How do I determine whether an array contains a particular value in Java

Question

I have a String   with values like so   public static final String   VALUES   new String     AB   BC   CD   AE      Given String s  is there a good way of testing whether VALUES contains s

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Developers often do   Set lt String gt  set   new HashSet lt String gt  Arrays asList arr    return set contains targetValue     The above code works  but there is no need to convert a list to set first  Converting a list to a set requires extra time  It can as simple as   Arrays asList arr  contains targetValue     or     for String s  arr           if s equals targetValue               return true         return false    The first one is more readable than the second one

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For what it s worth I ran a test comparing the 3 suggestions for speed  I generated random integers  converted them to a String and added them to an array  I then searched for the highest possible number string  which would be a worst case scenario for the asList   contains     When using a 10K array size the results were    Sort   Search     15 Binary Search     0 asList contains   0   When using a 100K array the results were    Sort   Search     156 Binary Search     0 asList contains   32   So if the array is created in sorted order the binary search is the fastest  otherwise the asList   contains would be the way to go  If you have many searches  then it may be worthwhile to sort the array so you can use the binary search  It all depends on your application     I would think those are the results most people would expect  Here is the test code   import java util     public class Test       public static void main String args                  long start   0          int size   100000          String   strings   new String size           Random random   new Random              for  int i   0  i  lt  size  i                strings i         random nextInt  size             start   System currentTimeMillis            Arrays sort strings           System out println Arrays binarySearch strings        size - 1              System out println  Sort  amp  Search        System currentTimeMillis   - start             start   System currentTimeMillis            System out println Arrays binarySearch strings        size - 1              System out println  Search               System currentTimeMillis   - start             start   System currentTimeMillis            System out println Arrays asList strings  contains        size - 1              System out println  Contains             System currentTimeMillis   - start

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Concise update for Java SE 9 Reference arrays are bad  For this case we are after a set  Since Java SE 9 we have Set of  private static final Set lt String gt  VALUES   Set of       quot AB quot   quot BC quot   quot CD quot   quot AE quot       quot Given String s  is there a good way of testing whether VALUES contains s  quot  VALUES contains s   O 1   The right type  immutable  O 1  and concise  Beautiful   Original answer details Just to clear the code up to start with  We have  corrected   public static final String   VALUES   new String     quot AB quot   quot BC quot   quot CD quot   quot AE quot     This is a mutable static which FindBugs will tell you is very naughty  Do not modify statics and do not allow other code to do so also  At an absolute minimum  the field should be private  private static final String   VALUES   new String     quot AB quot   quot BC quot   quot CD quot   quot AE quot      Note  you can actually drop the new String    bit   Reference arrays are still bad and we want a set  private static final Set lt String gt  VALUES   new HashSet lt String gt  Arrays asList       new String     quot AB quot   quot BC quot   quot CD quot   quot AE quot         Paranoid people  such as myself  may feel more at ease if this was wrapped in Collections unmodifiableSet - it could then even be made public     To be a little more on brand  the collections API is predictably still missing immutable collection types and the syntax is still far too verbose  for my tastes

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One possible solution   import java util Arrays  import java util List   public class ArrayContainsElement     public static final List lt String gt  VALUES   Arrays asList  AB    BC    CD    AE       public static void main String args             if  VALUES contains  AB                System out println  Contains            else             System out println  Not contains

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You can use ArrayUtils contains from Apache Commons Lang  public static boolean contains Object   array  Object objectToFind   Note that this method returns false if the passed array is null   There are also methods available for primitive arrays of all kinds   Example   String   fieldsToInclude      id    name    location      if   ArrayUtils contains  fieldsToInclude   id               Do some stuff

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the use of a Spliterator prevents the unnecessary generation of a List  boolean found   false      class variable  String search    AB   Spliterator lt String gt  spl   Arrays spliterator  VALUES  0  VALUES length    while     found   amp  amp  spl tryAdvance o - gt  found   o equals  search         found    true if search is contained in the array  this does work for arrays of primitives  public static final int   VALUES   new int    1  2  3  4   boolean found   false      class variable  int search   2  Spliterator lt Integer gt  spl   Arrays spliterator  VALUES  0  VALUES length

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Using a simple loop is the most efficient way of doing this    boolean useLoop String   arr  String targetValue        for String s  arr           if s equals targetValue               return true            return false      Courtesy to Programcreek

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Create a boolean initially set to false  Run a loop to check every value in the array and compare to the value you are checking against  If you ever get a match  set boolean to true and stop the looping  Then assert that the boolean is true

