Array or List in Java Which is faster

Question

I have to keep thousands of strings in memory to be accessed serially in Java  Should I store them in an array or should I use some kind of List    Since arrays keep all the data in a contiguous chunk of memory  unlike Lists   would the use of an array to store thousands of strings cause problems

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A List is more flexible     so better to List than array

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Which one to use depends on the problem  We need to look at the Big O   image source  https   github com egonSchiele grokking algorithms

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ArrayList internally uses  array  object to add or store  the elements  In other words  ArrayList is backed by Array data -structure The array of ArrayList is resizable  or dynamic    Array is faster than ArrayList because ArrayList internally uses an array  if we can directly add elements in Array and indirectly add an element in Array through ArrayList always directly mechanism is faster than an indirect mechanism  There is two overloaded add   methods in ArrayList class   add Object   adds an object to the end of the list  add int index  Object    inserts the specified object at the specified position in the list   How the size of ArrayList grows dynamically  public boolean add E e                        ensureCapacity size 1        elementData size      e                return true     An important point to note from the above code is that we are checking the capacity of the ArrayList  before adding the element  ensureCapacity    determines what is the current size of occupied elements and what is the maximum size of the array  If the size of the filled elements  including the new element to be added to the ArrayList class  is greater than the maximum size of the array then increase the size of the array  But the size of the array can not be increased dynamically  So what happens internally is new Array is created with the capacity Till Java 6 int newCapacity    oldCapacity   3  2   1    Update  From Java 7  int newCapacity   oldCapacity    oldCapacity  gt  gt  1    also  data from the old array is copied into the new array  Having overhead methods in ArrayList that s why Array is faster than ArrayList

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Remember that an ArrayList encapsulates an array  so there is little difference compared to using a primitive array  except for the fact that a List is much easier to work with in java    The pretty much the only time it makes sense to prefer an array to an ArrayList is when you are storing primitives  i e  byte  int  etc and you need the particular space-efficiency you get by using primitive arrays

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If you know in advance how large the data is then an array will be faster   A List is more flexible   You can use an ArrayList which is backed by an array

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A lot of microbenchmarks given here have found numbers of a few nanoseconds for things like array ArrayList reads  This is quite reasonable if everything is in your L1 cache   A higher level cache or main memory access can have order of magnitude times of something like 10nS-100nS  vs more like 1nS for L1 cache  Accessing an ArrayList has an extra memory indirection  and in a real application you could pay this cost anything from almost never to every time  depending on what your code is doing between accesses  And  of course  if you have a lot of small ArrayLists this might add to your memory use and make it more likely you ll have cache misses   The original poster appears to be using just one and accessing a lot of contents in a short time  so it should be no great hardship  But it might be different for other people  and you should watch out when interpreting microbenchmarks   Java Strings  however  are appallingly wasteful  especially if you store lots of small ones  just look at them with a memory analyzer  it seems to be   60 bytes for a string of a few characters   An array of strings has an indirection to the String object  and another from the String object to a char   which contains the string itself  If anything s going to blow your L1 cache it s this  combined with thousands or tens of thousands of Strings  So  if you re serious - really serious - about scraping out as much performance as possible then you could look at doing it differently  You could  say  hold two arrays  a char   with all the strings in it  one after another  and an int   with offsets to the starts  This will be a PITA to do anything with  and you almost certainly don t need it  And if you do  you ve chosen the wrong language

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Array is faster - all memory is pre-allocated in advance

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UPDATE   As Mark noted there is no significant difference after JVM warm up  several test passes   Checked with re-created array or even new pass starting with new row of matrix  With great probability this signs simple array with index access is not to be used in favor of collections   Still first 1-2 passes simple array is 2-3 times faster   ORIGINAL POST   Too much words for the subject too simple to check  Without any question array is several times faster than any class container  I run on this question looking for alternatives for my performance critical section  Here is the prototype code I built to check real situation   import java util List  import java util Arrays   public class IterationTest        private static final long MAX ITERATIONS   1000000000       public static void main String    args             Integer    array    1  5  3  5           List lt Integer gt  list   Arrays asList array            long start   System currentTimeMillis            int test sum   0          for  int i   0  i  lt  MAX ITERATIONS    i                  for  int e   array                for  int e   list                    test sum    e                                  long stop   System currentTimeMillis             long ms    stop - start           System out println  Time      ms             And here is the answer   Based on array  line 16 is active    Time  7064   Based on list  line 17 is active    Time  20950   Any more comment on  faster   This is quite understood  The question is when about 3 time faster is better for you than flexibility of List  But this is another question  By the way I checked this too based on manually constructed ArrayList  Almost the same result

