Getting Index of an item in an arraylist

Question

I have a Class called AuctionItem  The AuctionItem Class has a method called getName   that returns a String  If I have an ArrayList of type AuctionItem  what is the best way to return the index of an item in the ArrayList that has a specific name   I know that there is an  indexOf   function  The parameter for this function is an object  To find the item that has a name  should I just use a for loop  and when the item is found  return the element position in the ArrayList   Is there a better way

User · Answer

indexOf   works well  If you want an example here is one     ArrayList lt String gt  example   new ArrayList lt String gt       example add  AB      example add  CD      example add  EF      example add  GH      example add  IJ      example add  KL      example add  MN       System out println  Index of  AB     example indexOf  AB       System out println  Index of  KL     example indexOf  KL       System out println  Index of  AA     example indexOf  AA       System out println  Index of  EF     example indexOf  EF       will give you an output of  Index of  AB   0 Index of  KL   5 Index of  AA   -1 Index of  EF   2   Note  This method returns -1 if the specified element is not present in the list

User · Answer

You could implement hashCode equals of your AuctionItem so that two of them are equal if they have the same name  When you do this you can use the methods indexOf and contains of the ArrayList like this  arrayList indexOf new AuctionItem  The name     Or when you assume in the equals method that a String is passed  arrayList indexOf  The name    But that s not the best design   But I would also prefer using a HashMap to map the name to the item

User · Answer

for  int i   0  i  lt  list length  i         if  list get i   getName   equalsIgnoreCase  myName          System out println i       break

User · Answer

I think a for-loop should be a valid solution        public int getIndexByname String pName                for AuctionItem  item    yourArray                         if  item getName   equals pName                   return  yourarray  indexOf  item                    return -1

User · Answer

To find the item that has a name  should I just use a for loop  and when the item is found  return the element position in the ArrayList    Yes to the loop  either using indexes or an Iterator   On the return value  either return its index  or the item iteself  depending on your needs  ArrayList doesn t have an indexOf Object target  Comparator compare   or similar  Now that Java is getting lambda expressions  in Java 8   March 2014   I expect we ll see APIs get methods that accept lambdas for things like this

User · Answer

Basically you need to look up ArrayList element based on name getName  Two approaches to this problem   1- Don t use ArrayList  Use HashMap lt String AutionItem gt  where String would be name  2- Use getName to generate index and use index based addition into array list list add int index  E element   One way to generate index from name would be to use its hashCode and modulo by ArrayList current size  something similar what is used inside HashMap

User · Answer

Rather than a brute force loop through the list  eg 1 to 10000   rather use an iterative search approach   The List needs to be sorted by the element to be tested   Start search at the middle element size   2  eg 5000 if search item greater than element at 5000  then test the element at the midpoint between the upper 10000  and midpoint 5000  - 7500  keep doing this until you reach the match  or use a brute force loop through once you get down to a smaller range  eg 20 items   You can search a list of 10000 in around 13 to 14 tests  rather than potentially 9999 tests

User · Answer

Yes you have to loop it  public int getIndex String itemName        for  int i   0  i  lt  arraylist size    i                  AuctionItem auction   arraylist get i           if  itemName equals auction getname                           return i                        return -1

[java] Getting Index of an item in an arraylist;

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