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Actually  if you use HashSet lt String  as Tom Hawtin proposed you don t need to worry about sorting  and your speed is the same as with binary search on a presorted array  probably even faster   It all depends on how your code is set up  obviously  but from where I stand  the order would be   On an unsorted array    HashSet asList sort  amp  binary   On a sorted array    HashSet Binary asList   So either way  HashSet for the win

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Use the following  the contains   method is ArrayUtils in   in this code    ObjectUtils java  public class ObjectUtils                  A null safe method to detect if two objects are equal          param object1         param object2         return true if either both objects are null  or equal  else returns false              public static boolean equals Object object1  Object object2           return object1  null   object2  null   object1 equals object2              ArrayUtils java  public class ArrayUtils                  Find the index of of an object is in given array  starting from given inclusive index          param ts  Array to be searched in          param t  Object to be searched          param start  The index from where the search must start          return Index of the given object in the array if it is there  else -1              public static  lt T gt  int indexOf final T   ts  final T t  int start           for int i   start  i  lt  ts length    i              if ObjectUtils equals ts i   t                   return i          return -1                        Find the index of of an object is in given array  starting from 0          param ts  Array to be searched in          param t  Object to be searched          return  indexOf ts  t  0              public static  lt T gt  int indexOf final T   ts  final T t           return indexOf ts  t  0                         Detect if the given object is in the given array          param ts  Array to be searched in          param t  Object to be searched          return  If indexOf ts  t  is greater than -1              public static  lt T gt  boolean in final T   ts  final T t           return indexOf ts  t   gt  -1              As you can see in the code above  that there are other utility methods ObjectUtils equals   and ArrayUtils indexOf    that were used at other places as well

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For arrays of limited length use the following  as given by camickr    This is slow for repeated checks  especially for longer arrays  linear search       Arrays asList      contains       For fast performance if you repeatedly check against a larger set of elements   An array is the wrong structure   Use a TreeSet and add each element to it   It sorts elements and has a fast exist   method  binary search   If the elements implement Comparable  amp  you want the TreeSet sorted accordingly   ElementClass compareTo   method must be compatable with ElementClass equals    see Triads not showing up to fight   Java Set missing an item    TreeSet myElements   new TreeSet        Do this for each element  implementing  Comparable   myElements add nextElement        Alternatively   if an array is forceably provided from other code  myElements addAll Arrays asList myArray     Otherwise  use your own Comparator   class MyComparator implements Comparator lt ElementClass gt         int compareTo ElementClass element1  ElementClass element2                 Your comparison of elements              Should be consistent with object equality              boolean equals Object otherComparator                 Your equality of comparators               construct TreeSet with the comparator TreeSet myElements   new TreeSet new MyComparator         Do this for each element  implementing  Comparable   myElements add nextElement    The payoff  check existence of some element      Fast binary search through sorted elements  performance   log size    boolean containsElement   myElements exists someElement

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Check this  String   VALUES   new String     AB   BC   CD   AE    String s   for int i 0  i lt  VALUES length   i          if   VALUES i  equals s                      do your stuff            else                do your stuff

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If the array is not sorted  you will have to iterate over everything and make a call to equals on each   If the array is sorted  you can do a binary search  there s one in the Arrays class   Generally speaking  if you are going to do a lot of membership checks  you may want to store everything in a Set  not in an array

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Use Array BinarySearch array obj  for finding the given object in array or not   Example   if  Array BinarySearch str  i   gt  -1     true --exists   false --not exists

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You can use the Arrays class to perform a binary search for the value  If your array is not sorted  you will have to use the sort functions in the same class to sort the array  then search through it

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Just simply implement it by hand   public static  lt T gt  boolean contains final T   array  final T v        for  final T e   array          if  e    v    v    null  amp  amp  v equals e               return true       return false      Improvement   The v    null condition is constant inside the method  It always evaluates to the same Boolean value during the method call  So if the input array is big  it is more efficient to evaluate this condition only once  and we can use a simplified faster condition inside the for loop based on the result  The improved contains   method   public static  lt T gt  boolean contains2 final T   array  final T v        if  v    null            for  final T e   array              if  e    null                  return true             else           for  final T e   array              if  e    v    v equals e                   return true             return false