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I suggest that you use a profiler to test which is faster   My personal opinion is that you should use Lists   I work on a large codebase and a previous group of developers used arrays everywhere  It made the code very inflexible  After changing large chunks of it to Lists we noticed no difference in speed

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If you have thousands  consider using a trie  A trie is a tree-like structure that merges the common prefixes of the stored string   For example  if the strings were   intern international internationalize internet internets   The trie would store   intern  - gt   0  international  - gt   0  - gt  ize 0  net  - gt  0  - gt s 0   The strings requires 57 characters  including the null terminator    0   for storage  plus whatever the size of the String object that holds them   In truth  we should  probably round all sizes up to multiples of 16  but     Call it 57   5   62 bytes  roughly   The trie requires 29  including the null terminator    0   for storage  plus sizeof the trie nodes  which are a ref to an array and a list of child trie nodes   For this example  that probably comes out about the same  for thousands  it probably comes out less as long as you do have common prefixes   Now  when using the trie in other code  you ll have to convert to String  probably using a StringBuffer as an intermediary  If many of the strings are in use at once as Strings  outside the trie  it s a loss   But if you re only using a few at the time -- say  to look up things in a dictionary -- the trie can save you a lot of space  Definitely less space than storing them in a HashSet   You say you re accessing them  serially  -- if that means sequentially an alphabetically  the trie also obviously gives you alphabetical order for free  if you iterate it depth-first

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Arrays - It would always be better when we have to achieve faster fetching of results  Lists- Performs results on insertion and deletion since they can be done in O 1  and this also provides methods to add  fetch and delete data easily  Much easier to use   But always remember that the fetching of data would be fast when the index position in array where the data is stored - is known    This could be achieved well by sorting the array  Hence this increases the time to fetch the data  ie  storing the data   sorting the data   seek for the position where the data is found   Hence this increases additional latency to fetch the data from the array even they may be good at fetching the data sooner   Hence this could be solved with trie data structure or ternary data structure  As discussed above the trie data structure would be very efficient in searching for the data the search for a particularly word can be done in O 1  magnitude  When time matters ie  if you have to search and retrieve data quickly you may go with trie data structure    If you want your memory space to be consumed less and you wish to have a better performance then go with ternary data structure  Both these are suitable for storing huge number of strings  eg  like words contained in dictionary

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list is slower than arrays If you need efficiency use arrays If you need flexibility use list

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List is the preferred way in java 1 5 and beyond as it can use generics  Arrays cannot have generics  Also Arrays have a pre defined length  which cannot grow dynamically  Initializing an array with a large size is not a good idea  ArrayList is the the way to declare an array with generics and it can dynamically grow  But if delete and insert is used more frequently  then linked list is the fastest data structure to be used

