Convert ArrayList String to String array

Question

I m working in the android environment and have tried the following code  but it doesn t seem to be working   String    stockArr    String    stock list toArray      If I define as follows   String    stockArr     hello    world      it works  Is there something that I m missing

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The correct way to do this is     String   stockArr   stock list toArray new String stock list size        I d like to add to the other great answers here and explain how you could have used the Javadocs to answer your question   The Javadoc for toArray    no arguments  is here  As you can see  this method returns an Object   and not String   which is an array of the runtime type of your list      public Object   toArray           Returns an array containing all of the   elements in this collection  If the collection makes any guarantees as   to what order its elements are returned by its iterator  this method   must return the elements in the same order  The returned array will be    safe  in that no references to it are maintained by the collection     In other words  this method must allocate a new array even if the   collection is backed by an Array   The caller is thus free to modify   the returned array    Right below that method  though  is the Javadoc for toArray T   a   As you can see  this method returns a T   where T is the type of the array you pass in  At first this seems like what you re looking for  but it s unclear exactly why you re passing in an array  are you adding to it  using it for just the type  etc   The documentation makes it clear that the purpose of the passed array is essentially to define the type of array to return  which is exactly your use case         public  lt T gt  T   toArray T   a       Returns an array containing all of the   elements in this collection  the runtime type of the returned array is   that of the specified array  If the collection fits in the specified   array  it is returned therein  Otherwise  a new array is allocated   with the runtime type of the specified array and the size of this   collection  If the collection fits in the specified array with room to   spare  i e   the array has more elements than the collection   the   element in the array immediately following the end of the collection   is set to null  This is useful in determining the length of the   collection only if the caller knows that the collection does not   contain any null elements        If this collection makes any guarantees as to what order its elements   are returned by its iterator  this method must return the elements in   the same order       This implementation checks if the array is large enough to contain the   collection  if not  it allocates a new array of the correct size and   type  using reflection   Then  it iterates over the collection    storing each object reference in the next consecutive element of the   array  starting with element 0  If the array is larger than the   collection  a null is stored in the first location after the end of   the collection    Of course  an understanding of generics  as described in the other answers  is required to really understand the difference between these two methods  Nevertheless  if you first go to the Javadocs  you will usually find your answer and then see for yourself what else you need to learn  if you really do      Also note that reading the Javadocs here helps you to understand what the structure of the array you pass in should be  Though it may not really practically matter  you should not pass in an empty array like this     String    stockArr   stockList toArray new String 0        Because  from the doc  this implementation checks if the array is large enough to contain the collection  if not  it allocates a new array of the correct size and type  using reflection   There s no need for the extra overhead in creating a new array when you could easily pass in the size     As is usually the case  the Javadocs provide you with a wealth of information and direction     Hey wait a minute  what s reflection

User · Answer

An alternative in Java 8   String   strings   list stream   toArray String    new

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Use like this   List lt String gt  stockList   new ArrayList lt String gt     stockList add  stock1    stockList add  stock2     String   stockArr   new String stockList size     stockArr   stockList toArray stockArr    for String s   stockArr      System out println s

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What is happening is that stock list toArray   is creating an Object   rather than a String   and hence the typecast is failing1   The correct code would be     String    stockArr   stockList toArray new String stockList size        or even    String    stockArr   stockList toArray new String 0      For more details  refer to the javadocs for the two overloads of List toArray   The latter version uses the zero-length array to determine the type of the result array    Surprisingly  it is faster to do this than to preallocate     at least  for recent Java releases   See https   stackoverflow com a 4042464 139985 for details    From a technical perspective  the reason for this API behavior   design is that an implementation of the List lt T gt  toArray   method has no information of what the  lt T gt  is at runtime   All it knows is that the raw element type is Object   By contrast  in the other case  the array parameter gives the base type of the array    If the supplied array is big enough to hold the list elements  it is used   Otherwise a new array of the same type and a larger size is allocated and returned as the result      1 - In Java  an Object   is not assignment compatible with a String     If it was  then you could do this       Object   objects   new Object   new Cat  fluffy         Dog   dogs    Dog    objects      Dog d   dogs 0          Huh      This is clearly nonsense  and that is why array types are not generally assignment compatible

