Creating an array of objects in Java

Question

I am new to Java and for the time created an array of objects in Java    I have a class A for example -   A   arr   new A 4     But this is only creating pointers  references  to A and not 4 objects  Is this correct  I see that when I try to access functions variables in the objects created I get a null pointer exception  To be able to manipulate access the objects I had to do this   A   arr   new A 4   for  int i   0  i  lt  4  i          arr i    new A        Is this correct or am I doing something wrong  If this is correct its really odd   EDIT  I find this odd because in C   you just say new A 4  and it creates the four objects

User · Answer

This is correct  You can also do    A   a   new A     new A  args    new A  other args            This syntax can also be used to create and initialize an array anywhere  such as in a method argument   someMethod  new A     new A  args    new A  other args

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Yes  it creates only references  which are set to their default value null  That is why you get a NullPointerException You need to create objects separately and assign the reference  There are 3 steps to create arrays in Java -     Declaration     In this step  we specify the data type and the dimensions of the array that we are going to create  But remember  we don t mention the sizes of dimensions yet  They are left empty       Instantiation     In this step  we create the array  or allocate memory for the array  using the new keyword  It is in this step that we mention the sizes of the array dimensions       Initialization     The array is always initialized to the data type   s default value  But we can make our own initializations       Declaring Arrays In Java      This is how we declare a one-dimensional array in Java      int   array  int array          Oracle recommends that you use the former syntax for declaring arrays    Here are some other examples of legal declarations         One Dimensional Arrays int   intArray                 Good double   doubleArray      One Dimensional Arrays byte byteArray                 Ugly  long longArray        Two Dimensional Arrays int     int2DArray             Good double     double2DArray      Two Dimensional Arrays byte   byte2DArray             Ugly long   long2DArray          And these are some examples of illegal declarations      int 5  intArray           Don t mention size  double   doubleArray      Square Brackets please        Instantiation      This is how we    instantiate     or allocate memory for an array      int   array   new int 5         When the JVM encounters the new keyword  it understands that it must allocate memory for something  And by specifying int 5   we mean that we want an array of ints  of size 5    So  the JVM creates the memory and assigns the reference of the newly allocated memory to array which a    reference    of type int        Initialization      Using a Loop     Using a for loop to initialize elements of an array is the most common way to get the array going  There   s no need to run a for loop if you are going to assign the default value itself  because JVM does it for you       All in One        We can Declare  Instantiate and Initialize our array in one go  Here   s the syntax      int   arr    1  2  3  4  5         Here  we don   t mention the size  because JVM can see that we are giving 5 values    So  until we instantiate the references remain null  I hope my answer has helped you        Source - Arrays in Java

User · Answer

You are correct  Aside from that if we want to create array of specific size filled with elements provided by some  factory   since Java 8  which introduces stream API  we can use this one-liner   A   a   Stream generate    - gt  new A    limit 4  toArray A    new      Stream generate    - gt  new A    is like factory for separate A elements created in a way described by lambda     - gt  new A   which is implementation of Supplier lt A gt  - it describe how each new A instances should be created   limit 4  sets amount of elements which stream will generate toArray A    new   can also be rewritten as toArray size - gt  new A size    -  it lets us decide describe type of array which should be returned    For some primitive types you can use DoubleStream  IntStream  LongStream which additionally provide generators like range rangeClosed and few others

User · Answer

The genaral form to declare a new array in java is as follows   type arrayName     new type numberOfElements    Where type is a primitive type or Object  numberOfElements is the number of elements you will store into the array and this value can   t change because Java does not support dynamic arrays  if you need a flexible and dynamic structure for holding objects you may want to use some of the Java collections    Lets initialize an array to store the salaries of all employees in a small company of 5 people   int salaries     new int 5    The type of the array  in this case int  applies to all values in the array  You can not mix types in one array   Now that we have our salaries array initialized we want to put some values into it  We can do this either during the initialization like this   int salaries      50000  75340  110500  98270  39400    Or to do it at a later point like this   salaries 0    50000  salaries 1    75340  salaries 2    110500  salaries 3    98270  salaries 4    39400    More visual example of array creation    To learn more about Arrays  check out the guide

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For generic class it is necessary to create a wrapper  class  For Example   Set lt String gt    sets   new HashSet lt  gt  10    results in   Cannot create a generic array   Use instead           class SetOfS public Set lt String gt  set   new HashSet lt  gt              SetOfS   sets   new SetOfS 10

User · Answer

Yes it is correct in Java there are several steps to make an array of objects    Declaring and then Instantiating   Create memory to store  4  objects    A    arr   new A 4    Initializing the Objects  In this case you can Initialize 4 objects of class A   arr 0    new A    arr 1    new A    arr 2    new A    arr 3    new A      or  for  int i 0  i lt 4  i       arr i    new A       Now you can start calling existing methods from the objects you just made etc   For example     int x   arr 1  getNumber      or    arr 1  setNumber x

User · Answer

This is correct   A   a   new A 4        creates 4 A references  similar to doing this   A a1  A a2  A a3  A a4    Now you couldn t do a1 someMethod   without allocating a1 like this   a1   new A      Similarly  with the array you need to do this   a 0    new A         before using it

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Here is the clear example of creating array of 10 employee objects  with a constructor that takes parameter   public class MainClass         public static void main String args                  System out println  Hello  World               step1   first create array of 10 elements that holds object addresses          Emp   employees   new Emp 10             step2   now create objects in a loop          for int i 0  i lt employees length  i                 employees i    new Emp i 1    this will call constructor                     class Emp      int eno      public Emp int no           eno   no          System out println  emp constructor called  eno is    eno

[java] Creating an array of objects in Java

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