Get specific objects from ArrayList when objects were added anonymously

Question

I have created a short example of my problem  I m creating a list of objects anonymously and adding them to an ArrayList  Once items are in the ArrayList I later come back and add more information to each object within the list  Is there a way to extract a specific object from the list if you do not know its index   I know only the Object s  name  but you cannot do a list get ObjectName  or anything  What is the recommended way to handle this  I d rather not have to iterate through the entire list every time I want to retrieve one specific object   public class TestCode       public static void main  String args               Cave cave   new Cave                Loop adds several Parties to the cave s party list         cave parties add new Party  FirstParty        all anonymously added         cave parties add new Party  SecondParty             cave parties add new Party  ThirdParty                 How do I go about setting the  index  value of SecondParty for example           class Cave       ArrayList lt Party gt  parties   new ArrayList lt Party gt        class Party extends CaveElement      int index       public Party String n           name   n               getter and setter methods       public String toString              return name            class CaveElement       String name           int index   0       public String toString              return name        index

User · Answer

List indexOf   will give you what you want  provided you know precisely what you re after  and provided that the equals   method for Party is well-defined   Party searchCandidate   new Party  FirstParty    int index   cave parties indexOf searchCandidate     This is where it gets interesting - subclasses shouldn t be examining the private properties of their parents  so we ll define equals   in the superclass    Override public boolean equals Object o        if  this    o            return true            if    o instanceof CaveElement             return false             CaveElement that    CaveElement  o       if  index    that index            return false            if  name    null    name equals that name    that name    null            return false             return true      It s also wise to override hashCode if you override equals - the general contract for hashCode mandates that  if x equals y   then x hashCode      y hashCode      Override public int hashCode         int result   name    null   name hashCode     0      result   31   result   index      return result

User · Answer

You could use list indexOf Object  bug in all honesty what you re describing sounds like you d be better off using a Map   Try this   Map lt String  Object gt  mapOfObjects   new HashMap lt String  Object gt     mapOfObjects put  objectName   object     Then later when you want to retrieve the object  use   mapOfObjects get  objectName      Assuming you do know the object s name as you stated  this will be both cleaner and will have faster performance besides  particularly if the map contains large numbers of objects   If you need the objects in the Map to stay in order  you can use   Map lt String  Object gt  mapOfObjects   new LinkedHashMap lt String  Object gt       instead

User · Answer

I would suggest overriding the equals Object  of your Party class  It might look something like this   public boolean equals Object o       if o    null          return false      if o instanceof String          return name equalsIgnoreCase  String o       else if o instanceof Party          return equals   Party o  name       return false      After you do that  you could use the indexOf Object  method to retrieve the index of the party specified by its name  as shown below   int index   cave parties indexOf  SecondParty      Would return the index of the Party with the name SecondParty    Note  This only works because you are overriding the equals Object  method

User · Answer

If you want to lookup objects based on their String name  this is a textbook case for a Map  say a HashMap  You could use a LinkedHashMap and convert it to a List or Array later  Chris has covered this nicely in the comments below    LinkedHashMap because it lets you access the elements in the order you insert them if you want to do so  Otherwise HashMap or TreeMap will do   You could get this to work with List as the others are suggesting  but that feels Hacky to me   and this will be cleaner both in short and long run   If you MUST use a list for the object  you could still store a Map of the object name to the index in the array  This is a bit uglier  but you get almost the same performance as a plain Map

User · Answer

Given the use of List  there s no way to  lookup  a value without iterating through it     For example     Cave cave   new Cave        Loop adds several Parties to the cave s party list cave parties add new Party  FirstParty        all anonymously added cave parties add new Party  SecondParty     cave parties add new Party  ThirdParty      for  Party p   cave parties        if  p name equals  SecondParty             p index                break            Now  this will take time   If the element you are looking for is at the end of the list  you will have to iterate to the end of the list before you find a match   It might be better to use a Map of some kind     So  if we update Cave to look like     class Cave       Map lt String  Party gt  parties   new HashMap lt String  Party gt  25       We could do something like     Cave cave   new Cave        Loop adds several Parties to the cave s party list cave parties put  FirstParty   new Party  FirstParty        all anonymously added cave parties put  SecondParty   new Party  SecondParty     cave parties put  ThirdParty   new Party  ThirdParty      if  cave parties containsKey  SecondParty          cave parties get  SecondParty   index           Instead     Ultimately  this will all depend on what it is you want to achieve

User · Answer

You could simply create a method to get the object by it s name   public Party getPartyByName String name        for Party party   parties            if name equalsIgnoreCase party name                 return party                      return null

User · Answer

As per your question requirement   I would like to suggest that Map will solve your problem very efficient and without any hassle   In Map you can give the name as key and your original object as value     Map lt String Cave gt  myMap new HashMap lt String Cave gt

[java] Get specific objects from ArrayList when objects were added anonymously?

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