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I can see many answers showing how to solve problem  but only Stephen s answer is trying to explain why problem occurs so I will try to add something more on this subject  It is a story about possible reasons why Object   toArray wasn t changed to T   toArray where generics ware introduced to Java   Why String   stockArr    String    stock list toArray    wont work  In Java  generic type exists at compile-time only  At runtime information about generic type  like in your case  lt String gt   is removed and replaced with Object type  take a look at type erasure   That is why at runtime toArray   have no idea about what precise type to use to create new array  so it uses Object as safest type  because each class extends Object so it can safely store instance of any class  Now the problem is that you can t cast instance of Object   to String    Why  Take a look at this example  lets assume that class B extends A     B extends A A a   new A    B b    B a   Although such code will compile  at runtime we will see thrown ClassCastException because instance held by reference a is not actually of type B  or its subtypes   Why is this problem  why this exception needs to be cast   One of the reasons is that B could have new methods fields which A doesn t  so it is possible that someone will try to use these new members via b reference even if held instance doesn t have  doesn t support  them  In other words we could end up trying to use data which doesn t exist  which could lead to many problems  So to prevent such situation JVM throws exception  and stop further potentially dangerous code  You could ask now  quot So why aren t we stopped even earlier  Why code involving such casting is even compilable  Shouldn t compiler stop it  quot   Answer is  no because compiler can t know for sure what is the actual type of instance held by a reference  and there is a chance that it will hold instance of class B which will support interface of b reference  Take a look at this example  A a   new B                ------ Here reference  quot a quot  holds instance of type B B b    B a        so now casting is safe  now JVM is sure that  b  reference can                    safely access all members of B class   Now lets go back to your arrays  As you see in question  we can t cast instance of Object   array to more precise type String   like Object   arr   new Object      quot ab quot    quot cd quot     String   arr2    String    arr   ClassCastException will be thrown  Here problem is a little different  Now we are sure that String   array will not have additional fields or methods because every array support only      operator  length filed  methods inherited from Object supertype   So it is not arrays interface which is making it impossible  Problem is that Object   array beside Strings can store any objects  for instance Integers  so it is possible that one beautiful day we will end up with trying to invoke method like strArray i  substring 1 3  on instance of Integer which doesn t have such method  So to make sure that this situation will never happen  in Java array references can hold only  instances of array of same type as reference  reference String   strArr can hold String    instances of array of subtype  Object   can hold String   because String is subtype of Object    but can t hold  array of supertype of type of array from reference  String   can t hold Object    array of type which is not related to type from reference  Integer   can t hold String     In other words something like this is OK Object   arr   new String      quot ab quot    quot cd quot       OK - because                                               arr  holds array of subtype of Object  String  String   arr2    String    arr    OK -  arr2  reference will hold same array of same type as                                         reference   You could say that one way to resolve this problem is to find at runtime most common type between all list elements and create array of that type  but this wont work in situations where all elements of list will be of one type derived from generic one  Take a look   B extends A List lt A gt  elements   new ArrayList lt A gt     elements add new B     elements add new B      now most common type is B  not A so toArray   A   arr   elements toArray     would return array of B class new B    Problem with this array is that while compiler would allow you to edit its content by adding new A   element to it  you would get ArrayStoreException because B   array can hold only elements of class B or its subclass  to make sure that all elements will support interface of B  but instance of A may not have all methods fields of B  So this solution is not perfect   Best solution to this problem is explicitly tell what type of array toArray   should be returned by passing this type as method argument like String   arr   list toArray new String list size       or String   arr   list toArray new String 0      if size of array is smaller then list it will be automatically adjusted

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Try this  String   arr   list toArray new String list size

[java] Convert ArrayList<String> to String[] array

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