Java dynamic array sizes

Question

I have a class - xClass   that I want to load into an array of xClass so I the declaration   xClass mysclass     new xClass 10   myclass 0    new xClass    myclass 9    new xClass      However  I don t know if I will need 10  I may need 8 or 12 or any other number for that matter  I won t know until runtime   Can I change the number of elements in an array on the fly   If so  how

User · Answer

I recommend using vectors instead  Very easy to use and has many predefined methods for implementation   import java util     Vector lt Integer gt  v new Vector lt Integer gt  5 2     to add an element simply use   v addElement int     In the  5 2  the first 5 is the initial size of the vector  If you exceed the initial size the vector will grow by 2 places  If it exceeds again  then it will again increase by 2 places and so on

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It is a good practice get the amount you need to store first then initialize the array   for example  you would ask the user how many data he need to store and then initialize it  or query the component or argument of how many you need to store  if you want a dynamic array you could use ArrayList   and use al add    function to keep adding  then you can transfer it to a fixed array     Initialize ArrayList and cast string so ArrayList accepts strings  or anything ArrayList lt string gt  al   new ArrayList       add a certain amount of data for int i 0 i lt x i        al add  data   i         get size of data inside int size   al size       initialize String array with the size you have String strArray     new String size      insert data from ArrayList to String array for int i 0 i lt size i        strArray i    al get i       doing so is redundant but just to show you the idea  ArrayList can hold objects unlike other primitive data types and are very easy to manipulate  removing anything from the middle is easy as well  completely dynamic same with List and Stack

User · Answer

No you can t change the size of an array once created  You either have to allocate it bigger than you think you ll need or accept the overhead of having to reallocate it needs to grow in size  When it does you ll have to allocate a new one and copy the data from the old to the new   int   oldItems   new int 10   for  int i   0  i  lt  10  i          oldItems i    i   10    int   newItems   new int 20   System arraycopy oldItems  0  newItems  0  10   oldItems   newItems    If you find yourself in this situation  I d highly recommend using the Java Collections instead  In particular ArrayList essentially wraps an array and takes care of the logic for growing the array as required   List lt XClass gt  myclass   new ArrayList lt XClass gt     myclass add new XClass     myclass add new XClass       Generally an ArrayList is a preferable solution to an array anyway for several reasons  For one thing  arrays are mutable  If you have a class that does this   class Myclass       private int   items       public int   getItems             return items            you ve created a problem as a caller can change your private data member  which leads to all sorts of defensive copying  Compare this to the List version   class Myclass       private List lt Integer gt  items       public List lt Integer gt  getItems             return Collections unmodifiableList items

User · Answer

Java Array sizes are fixed   You cannot make dynamic Arrays as that of in C

User · Answer

Since ArrayList takes to much memory when I need array of primitive types  I prefer using IntStream builder   for creating int array  You can also use LongStream and DoubleStream builders    Example   Builder builder   IntStream builder    int arraySize   new Random   nextInt    for int i   0  i lt arraySize  i           builder add i     int   array   builder build   toArray      Note  available since Java 8

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Arrays copyOf   method has many options to fix the problem with Array length increasing dynamically   Java API

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In java array length is fixed   You can use a List to hold the values and invoke the toArray method if needed See the following sample   import java util List  import java util ArrayList  import java util Random   public class A         public static void main  String    args                dynamically hold the instances         List lt xClass gt  list   new ArrayList lt xClass gt                 fill it with a random number between 0 and 100         int elements   new Random   nextInt 100             for  int i   0   i  lt  elements   i                   list add  new xClass                            convert it to array         xClass    array   list toArray  new xClass  list size                  System out println   size of array       array length            class xClass

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Where you declare the myclass   array as    xClass myclass     new xClass 10      simply pass in as an argument the number of XClass elements you ll need   At that point do you know how many you will need   By declaring the array as having 10 elements  you are not declaring 10 XClass objects  you re simply creating an array with 10 elements of type xClass

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You set the number of elements to anything you want at the time you create it   xClass   mysclass   new xClass n     Then you can initialize the elements in a loop  I am guessing that this is what you need   If you need to add or remove elements to the array after you create it  then you would have to use an ArrayList

User · Answer

Yes  wrap it and use the Collections framework   List l   new ArrayList    l add new xClass        do stuff l add new xClass       Then use List toArray   when necessary  or just iterate over said List

User · Answer

You can use ArrayList   import java util ArrayList  import java util Iterator         ArrayList lt String gt  arr   new ArrayList lt String gt     arr add  neo    arr add  morpheus    arr add  trinity    Iterator lt String gt  foreach   arr iterator    while  foreach hasNext    System out println foreach next

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You can do some thing   private  static Person     addPersons Person   persons  Person personToAdd        int currentLenght   persons length       Person    personsArrayNew   Arrays copyOf persons  currentLenght  1       personsArrayNew currentLenght     personToAdd       return personsArrayNew

