What causes a java lang ArrayIndexOutOfBoundsException and how do I prevent it

Question

What does ArrayIndexOutOfBoundsException mean and how do I get rid of it    Here is a code sample that triggers the exception   String   names      tom    bob    harry     for  int i   0  i  lt   names length  i          System out println names i

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ArrayIndexOutOfBoundsException name itself explains that If you trying to access the value at the index which is out of the scope of Array size then such kind of exception occur    In your case  You can just remove equal sign from your for loop   for int i   0  i lt name length  i      The better option is to iterate an array    for String i   name         System out println i

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This error is occurs at runs loop  overlimit times Let s consider simple example like this   class demo    public static void main String a          int   numberArray  4 8 2 3 89 5        int i       for i 0 i lt numberArray length i             System out print numberArray i 1                   At first  I have initialized an array as  numberArray   then   some array elements are printed using for loop  When loop is running  i  time   print the  numberArray i 1  element   when i value is 1  numberArray i 1  element is printed    Suppose that  when i  numberArray length-2   last element of array is printed  When  i  value goes to  numberArray length-1    no value for printing  In that point    ArrayIndexOutOfBoundsException  is occur I hope to you could get idea thank you

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According to your Code    String   name     tom    dick    harry    for int i   0  i lt  name length  i        System out print name i     n        If You check System out print name length    you will get 3   that mean your name length is 3  your loop is running from 0 to 3  which should be running either  0 to 2  or  1 to 3   Answer  String   name     tom    dick    harry    for int i   0  i lt name length  i        System out print name i     n

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The most common case I ve seen for seemingly mysterious ArrayIndexOutOfBoundsExceptions  i e  apparently not caused by your own array handling code  is the concurrent use of SimpleDateFormat  Particularly in a servlet or controller   public class MyController     SimpleDateFormat dateFormat   new SimpleDateFormat  MM dd yyyy       public void handleRequest ServletRequest req  ServletResponse res        Date date   dateFormat parse req getParameter  date             If two threads enter the SimplateDateFormat parse   method together you will likely see an ArrayIndexOutOfBoundsException  Note the synchronization section of the class javadoc for SimpleDateFormat   Make sure there is no place in your code that are accessing thread unsafe classes like SimpleDateFormat in a concurrent manner like in a servlet or controller  Check all instance variables of your servlets and controllers for likely suspects

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In most of the programming language indexes is start from 0 So you must have to write i lt names length or i lt  names length-1 instead of i lt  names length

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You can use Optional in functional style to avoid NullPointerException and ArrayIndexOutOfBoundsException    String   array   new String    aaa   null   ccc    for  int i   0  i  lt  4  i          String result   Optional ofNullable array length  gt  i   array i    null               map x - gt  x toUpperCase      some operation here              orElse  NO DATA        System out println result       Output   AAA NO DATA CCC NO DATA

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You could not iterate or store more data than the length of your array  In this case you could do like this   for  int i   0  i  lt   name length - 1  i                      Or this   for  int i   0  i  lt  name length  i

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If you use an array s length to control iteration of a for loop  always remember that the index of the first item in an array is 0  So the index of the last element in an array is one less than the array s length

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if  index  lt  0    index  gt   array length           Don t use this index  This is out of bounds  borders  limits  whatever     else          Yes  you can safely use this index  The index is present in the array      Object element   array index       See also    The Java Tutorials - Language Basics - Arrays     Update  as per your code snippet    for  int i   0  i lt  name length  i        The index is inclusive the array s length  This is out of bounds  You need to replace  lt   by  lt    for  int i   0  i  lt  name length  i

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ArrayIndexOutOfBoundsException whenever this exception is coming it mean you are trying to use an index of array which is out of its bounds or in lay man terms you are requesting more than than you have initialised   To prevent this always make sure that you are not requesting a index which is not present in array i e  if array length is 10 then your index must range between 0 to 9

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ArrayIndexOutOfBounds means you are trying to index a position within an array that is not allocated   In this case   String   name      tom    dick    harry     for  int i   0  i  lt   name length  i          System out println name i         name length is 3 since the array has been defined with 3 String objects  When accessing the contents of an array  position starts from 0  Since there are 3 items  it would mean name 0   tom   name 1   dick  and name 2   harry When you loop  since i can be less than or equal to name length  you are trying to access name 3  which is not available    To get around this       In your for loop  you can do i  lt  name length  This would prevent looping to name 3  and would instead stop at name 2   for int i   0  i lt name length  i    Use a for each loop  String   name      tom    dick    harry     for String n   name        System out println n     Use list forEach Consumer action   requires Java8   String   name      tom    dick    harry     Arrays asList name  forEach System out  println   Convert array to stream - this is a good option if you want to perform additional  operations  to your array e g  filter  transform the text  convert to a map etc  requires Java8   String   name      tom    dick    harry     ---  Arrays asList name  stream   forEach System out  println   ---  Stream of name  forEach System out  println

