Where is Java s Array indexOf

Question

I must be missing something very obvious  but I ve searched all over and can t find this method

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Java ArrayList has an indexOf method   Java arrays have no such method

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Arrays themselves do not have that method   A List  however  does  indexOf

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I don t recall of a  indexOf  on arrays other than coding it for yourself    though you could probably use one of the many java util Arrays binarySearch      methods  see the Arrays javadoc  if your array contains primitive types

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Jeffrey Hantin s answer is good but it has some constraints  if its this do this or else to that     You can write your own extension method and it always works the way you want   Lists indexOf array  x - gt  item    x      compare in the way you want   And here is your extension  public final class Lists       private Lists                public static  lt T gt  int indexOf T   array  Predicate lt T gt  predicate            for  int i   0  i  lt  array length  i                  if  predicate test array i    return i                    return -1             public static  lt T gt  int indexOf List lt T gt  list  Predicate lt T gt  predicate            for  int i   0  i  lt  list size    i                  if  predicate test list get i    return i                    return -1             public interface Predicate lt T gt            boolean test T t

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Unlike in C  where you have the Array IndexOf method  and JavaScript where you have the indexOf method  Java s API  the Array and Arrays classes in particular  have no such method    This method indexOf  together with its complement lastIndexOf  is defined in the java util List interface  Note that indexOf and lastIndexOf are not overloaded and only take an Object as a parameter   If your array is sorted  you are in luck because the Arrays class defines a series of overloads of the binarySearch method that will find the index of the element you are looking for with best possible performance  O log n  instead of O n   the latter being what you can expect from a sequential search done by indexOf   There are four considerations    The array must be sorted either in natural order or in the order of a Comparator that you provide as an argument  or at the very least all elements that are  less than  the key must come before that element in the array and all elements that are  greater than  the key must come after that element in the array  The test you normally do with indexOf to determine if a key is in the array  verify if the return value is not -1  does not hold with binarySearch  You need to verify that the return value is not less than zero since the value returned will indicate the key is not present but the index at which it would be expected if it did exist  If your array contains multiple elements that are equal to the key  what you get from binarySearch is undefined  this is different from indexOf that will return the first occurrence and lastIndexOf that will return the last occurrence  An array of booleans might appear to be sorted if it first contains all falses and then all trues  but this doesn t count  There is no override of the binarySearch method that accepts an array of booleans and you ll have to do something clever there if you want O log n  performance when detecting where the first true appears in an array  for instance using an array of Booleans and the constants Boolean FALSE and Boolean TRUE    If your array is not sorted and not primitive type  you can use List s indexOf and lastIndexOf methods by invoking the asList method of java util Arrays  This method will return an AbstractList interface wrapper around your array  It involves minimal overhead since it does not create a copy of the array  As mentioned  this method is not overloaded so this will only work on arrays of reference types   If your array is not sorted and the type of the array is primitive  you are out of luck with the Java API  Write your own for loop  or your own static utility method  which will certainly have performance advantages over the asList approach that involves some overhead of an object instantiation  In case you re concerned that writing a brute force for loop that iterates over all of the elements of the array is not an elegant solution  accept that that is exactly what the Java API is doing when you call indexOf  You can make something like this   public static int indexOfIntArray int   array  int key        int returnvalue   -1      for  int i   0  i  lt  array length    i            if  key    array i                 returnvalue   i              break                      return returnvalue      If you want to avoid writing your own method here  consider using one from a development framework like Guava  There you can find an implementation of indexOf and lastIndexOf

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You re probably thinking of the java util ArrayList  not the array

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There is no direct indexOf function in java arrays

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int findIndex int myElement  int   someArray    int index   0   for int n  someArray      if myElement    n  return index     else index           Note  you can use this method for arrays of type int  you can also use this algorithm for other types with minor changes

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Array has no indexOf   method   Maybe this Apache Commons Lang ArrayUtils method is what you are looking for  import org apache commons lang3 ArrayUtils   String   colours      Red    Orange    Yellow    Green      int indexOfYellow   ArrayUtils indexOf colours   Yellow

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For primitives  if you want to avoid boxing  Guava has helpers for primitive arrays e g  Ints indexOf int   array  int target

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The List interface has an indexOf   method  and you can obtain a List from your array with Array s asList   method  Other than that  Array itself has no such method  It does have a binarySearch   method for sorted arrays

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There are a couple of ways to accomplish this using the Arrays utility class   If the array is not sorted and is not an array of primitives   java util Arrays asList theArray  indexOf o    If the array is primitives and not sorted  one should use a solution offered by one of the other answers such as Kerem Baydogan s  Andrew McKinlay s or Mishax s   The above code will compile even if theArray is primitive  possibly emitting a warning  but you ll get totally incorrect results nonetheless   If the array is sorted  you can make use of a binary search for performance   java util Arrays binarySearch theArray  o

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There is none  Either use a java util List   or you can write your own indexOf     public static  lt T gt  int indexOf T needle  T   haystack        for  int i 0  i lt haystack length  i                  if  haystack i     null  amp  amp  haystack i  equals needle                 needle    null  amp  amp  haystack i     null  return i             return -1         you can make one from your array using Arrays asList

[java] Where is Java's Array indexOf?

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