How to use Collections sort in Java

Question

I got an object Recipe that implements Comparable lt Recipe gt     public int compareTo Recipe otherRecipe        return this inputRecipeName compareTo otherRecipe inputRecipeName       I ve done that so I m able to sort the List alphabetically in the following method   public static Collection lt Recipe gt  getRecipes        List lt Recipe gt  recipes   new ArrayList lt Recipe gt  RECIPE MAP values         Collections sort recipes       return recipes      But now  in a different method  lets call it getRecipesSort    I want to sort the same list but numerically  comparing a variable that contains their ID  To make things worse  the ID field is of the type String   How do I use Collections sort   to perform the sorts in Java

User · Answer

The answer given by NINCOMPOOP can be made simpler using Lambda Expressions     Collections sort recipes   Recipe r1  Recipe r2  - gt  r1 getID   compareTo r2 getID       Also introduced after Java 8 is the comparator construction methods in the Comparator interface  Using these  one can further reduce this to 1     recipes sort comparingInt Recipe  getId        1 Bloch  J  Effective Java  3rd Edition   2018  Item 42  p  194

User · Answer

Use this method Collections sort List Comparator    Implement a Comparator and pass it to Collections sort     class RecipeCompare implements Comparator lt Recipe gt          Override     public int compare Recipe o1  Recipe o2               write comparison logic here like below   it s just a sample         return o1 getID   compareTo o2 getID               Then use the Comparator as   Collections sort recipes new RecipeCompare

User · Answer

Use the method that accepts a Comparator when you want to sort in something other than natural order   Collections sort List  Comparator

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Sort the unsorted hashmap in ascending order      Sorting the list based on values Collections sort list  new Comparator lt Entry lt String  Integer gt  gt      public int compare Entry lt String  Integer gt  o1  Entry lt String  Integer gt  o2                     return o2 getValue   compareTo o1 getValue                               Maintaining insertion order with the help of LinkedList     Map lt String  Integer gt  sortedMap   new LinkedHashMap lt String  Integer gt         for  Entry lt String  Integer gt  entry   list            sortedMap put entry getKey    entry getValue

User · Answer

Create a comparator which accepts the compare mode in its constructor and pass different modes for different scenarios based on your requirement  public class RecipeComparator implements Comparator lt Recipe gt     public static final int COMPARE BY ID   0  public static final int COMPARE BY NAME   1   private int compare mode   COMPARE BY NAME   public RecipeComparator        public RecipeComparator int compare mode        this compare mode   compare mode      Override public int compare Recipe o1  Recipe o2        switch  compare mode        case COMPARE BY ID          return o1 getId   compareTo o2 getId         default          return o1 getInputRecipeName   compareTo o2 getInputRecipeName                  Actually for numbers you need to handle them separately check below  public static void main String   args        String string1    1       String string2    2       String string11    11        System out println string1 compareTo string2         System out println string2 compareTo string11      expected -1 returns 1       to compare numbers you actually need to do something like this      int number2   Integer valueOf string1       int number11   Integer valueOf string11        int compareTo   number2  gt  number11   1    number2  lt  number11   -1   0        System out println compareTo     prints -1

[java] How to use Collections.sort() in Java?

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