Use of contains in Java ArrayList String

Question

If I have an ArrayList of String forming part of a class in Java like so   private ArrayList lt String gt  rssFeedURLs     If I want to use a method in the class containing the above ArrayList  using ArrayList  contains to check if a String is contained in this ArrayList  I believe I should be able to do so as follows   if  this rssFeedURLs contains rssFeedURL       Where rssFeedURL is a String   Am I right or wrong

User · Answer

Thanks to you all for answering so quickly. I could always use a set but I have the ArrayList working now. The problem was that in the constructor of the class containing the ArrayList, I was not saying:

public RSS_Feed_Miner() {
    ...
    this.rssFeedURLs = new ArrayList<String>();
    ... 
}

D'Oh! for a Friday afternoon.

User · Answer

Yes  that should work for Strings  but if you are worried about duplicates use a Set  This collection prevents duplicates without you having to do anything  A HashSet is OK to use  but it is unordered so if you want to preserve insertion order you use a LinkedHashSet

User · Answer

Your question is not very clear    What s your code exactly doing  Give more code  What s the error you re getting    You say you get a null-pointer  You cannot get a null pointer as a value returned by contains     However you can get a NullPointerException if your list has not been initialized  By reading your question now  I d say that what you show here is correct  but maybe you just didn t instantiate the list   For this to work  to add a feed URL if it isn t already in the list    if   this rssFeedURLs contains rssFeedURL         this rssFeedURLs add rssFeedUrl       then this declaration would do   private ArrayList lt String gt  rssFeedURLs   new ArrayList lt String gt       or initialize your list later on  but before trying to access its methods   rssFeedUrls   new ArrayList lt String gt         Finally    Do you really need a List  Maybe a Set would be better if you don t want duplicates  Use a LinkedHashSet if preserving the ordering matters

User · Answer

Perhaps you need to post the code that caused your exception  If the above is all you have  perhaps you just failed to actually initialise the array   Using contains here should work though

User · Answer

You are right that it should work  perhaps you forgot to instantiate something   Does your code look something like this   String rssFeedURL    http   stackoverflow com   this rssFeedURLS   new ArrayList lt String gt     this rssFeedURLS add rssFeedURL   if this rssFeedURLs contains rssFeedURL        this code will execute     For reference  note that the following conditional will also execute if you append this code to the above   String copyURL   new String rssFeedURL   if this rssFeedURLs contains copyURL        code will still execute because contains   checks equals       Even though  rssFeedURL    copyURL  is false  rssFeedURL equals copyURL  is true   The contains method cares about the equals method

User · Answer

Right   with strings   the moment you deviate from primitives or strings things change and you need to implement hashcode equals to get the desired effect   EDIT  Initialize your ArrayList lt String gt  then attempt to add an item

User · Answer

You are right  ArrayList contains   tests equals    not object identity      returns true if and only if this list   contains at least one element e such   that  o  null   e  null   o equals e     If you got a NullPointerException  verify that you initialized your list  either in a constructor or the declaration  For example   private List lt String gt  rssFeedURLs   new ArrayList lt String gt

User · Answer

ArrayList lt String gt  newlyAddedTypes new ArrayList lt String gt             newlyAddedTypes add  test1     newlyAddedTypes add  test1     newlyAddedTypes add  test2     if newlyAddedTypes contain  test      called here   else

User · Answer

You re correct  As others said according to your comments  you probably did not initialize your ArrayList   My point is different  you claimed that you re checking for duplicates and this is why you call the contains method  Try using HashSet  It should be more efficient - unless you need to keep the order of URLs for any reason

[java] Use of contains in Java ArrayList<String>

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