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You can check it by two methods   A By converting the array into string and then check the required string by  contains method    String a Arrays toString VALUES       System out println a contains  AB         System out println a contains  BC         System out println a contains  CD         System out println a contains  AE       B this is a more efficent method   Scanner s new Scanner System in        String u s next       boolean d true      for int i 0 i lt VAL length i                  if VAL i  equals u   d              System out println VAL i       u VAL i  equals u

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Try using Java 8 predicate test method  Here is a full example of it   import java util Arrays  import java util List  import java util function Predicate  public class Test    public static final List lt String gt  VALUES   Arrays asList  AA    AB    BC    CD    AE      public static void main String args        Predicate lt String gt  containsLetterA   VALUES - gt  VALUES contains  AB      for  String i   VALUES        System out println containsLetterA test i                http   mytechnologythought blogspot com 2019 10 java-8-predicate-test-method-example html  https   github com VipulGulhane1 java8 blob master Test java

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Arrays asList   -  then calling the contains   method will always work  but a search algorithm is much better since you don t need to create a lightweight list wrapper around the array  which is what Arrays asList   does   public boolean findString String   strings  String desired      for  String str   strings          if  desired equals str                return true                   return false    if we get here    there is no desired String  return false

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Try this   ArrayList lt Integer gt  arrlist   new ArrayList lt Integer gt  8       use add   method to add elements in the list arrlist add 20   arrlist add 25   arrlist add 10   arrlist add 15    boolean retval   arrlist contains 10   if  retval    true        System out println  10 is contained in the list      else       System out println  10 is not contained in the list

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If you don t want it to be case sensitive   Arrays stream VALUES  anyMatch s  equalsIgnoreCase

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With Java 8 you can create a stream and check if any entries in the stream matches  s    String   values     AB   BC   CD   AE    boolean sInArray   Arrays stream values  anyMatch  s   equals     Or as a generic method   public static  lt T gt  boolean arrayContains T   array  T value        return Arrays stream array  anyMatch value  equals

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ObStupidAnswer  but I think there s a lesson in here somewhere    enum Values       AB  BC  CD  AE    try       Values valueOf s       return true    catch  IllegalArgumentException exc        return false

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As I m dealing with low level Java using primitive types byte and byte    the best so far I got is from bytes-java https   github com patrickfav bytes-java seems a fine piece of work

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If you have the google collections library  Tom s answer can be simplified a lot by using ImmutableSet  http   google-collections googlecode com svn trunk javadoc com google common collect ImmutableSet html   This really removes a lot of clutter from the initialization proposed  private static final Set lt String gt  VALUES    ImmutableSet of  AB   BC   CD   AE

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Instead of using the quick array initialisation syntax too  you could just initialise it as a List straight away in a similar manner using the Arrays asList method  e g    public static final List lt String gt  STRINGS   Arrays asList  firstString    secondString         lastString      Then you can do  like above     STRINGS contains  the string you want to find

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Arrays asList yourArray  contains yourValue    Warning  this doesn t work for arrays of primitives  see the comments      Since java-8 you can now use Streams   String   values     AB   BC   CD   AE    boolean contains   Arrays stream values  anyMatch  s   equals     To check whether an array of int  double or long contains a value use IntStream  DoubleStream or LongStream respectively   Example  int   a    1 2 3 4   boolean contains   IntStream of a  anyMatch x - gt  x    4

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Four Different Ways to Check If an Array Contains a Value   Using List   public static boolean useList String   arr  String targetValue        return Arrays asList arr  contains targetValue      Using Set   public static boolean useSet String   arr  String targetValue        Set lt String gt  set   new HashSet lt String gt  Arrays asList arr        return set contains targetValue      Using a simple loop   public static boolean useLoop String   arr  String targetValue        for  String s  arr            if  s equals targetValue               return true            return false     Using Arrays binarySearch     The code below is wrong  it is listed here for completeness  binarySearch   can ONLY be used on sorted arrays  You will find the result is weird below  This is the best option when array is sorted   public static boolean binarySearch String   arr  String targetValue          return Arrays binarySearch arr  targetValue   gt   0       Quick Example   String testValue  test   String newValueNotInList  newValue   String   valueArray      this    is    java     test     Arrays asList valueArray  contains testValue      returns true Arrays asList valueArray  contains newValueNotInList      returns false

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In Java 8 use Streams   List lt String gt  myList   Arrays asList  a1    a2    b1    c2    c1     myList  stream    filter s - gt  s startsWith  c     map String  toUpperCase   sorted    forEach System out  println

[java] How do I determine whether an array contains a particular value in Java?

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