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In your code you have accessed the elements from index 0 to the length of the string array  name length gives the number of string objects in your array of string objects i e  3  but you can access only up to index 2 name 2   because the array can be accessed from index 0 to name length - 1 where you get name length number of objects  Even while using a for loop you have started with index zero and you should end with name length - 1  In an array a n  you can access form a 0  to a n-1   For example  String   a   quot str1 quot    quot str2 quot    quot str3 quot        quot strn quot     for int i 0  i lt a length    i        System out println a i     In your case  String   name     quot tom quot    quot dick quot    quot harry quot     for int i   0  i lt  name length  i          System out print name i     n

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Each item in an array is called an element  and each element is accessed by its numerical index  As shown in the preceding illustration  numbering begins with 0  The 9th element  for example  would therefore be accessed at index 8   IndexOutOfBoundsException is thrown to indicate that an index of some sort  such as to an array  to a string  or to a vector  is out of range   Any array X  can be accessed from  0 to  X length - 1

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For your given array the length of the array is 3 i e  name length   3   But as it stores element starting from index 0  it has max index 2   So  instead of   i   lt  name length  you should write  i lt   name length  to avoid  ArrayIndexOutOfBoundsException

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From this excellent article  ArrayIndexOutOfBoundsException in for loop  To put it briefly   In the last iteration of  for  int i   0  i  lt   name length  i        i will equal name length which is an illegal index  since array indices are zero-based   Your code should read  for  int i   0  i  lt  name length  i

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To avoid an array index out-of-bounds exception  one should use the enhanced-for statement where and when they can   The primary motivation  and use case  is when you are iterating and you do not require any complicated iteration steps   You would not be able to use an enhanced-for to move backwards in an array or only iterate on every other element   You re guaranteed not to run out of elements to iterate over when doing this  and your  corrected  example is easily converted over   The code below   String   name     tom    dick    harry    for int i   0  i lt  name length  i          System out print name i      n           is equivalent to this   String   name     tom    dick    harry    for String firstName   name        System out println firstName     n

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Here 0-- gt  tom   1-- gt bob  2-- gt harry  But there is no index for 3  Here   value of names length is  3   So by for loop you are accessing index 3  So  the error is coming  just change String   names      quot tom quot    quot bob quot    quot harry quot     for  int i   0  i  lt   names length -1   i          System out println names i       to avoid ArrayIndexOutOfBoundsException

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ArrayIndexOutOfBoundsException means that you are trying to access an index of the array that does not exist or out of the bound of this array  Array indexes start from 0 and end at length - 1    In your case   for int i   0  i lt  name length  i          System out print name i     n       i goes from 0 to length  Not correct     ArrayIndexOutOfBoundsException happens when you are trying to access  the name length indexed element which does not exist  array index ends at  length -1   just replacing  lt   with  lt  would solve this problem    for int i   0  i  lt  name length  i          System out print name i     n        i goes from 0 to length - 1  Correct

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For any array of length n  elements of the array will have an index from 0 to n-1   If your program is trying to access any element  or memory  having array index greater than n-1  then Java will throw ArrayIndexOutOfBoundsException  So here are two solutions that we can use in a program   Maintaining count   for int count   0  count  lt  array length  count          System out println array count        Or some other looping statement like  int count   0  while count  lt  array length        System out println array count        count       A better way go with a for each loop  in this method a programmer has no need to bother about the number of elements in the array   for String str   array        System out println str

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So much for this simple question  but I just wanted to highlight a new feature in Java which will avoid all confusions around indexing in arrays even for beginners  Java-8 has abstracted the task of iterating for you   int   array   new int 5      If you need just the items Arrays stream array  forEach item - gt    println item          If you need the index as well IntStream range 0  array length  forEach index - gt    println array index         What s the benefit  Well  one thing is the readability like English  Second  you need not worry about the ArrayIndexOutOfBoundsException

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It means that you are trying to access an index of an array which is not valid as it is not in between the bounds   For example this would initialize a primitive integer array with the upper bound 4   int intArray     new int 5     Programmers count from zero  So this for example would throw an ArrayIndexOutOfBoundsException as the upper bound is 4 and not 5   intArray 5