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Although the answers proposing to use ArrayList do make sense in most scenario  the actual question of relative performance has not really been answered  There are a few things you can do with an array   create it set an item get an item clone copy it  General conclusion Although get and set operations are somewhat slower on an ArrayList  resp  1 and 3 nanosecond per call on my machine   there is very little overhead of using an ArrayList vs  an array for any non-intensive use  There are however a few things to keep in mind   resizing operations on a list  when calling list add       are costly and one should try to set the initial capacity at an adequate level when possible  note that the same issue arises when using an array  when dealing with primitives  arrays can be significantly faster as they will allow one to avoid many boxing unboxing conversions an application that only gets sets values in an ArrayList  not very common   could see a performance gain of more than 25  by switching to an array  Detailed results Here are the results I measured for those three operations using the jmh benchmarking library  times in nanoseconds  with JDK 7 on a standard x86 desktop machine  Note that ArrayList are never resized in the tests to make sure results are comparable  Benchmark code available here  Array ArrayList Creation I ran 4 tests  executing the following statements   createArray1  Integer   array   new Integer 1   createList1  List lt Integer gt  list   new ArrayList lt  gt   1   createArray10000  Integer   array   new Integer 10000   createList10000  List lt Integer gt  list   new ArrayList lt  gt   10000    Results  in nanoseconds per call  95  confidence   a p g a ArrayVsList CreateArray1          10 933  11 097  a p g a ArrayVsList CreateList1           10 799  11 046  a p g a ArrayVsList CreateArray10000     394 899  404 034  a p g a ArrayVsList CreateList10000      396 706  401 266   Conclusion  no noticeable difference  get operations I ran 2 tests  executing the following statements   getList  return list get 0   getArray  return array 0    Results  in nanoseconds per call  95  confidence   a p g a ArrayVsList getArray    2 958  2 984  a p g a ArrayVsList getList     3 841  3 874   Conclusion  getting from an array is about 25  faster than getting from an ArrayList  although the difference is only on the order of one nanosecond  set operations I ran 2 tests  executing the following statements   setList  list set 0  value   setArray  array 0    value   Results  in nanoseconds per call   a p g a ArrayVsList setArray    4 201  4 236  a p g a ArrayVsList setList     6 783  6 877   Conclusion  set operations on arrays are about 40  faster than on lists  but  as for get  each set operation takes a few nanoseconds - so for the difference to reach 1 second  one would need to set items in the list array hundreds of millions of times  clone copy ArrayList s copy constructor delegates to Arrays copyOf so performance is identical to array copy  copying an array via clone  Arrays copyOf or System arrayCopy makes no material difference performance-wise

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It you can live with a fixed size  arrays will will be faster and need less memory   If you need the flexibility of the List interface with adding and removing elements  the question remains which implementation you should choose  Often ArrayList is recommended and used for any case  but also ArrayList has its performance problems if elements at the beginning or in the middle of the list must be removed or inserted   You therefore may want to have a look at http   java dzone com articles gaplist- E2 80 93-lightning-fast-list which introduces GapList  This new list implementation combines the strengths of both ArrayList and LinkedList resulting in very good performance for nearly all operations

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Don t get into the trap of optimizing without proper benchmarking  As others have suggested use a profiler before making any assumption   The different data structures that you have enumerated have different purposes  A list is very efficient at inserting elements in the beginning and at the end but suffers a lot when accessing random elements  An array has fixed storage but provides fast random access  Finally an ArrayList improves the interface to an array by allowing it to grow  Normally the data structure to be used should be dictated by how the data stored will be access or added   About memory consumption  You seem to be mixing some things  An array will only give you a continuous chunk of memory for the type of data that you have  Don t forget that java has a fixed data types  boolean  char  int  long  float and Object  this include all objects  even an array is an Object   It means that if you declare an array of String strings  1000  or MyObject myObjects  1000  you only get a 1000 memory boxes big enough to store the location  references or pointers  of the objects  You don t get a 1000 memory boxes big enough to fit the size of the objects  Don t forget that your objects are first created with  new   This is when the memory allocation is done and later a reference  their memory address  is stored in the array  The object doesn t get copied into the array only it s reference

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Since there are already a lot of good answers here  I would like to give you some other information of practical view  which is insertion and iteration performance comparison   primitive array vs Linked-list in Java   This is actual simple performance check  So  the result will depend on the machine performance   Source code used for this is below     import java util Iterator  import java util LinkedList   public class Array vs LinkedList        private final static int MAX SIZE   40000000       public static void main String   args             LinkedList lList   new LinkedList                 insertion performance check             long startTime   System currentTimeMillis             for  int i 0  i lt MAX SIZE  i                  lList add i                      long stopTime   System currentTimeMillis            long elapsedTime   stopTime - startTime          System out println   Insert LinkedList insert operation with     MAX SIZE     number of integer elapsed time is     elapsedTime     millisecond              int   arr   new int MAX SIZE            startTime   System currentTimeMillis            for int i 0  i lt MAX SIZE  i                 arr i    i                      stopTime   System currentTimeMillis            elapsedTime   stopTime - startTime          System out println   Insert Array Insert operation with     MAX SIZE     number of integer elapsed time is     elapsedTime     millisecond                  iteration performance check             startTime   System currentTimeMillis             Iterator itr   lList iterator             while itr hasNext                  itr next                   System out println  Linked list running       itr next                        stopTime   System currentTimeMillis            elapsedTime   stopTime - startTime          System out println   Loop LinkedList iteration with     MAX SIZE     number of integer elapsed time is     elapsedTime     millisecond               startTime   System currentTimeMillis             int t   0          for  int i 0  i  lt  MAX SIZE  i                  t   arr i                  System out println  array running       i                      stopTime   System currentTimeMillis            elapsedTime   stopTime - startTime          System out println   Loop Array iteration with     MAX SIZE     number of integer elapsed time is     elapsedTime     millisecond               Performance Result is below

