cannot be cast to java lang Comparable

Question

import java util HashMap  import java io    import java util    public class Fegan implements Comparable  HashMap lt String  Integer gt  cart   new HashMap lt String  Integer gt     List list   new ArrayList lt FoodItems gt     int x  0  public void addToCart FoodItems f        cart put f name  f valueOfFood     public String display FoodItems f        return f name     costs     f valueOfFood     public void addToList FoodItems f          FoodItems temp    FoodItems  f      list add f     public int compareTo Object o          FoodItems temp    FoodItems  o      if this x      FoodItems o  valueOfFood          return 0      else if  this x  lt   FoodItems o  valueOfFood          return 1      else          return -1    public void sortMap List list        for int i  0  i lt  list size    i                  FoodItems temp    FoodItems  list get i           cart put temp name  temp valueOfItem                  import java util Collections  import java util HashMap  import java util Iterator  import java io    import java util    public class Test   public static void main String    args        HashMap lt String  Integer gt  cart   new HashMap lt String  Integer gt         FoodItems firts   new FoodItems  Chocolate    50       FoodItems second   new FoodItems  Juice   79       FoodItems third   new FoodItems  Apple   200       FoodItems forth   new FoodItems  Orange   300       FoodItems fifth   new FoodItems  Milk    400       ArrayList items   new ArrayList        items add firts       items add second       items add third       items add forth       items add fifth       Collections sort items       Iterator itr   items iterator        Fegan myFegan   new Fegan        myFegan sortMap items        while itr hasNext             Object element   itr next            System out println element     n                Why it is writing   Exception in thread  main  java lang ClassCastException  FoodItems cannot be cast to java lang Comparable at java util ComparableTimSort countRunAndMakeAscending Unknown Source  at java util ComparableTimSort sort Unknown Source  at java util ComparableTimSort sort Unknown Source  at java util Arrays sort Unknown Source  at java util Collections sort Unknown Source  at Test main Test java 21

User · Answer

I faced a similar kind of issue while using a custom object as a key in Treemap  Whenever you are using a custom object as a key in hashmap then you override two function equals and hashcode  However if you are using ContainsKey method of Treemap on this object then you need to override CompareTo method as well otherwise you will be getting this error Someone using a custom object as a key in hashmap in kotlin should do like following    data class CustomObjectKey var key1 String        var   key2 String       Comparable lt CustomObjectKey  gt     override fun compareTo other  CustomObjectKey    Int       if other    null          return -1       suppose you want to do comparison based on key 1      return this key1 compareTo  other key1     override fun equals other  Any    Boolean       if other    null          return false     return this key1     other as CustomObjectKey  key1    override fun hashCode    Int       return this key1 hashCode

User · Answer

the object which implements Comparable is Fegan    The method compareTo you are overidding in it should have a Fegan object as a parameter whereas you are casting it to a FoodItems  Your compareTo implementation should describe how a Fegan compare to another Fegan    To actually do your sorting  you might want to make your FoodItems implement Comparable aswell and copy paste your actual compareTo logic in it

[java] cannot be cast to java.lang.Comparable

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