User · Answer

As others have said  you cannot change the size of an existing Java array     ArrayList is the closest that standard Java has to a dynamic sized array   However  there are some things about ArrayList  actually the List interface  that are not  array like    For example    You cannot use         to index a list   You have to use the get int  and set int  E  methods  An ArrayList is created with zero elements   You cannot simple create an ArrayList with 20 elements and then call set 15  foo   You cannot directly change the size of an ArrayList   You do it indirectly using the various add  insert and remove methods    If you want something more array-like  you will need to design your own API    Maybe someone could chime in with an existing third party library     I couldn t find one with 2 minutes  research  using Google  -     If you only really need an array that grows as you are initializing it  then the solution is something like this   ArrayList lt T gt  tmp   new ArrayList lt T gt     while             tmp add new T             This creates a new array and copies the element of  tmp  to it  T   array   tmp toArray new T tmp size

User · Answer

As other users say  you probably need an implementation of java util List   If  for some reason  you finally need an array  you can do two things    Use a List and then convert it to an array with myList toArray   Use an array of certain size  If you need more or less size  you can modify it with java util Arrays methods    Best solution will depend on your problem

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In Java Array Sizes are always of Fixed Length But there is way in which you can Dynamically increase the Size of the Array at Runtime Itself  This is the most  used  as well as preferred way to do it-      int temp   new int stck length 1       for int i 0 i lt stck length i   temp i  stck i       stck temp    In the above code we are initializing a new temp   array  and further using a for loop to initialize the contents of the temp with the contents of the original array ie  stck    And then again copying it back to the original one  giving us a new array of new SIZE   No doubt it generates a CPU Overhead due to reinitializing an array using for loop repeatedly  But you can still use and implement it in your code  For the best practice use  Linked List  instead of Array  if you want the data to be stored dynamically in the memory  of variable length    Here s a Real-Time Example based on Dynamic Stacks to INCREASE ARRAY SIZE at Run-Time  File-name  DStack java  public class DStack   private int stck    int tos   void Init Stck int size        stck new int size       tos -1    int Change Stck int size       return stck size      public void push int item       if tos  stck length-1           int temp   new int stck length 1           for int i 0 i lt stck length i   temp i  stck i           stck temp          stck   tos  item            else         stck   tos  item    public int pop        if tos lt 0           System out println  Stack Underflow            return 0            else return stck tos--      public void display        for int x 0 x lt stck length x             System out print stck x                  System out println           File-name  Exec java  with the main class   import java util    public class Exec    private static Scanner in   public static void main String   args        in   new Scanner System in       int option item i 1      DStack obj new DStack        obj Init Stck 1       do          System out println            System out println  --MENU--            System out println  1  Push a Value in The Stack            System out println  2  Pop a Value from the Stack            System out println  3  Display Stack            System out println  4  Exit            option in nextInt            switch option           case 1              System out println  Enter the Value to be Pushed                item in nextInt                obj push item               break          case 2              System out println  Popped Item    obj pop                 obj Change Stck obj tos               break          case 3              System out println  Displaying                   obj display                break          case 4              System out println  Exiting                   i 0              break          default              System out println  Enter a Valid Value                    while i  1           Hope this solves your query

User · Answer

I don t know if you can change the size at runtime but you can allocate the size at runtime  Try using this code   class MyClass       void myFunction              Scanner s   new Scanner  System in           int myArray             int x           System out print   Enter the size of the array              x   s nextInt             myArray   new int x             this assigns your array size to be the one entered at run time into x

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Here s a method that doesn t use ArrayList  The user specifies the size and you can add a do-while loop for recursion   import java util Scanner      public class Dynamic           public static Scanner value          public static void main String  args               value new Scanner System in               System out println  Enter the number of tests to calculate average n                int limit value nextInt                int index 0              int    marks new int limit               float sum ave              sum 0                    while index lt limit                                int test index 1                  System out println  Enter the marks on test    test                   marks index  value nextInt                    sum  marks index                   index                              ave sum limit              System out println  The average is      ave

User · Answer

Yes  we can do this way   import java util Scanner   public class Collection Basic        private static Scanner sc       public static void main String   args             Object   obj new Object 4           sc   new Scanner System in               Storing element         System out println  enter your element            for int i 0 i lt 4 i                 obj i  sc nextInt                                     here  size reaches with its maximum capacity so u can not store more element                         for storing more element we have to create new array Object with required size                      Object   tempObj new Object 10              copying old array to new Array          int oldArraySize obj length          int i 0          for  i lt oldArraySize i                  tempObj i  obj i                                    storing new element to the end of new Array objebt                     tempObj i  90             assigning new array Object refeence to the old one          obj tempObj           for int j 0 j lt obj length j                 System out println  obj   j    -  obj j

[java] Java dynamic array sizes?

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