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I don t think it makes a real difference for Strings  What is contiguous in an array of strings is the references to the strings  the strings themselves are stored at random places in memory   Arrays vs  Lists can make a difference for primitive types  not for objects  IF you know in advance the number of elements  and don t need flexibility  an array of millions of integers or doubles will be more efficient in memory and marginally in speed than a list  because indeed they will be stored contiguously and accessed instantly  That s why Java still uses arrays of chars for strings  arrays of ints for image data  etc

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Thousands  is not a large number  A few thousand paragraph-length strings are on the order of a couple of megabytes in size  If all you want to do is access these serially  use an immutable singly-linked List

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I came here to get a better feeling for the performance impact of using lists over arrays  I had to adapt code here for my scenario  array list of  1000 ints using mostly getters  meaning array j  vs  list get j   Taking the best of 7 to be unscientific about it  first few with list where 2 5x slower  I get this   array Integer   best 643ms iterator ArrayList lt Integer gt  best 1014ms iterator  array Integer   best 635ms getter ArrayList lt Integer gt  best 891ms getter  strange though    - so  very roughly 30  faster with array   The second reason for posting now is that no-one mentions the impact if you do math matrix simulation optimization code with nested loops   Say you have three nested levels and the inner loop is twice as slow you are looking at 8 times performance hit  Something that would run in a day now takes a week    EDIT Quite shocked here  for kicks I tried declaring int 1000  rather than Integer 1000   array int   best 299ms iterator array int   best 296ms getter   Using Integer   vs  int   represents a double performance hit  ListArray with iterator is 3x slower than int    Really thought Java s list implementations were similar to native arrays     Code for reference  call multiple times        public static void testArray                 final long MAX ITERATIONS   1000000          final int MAX LENGTH   1000           Random r   new Random               Integer   array   new Integer MAX LENGTH           int   array   new int MAX LENGTH            List lt Integer gt  list   new ArrayList lt Integer gt                           for  int i   0  i  lt  MAX LENGTH    i                                int val   r nextInt                    add val                   array i    val                                     long start   System currentTimeMillis            int test sum   0          for  int i   0  i  lt  MAX ITERATIONS    i                        for  int e   array              for  int e   list                        for  int j   0  j  lt  MAX LENGTH    j                                int e   array j                   int e   list get j                   test sum    e                                   long stop   System currentTimeMillis             long ms    stop - start           System out println  Time      ms

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Array vs  List choice is not so important  considering performance  in the case of storing string objects  Because both array and list will store string object references  not the actual objects    If the number of strings is almost constant then use an array  or ArrayList   But if the number varies too much then you d better use LinkedList  If there is  or will be  a need for adding or deleting elements in the middle  then you certainly have to use LinkedList