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That s how this type of exception looks when thrown in Eclipse  The number in red signifies the index you tried to access  So the code would look like this   myArray 5    The error is thrown when you try to access an index which doesn t exist in that array  If an array has a length of 3   int   intArray   new int 3     then the only valid indexes are   intArray 0  intArray 1  intArray 2    If an array has a length of 1   int   intArray   new int 1     then the only valid index is   intArray 0    Any integer equal to the length of the array  or bigger than it  is out of bounds   Any integer less than 0  is out of bounds   P S   If you look to have a better understanding of arrays and do some practical exercises  there s a video here  tutorial on arrays in Java

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I see all the answers here explaining how to work with arrays and how to avoid the index out of bounds exceptions   I personally avoid arrays at all costs   I use the Collections classes  which avoids all the silliness of having to deal with array indices entirely   The looping constructs work beautifully with collections supporting code that is both easier to write  understand and maintain

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What causes ArrayIndexOutOfBoundsException   If you think of a variable as a  box  where you can place a value  then an array is a series of boxes placed next to eachother  where the number of boxes is a finite and explicit integer   Creating an array like this   final int   myArray   new int 5    creates a row of 5 boxes  each holding an int  Each of the boxes have an index  a position in the series of boxes  This index starts at 0  and ends at N-1  where N is the size of the array  the number of boxes    To retrieve one of the values from this series of boxes  you can refer to it through its index  like this   myArray 3    Which will give you the value of the 4th box in the series  since the first box has index 0    An ArrayIndexOutOfBoundsException is caused by trying to retrive a  box  that does not exist  by passing an index that is higher than the index of last  box   or negative   With my running example  these code snippets would produce such an exception   myArray 5    tries to retrieve the 6th  box  when there is only 5 myArray -1    just makes no sense myArray 1337    waay to high   How to avoid ArrayIndexOutOfBoundsException  In order to prevent ArrayIndexOutOfBoundsException  there are some key points to consider   Looping  When looping through an array  always make sure that the index you are retrieving is strictly smaller than the length of the array  the number of boxes   For instance   for  int i   0  i  lt  myArray length  i        Notice the  lt   never mix a   in there    You might want to be tempted to do something like this   for  int i   1  i  lt   myArray length  i          final int someint   myArray i - 1    Just don t  Stick to the one above  if you need to use the index  and it will save you a lot of pain   Where possible  use foreach   for  int value   myArray      This way you won t have to think about indexes at all   When looping  whatever you do  NEVER change the value of the loop iterator  here  i   The only place this should change value is to keep the loop going  Changing it otherwise is just risking an exception  and is in most cases not neccessary   Retrieval update  When retrieving an arbitrary element of the array  always check that it is a valid index against the length of the array   public Integer getArrayElement final int index        if  index  lt  0    index  gt   myArray length            return null    although I would much prefer an actual exception being thrown when this happens            return myArray index

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For multidimensional arrays  it can be tricky to make sure you access the length property of the right dimension  Take the following code for example   int        a    new int  2  3  4    for int i   0  i  lt  a length  i         for int j   0  j  lt  a i  length  j             for int k   0  k  lt  a j  length  k                 System out print a i  j  k                      System out println              System out println        Each dimension has a different length  so the subtle bug is that the middle and inner loops use the length property of the same dimension  because a i  length is the same as a j  length    Instead  the inner loop should use a i  j  length  or a 0  0  length  for simplicity

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Your first port of call should be the documentation which explains it reasonably clearly      Thrown to indicate that an array has been accessed with an illegal index  The index is either negative or greater than or equal to the size of the array    So for example   int   array   new int 5   int boom   array 10      Throws the exception   As for how to avoid it    um  don t do that  Be careful with your array indexes   One problem people sometimes run into is thinking that arrays are 1-indexed  e g   int   array   new int 5          populate the array here     for  int index   1  index  lt   array length  index          System out println array index        That will miss out the first element  index 0  and throw an exception when index is 5  The valid indexes here are 0-4 inclusive  The correct  idiomatic for statement here would be   for  int index   0  index  lt  array length  index       That s assuming you need the index  of course  If you can use the enhanced for loop instead  do so

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You are getting ArrayIndexOutOfBoundsException due to i lt  name length part  name length return the length of the string name  which is 3  Hence when you try to access name 3   it s illegal and throws an exception   Resolved code   String   name     tom    dick    harry    for int i   0  i  lt  name length  i        use  lt  insteadof  lt     System out print name i     n        It s defined in the Java language specification      The public final field length  which contains the number of components   of the array  length may be positive or zero

[java] What causes a java.lang.ArrayIndexOutOfBoundsException and how do I prevent it?

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