Java method Finding object in array list given a known attribute value

Question

I have a couple of questions actually   I have a class Dog with the following instance fields   private int id  private int id mother  private int id father  private String name     private String owner     private String bDate       I also have a class Archive which can instantiate Dog and put Dog objects into an ArrayList   I am trying to write a method in Archive which takes an integer as ID and looks through the ArrayList  and returns the object containing that ID   private Dog getDog int id       Dog dog   new Dog        int length getSize        int i 0       dog al get i       i         while dog getId    id  amp  amp  i lt length          dog al get i           i         if dog getId    id          dog null      return dog     end getDog   There are two problems with this method  the other methods I use work   First of all it s not working  and I can t see why  I m while-looping through  potentially  all the objects in the arraylist  for then after the loop is finished  checking whether the loop finished because it ran out of objects to search through  or because it found an object with the given ID  Secondly  that seems like an immensely time-consuming process  Is there some way to speed this up

User · Answer

I solved this using java 8 lambdas  int dogId   2   return dogList stream   filter dog- gt  dogId    dog getId    collect Collectors toList    get 0

User · Answer

If you have to get an attribute that is not the ID  I would use CollectionUtils   Dog someDog   new Dog    Dog dog   CollectionUtils dogList  new Predicate       Override public boolean evaluate Object o        Dog d    Dog o      return someDog getName   equals d getName

User · Answer

To improve performance of the operation  if you re always going to want to look up objects by some unique identifier  then you might consider using a Map lt Integer Dog gt   This will provide constant-time lookup by key  You can still iterate over the objects themselves using the map values     A quick code fragment to get you started      Populate the map Map lt Integer Dog gt  dogs   new HashMap lt Integer Dog gt     for  Dog dog      dog source            dogs put  dog getId    dog          Perform a lookup Dog dog   dogs get  id      This will help speed things up a bit if you re performing multiple lookups of the same nature on the list  If you re just doing the one lookup  then you re going to incur the same loop overhead regardless

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List lt YourClass gt  list   ArrayList lt YourClass gt       List lt String gt  userNames   list stream   map m - gt  m getUserName    collect Collectors toList       output     John   Alex

User · Answer

You have to loop through the entire array  there s no changing that  You can however  do it a little easier  for  Dog dog   list      if  dog getId      id        return dog    gotcha        return null     dog not found    or without the new for loop  for  int i   0  i  lt  list size    i        if  list get i  getId      id        return list get i

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A while applies to the expression or block after the while   You dont have a block  so your while ends with the expression dog al get i    while dog getId    id  amp  amp  i lt length                  dog al get i     Everything after that happens only once   There s no reason to new up a Dog  as you re never using the dog you new d up  you immediately assign a Dog from the array to your dog reference   And if you need to get a value for a key  you should use a Map  not an Array   Edit  this was donwmodded why    Comment from OP      One further question with regards to not having to make a new instance of a Dog  If I am just taking out copies of the objects from the array list  how can I then take it out from the array list without having an object in which I put it  I just noticed as well that I didn t bracket the while-loop    A Java reference and the object it refers to are different things  They re very much like a C   reference and object  though a Java reference can be re-pointed like a C   pointer   The upshot is that Dog dog  or Dog dog   null gives you a reference that points to no object  new Dog   creates an object that can be pointed to   Following that with a dog   al get i  means that the reference now points to the dog reference returned by al get i   Understand  in Java  objects are never returned  only references to objects  which are addresses of the object in memory     The pointer reference address of the Dog you newed up is now lost  as no code refers to it  as the referent was replaced with the referent you got from al get    Eventually the Java garbage collector will destroy that object  in C   you d have  leaked  the memory   The upshot is that you do need to create a variable that can refer to a Dog  you don t need to create a Dog with new      In truth you don t need to create a reference  as what you really ought to be doing is returning what a Map returns from its get   function  If the Map isn t parametrized on Dog  like this  Map lt Dog gt   then you ll need to cast the return from get  but you won t need a reference  return  Dog  map get id   or if the Map is parameterized  return map get id   And that one line is your whole function  and it ll be faster than iterating an array for most cases

User · Answer

I was interested to see that the original poster used a style that avoided early exits  Single Entry  Single Exit  SESE  is an interesting style that I ve not really explored  It s late and I ve got a bottle of cider  so I ve written a solution  not tested  without an early exit   I should have used an iterator  Unfortunately java util Iterator has a side-effect in the get method   I don t like the Iterator design due to its exception ramifications    private Dog findDog int id        int i   0      for    i  dogs length    amp  amp  dogs get i  getID    id    i                         return i  dogs length     dogs get i    null      Note the duplication of the i  dogs length   expression  could have chosen dogs get i  getID    id

User · Answer

Assuming that you ve written an equals method for Dog correctly that compares based on the id of the Dog the easiest and simplest way to return an item in the list is as follows   if  dogList contains dog        return dogList get dogList indexOf dog        That s less performance intensive that other approaches here   You don t need a loop at all in this case   Hope this helps   P S You can use Apache Commons Lang to write a simple equals method for Dog as follows    Override public boolean equals Object obj            EqualsBuilder builder   new EqualsBuilder   append this getId    obj getId                       return builder isEquals

[java] Java method: Finding object in array list given a known attribute value

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