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None of the answers had information that I was interested in - repetitive scan of the same array many many times  Had to create a JMH test for this   Results  Java 1 8 0 66 x32  iterating plain array is at least 5 times quicker than ArrayList    Benchmark                    Mode  Cnt   Score   Error  Units MyBenchmark testArrayForGet  avgt   10   8 121   0 233  ms op MyBenchmark testListForGet   avgt   10  37 416   0 094  ms op MyBenchmark testListForEach  avgt   10  75 674   1 897  ms op   Test  package my jmh test   import java util ArrayList  import java util List  import java util concurrent TimeUnit  import org openjdk jmh annotations Benchmark  import org openjdk jmh annotations BenchmarkMode  import org openjdk jmh annotations Fork  import org openjdk jmh annotations Measurement  import org openjdk jmh annotations Mode  import org openjdk jmh annotations OutputTimeUnit  import org openjdk jmh annotations Scope  import org openjdk jmh annotations State  import org openjdk jmh annotations Warmup    State Scope Benchmark   Fork 1   Warmup iterations   5  timeUnit   TimeUnit SECONDS   Measurement iterations   10   BenchmarkMode Mode AverageTime   OutputTimeUnit TimeUnit MILLISECONDS  public class MyBenchmark        public final static int ARR SIZE   100      public final static int ITER COUNT   100000       String arr     new String ARR SIZE       List lt String gt  list   new ArrayList lt  gt  ARR SIZE        public MyBenchmark             for  int i   0  i  lt  ARR SIZE  i                   list add null                         Benchmark     public void testListForEach             int count   0          for  int i   0  i  lt  ITER COUNT  i                   for  String str   list                     if  str    null                       count                                    if  count  gt  0               System out print count               Benchmark     public void testListForGet             int count   0          for  int i   0  i  lt  ITER COUNT  i                   for  int j   0  j  lt  ARR SIZE  j                       if  list get j     null                       count                                    if  count  gt  0               System out print count               Benchmark     public void testArrayForGet             int count   0          for  int i   0  i  lt  ITER COUNT  i                   for  int j   0  j  lt  ARR SIZE  j                       if  arr j     null                       count                                    if  count  gt  0               System out print count

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The Java way is that you should consider what data abstraction most suits your needs   Remember that in Java a List is an abstract  not a concrete data type   You should declare the strings as a List  and then initialize it using the ArrayList implementation   List lt String gt  strings   new ArrayList lt String gt       This separation of Abstract Data Type and specific implementation is one the key aspects of object oriented programming   An ArrayList implements the List Abstract Data Type using an array as its underlying implementation   Access speed is virtually identical to an array  with the additional advantages of being able to add and subtract elements to a List  although this is an O n  operation with an ArrayList  and that if you decide to change the underlying implementation later on you can   For example  if you realize you need synchronized access  you can change the implementation to a Vector without rewriting all your code   In fact  the ArrayList was specifically designed to replace the low-level array construct in most contexts   If Java was being designed today  it s entirely possible that arrays would have been left out altogether in favor of the ArrayList construct      Since arrays keep all the data in a contiguous chunk of memory  unlike Lists   would the use of an array to store thousands of strings cause problems     In Java  all collections store only references to objects  not the objects themselves    Both arrays and ArrayList will store a few thousand references in a contiguous array  so they are essentially identical   You can consider that a contiguous block of a few thousand 32-bit references will always be readily available on modern hardware   This does not guarantee that you will not run out of memory altogether  of course  just that the contiguous block of memory requirement is not difficult to fufil

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I agree that in most cases you should choose the flexibility and elegance of ArrayLists over arrays - and in most cases the impact to program performance will be negligible   However  if you re doing constant  heavy iteration with little structural change  no adds and removes  for  say  software graphics rendering or a custom virtual machine  my sequential access benchmarking tests show that ArrayLists are 1 5x slower than arrays on my system  Java 1 6 on my one year-old iMac    Some code   import java util     public class ArrayVsArrayList       static public void main  String   args              String   array   new String 300           ArrayList lt String gt  list   new ArrayList lt String gt  300            for  int i 0  i lt 300    i                if  Math random    gt  0 5                    array i     abc                 else                   array i     xyz                              list add  array i                        int iterations   100000000          long start ms          int sum           start ms   System currentTimeMillis            sum   0           for  int i 0  i lt iterations    i              for  int j 0  j lt 300    j  sum    array j  length                       System out println   System currentTimeMillis   - start ms      ms  array                 Prints  13 500 ms on my system          start ms   System currentTimeMillis            sum   0           for  int i 0  i lt iterations    i              for  int j 0  j lt 300    j  sum    list get j  length                       System out println   System currentTimeMillis   - start ms      ms  ArrayList                 Prints  20 800 ms on my system - about 1 5x slower than direct array access

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You should prefer generic types over arrays  As mentioned by others  arrays are inflexible and do not have the expressive power of generic types   They do however support runtime typechecking  but that mixes badly with generic types    But  as always  when optimizing you should always follow these steps    Don t optimize until you have a nice  clean  and working version of your code  Changing to generic types could very well be motivated at this step already  When you have a version that is nice and clean  decide if it is fast enough  If it isn t fast enough  measure its performance  This step is important for two reasons  If you don t measure you won t  1  know the impact of any optimizations you make and  2  know where to optimize  Optimize the hottest part of your code  Measure again  This is just as important as measuring before  If the optimization didn t improve things  revert it  Remember  the code without the optimization was clean  nice  and working

