream using a lambda   list stream   sorted Comparator comparing customObject  getId                 foreach object - gt  System out println object

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With Java 8 you have several options  combining method references and the built-in comparing comparator   import static java util Comparator comparing   Collection lt CustomObject gt  list   new ArrayList lt CustomObject gt      Collections sort list  comparing CustomObject  getId      or list sort comparing CustomObject  getId

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Comparator is the way  Also See    sorting-of-arraylist

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Use a Comparator   List lt CustomObject gt  list   new ArrayList lt CustomObject gt     Comparator lt CustomObject gt  comparator   new Comparator lt CustomObject gt           Override     public int compare CustomObject left  CustomObject right            return left getId   - right getId       use your logic           Collections sort list  comparator      use the comparator as much as u want System out println list     Additionally  if CustomObjectimplements Comparable  then just use Collections sort list   With JDK 8 the syntax is much simpler   List lt CustomObject gt  list   getCustomObjectList    Collections sort list   left  right  - gt  left getId   - right getId     System out println list     Much simplier  List lt CustomObject gt  list   getCustomObjectList    list sort  left  right  - gt  left getId   - right getId     System out println list     Simplest  List lt CustomObject gt  list   getCustomObjectList    list sort Comparator comparing CustomObject  getId    System out println list     Obviously the initial code can be used for JDK 8 too

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You can use java Custom Class for the purpose of sorting

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SortedSet and Comparator  Comparator should honour the id field

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The question is   Sort Collection   So you can t use Collections sort List lt T gt  l  Comparator lt   super T gt  comparator    Some tips   For Collection type   Comparator lt String gt  defaultComparator   new Comparator lt String gt          Override    public int compare String o1  String o2           return o1 compareTo o2            Collection lt String gt  collection   getSomeStringCollection    String   strings   collection toArray new String collection size      Arrays sort strings  defaultComparator   List lt String gt  sortedStrings   Arrays asList strings    Collection lt String gt  collection   getSomeStringCollection    List lt String gt  list   new ArrayList collection   Collections sort list  defaultComparator   collection   list     if you wish   For List type   List lt String gt  list   getSomeStringList    Collections sort list  defaultComparator     For Set type   Set lt String gt  set   getSomeStringSet       Than steps like in  For Collection type  section or use java util TreeSet    TreeSet sample     Sorted using java lang Comparable  Set lt String gt  naturalSorted   new TreeSet set    Set lt String gt  set   getSomeStringSet    Set lt String gt  sortedSet   new TreeSet defaultComparator   sortedSet addAll set     Java 8 version  There is java util List sort Comparator lt   super E gt  c  method  List lt String gt  list   getSomeStringList    list sort defaultComparator     or  List lt String gt  list   getSomeStringList    list sort  String o1  String o2  - gt  o1 compareTo o2      or for types that implements Comparable   List lt String gt  list   getSomeStringList    list sort String  compareTo

[java] Sort Java Collection

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