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Depending on the implementation  it s possible that an array of primitive types will be smaller and more efficient than ArrayList  This is because the array will store the values directly in a contiguous block of memory  while the simplest ArrayList implementation will store pointers to each value  On a 64-bit platform especially  this can make a huge difference   Of course  it s possible for the jvm implementation to have a special case for this situation  in which case the performance will be the same

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I wrote a little benchmark to compare ArrayLists with Arrays  On my old-ish laptop  the time to traverse through a 5000-element arraylist  1000 times  was about 10 milliseconds slower than the equivalent array code    So  if you re doing nothing but iterating the list  and you re doing it a lot  then maybe it s worth the optimisation  Otherwise I d use the List  because it ll make it easier when you do need to optimise the code   n b  I did notice that using for String s  stringsList was about 50  slower than using an old-style for-loop to access the list  Go figure    Here s the two functions I timed  the array and list were filled with 5000 random  different  strings   private static void readArray String   strings        long totalchars   0      for  int j   0  j  lt  ITERATIONS  j              totalchars   0          for  int i   0  i  lt  strings length  i                  totalchars    strings i  length                        private static void readArrayList List lt String gt  stringsList        long totalchars   0      for  int j   0  j  lt  ITERATIONS  j              totalchars   0          for  int i   0  i  lt  stringsList size    i                  totalchars    stringsList get i  length

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It depends on how you have to access it   After storing  if you mainly want to do search operation  with little or no insert delete  then go for Array  as search is done in O 1  in arrays  whereas add delete may need re-ordering of the elements    After storing  if your main purpose is to add delete strings  with little or no search operation  then go for List

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Arrays recommended everywhere you may use them instead of list  especially in case if you know items count and size would not be changing    See Oracle Java best practice  http   docs oracle com cd A97688 16 generic 903 bp java htm 1007056  Of course  if you need add and remove objects from collection many times easy use lists

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No  because technically  the array only stores the reference to the strings  The strings themselves are allocated in a different location  For a thousand items  I would say a list would be better  it is slower  but it offers more flexibility and it s easier to use  especially if you are going to resize them

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Well firstly it s worth clarifying do you mean  list  in the classical comp sci data structures sense  ie a linked list  or do you mean java util List  If you mean a java util List  it s an interface  If you want to use an array just use the ArrayList implementation and you ll get array-like behaviour and semantics  Problem solved   If you mean an array vs a linked list  it s a slightly different argument for which we go back to Big O  here is a plain English explanation if this is an unfamiliar term   Array    Random Access  O 1   Insert  O n   Delete  O n     Linked List    Random Access  O n   Insert  O 1   Delete  O 1     So you choose whichever one best suits how you resize your array  If you resize  insert and delete a lot then maybe a linked list is a better choice  Same goes for if random access is rare  You mention serial access  If you re mainly doing serial access with very little modification then it probably doesn t matter which you choose   Linked lists have a slightly higher overhead since  like you say  you re dealing with potentially non-contiguous blocks of memory and  effectively  pointers to the next element  That s probably not an important factor unless you re dealing with millions of entries however

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I m guessing the original poster is coming from a C   STL background which is causing some confusion  In C   std  list is a doubly linked list   In Java  java util  List is an implementation-free interface  pure abstract class in C   terms   List can be a doubly linked list - java util LinkedList is provided  However  99 times out of 100 when you want a make a new List  you want to use java util ArrayList instead  which is the rough equivalent of C   std  vector  There are other standard implementations  such as those returned by java util Collections emptyList   and java util Arrays asList     From a performance standpoint there is a very small hit from having to go through an interface and an extra object  however runtime inlining means this rarely has any significance  Also remember that String are typically an object plus array  So for each entry  you probably have two other objects  In C   std  vector lt std  string gt   although copying by value without a pointer as such  the character arrays will form an object for string  and these will not usually be shared    If this particular code is really performance-sensitive  you could create a single char   array  or even byte    for all the characters of all the strings  and then an array of offsets  IIRC  this is how javac is implemented

[java] Array or List in Java. Which is faster?

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