Creating an array of objects in Java

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.

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"), . . } )

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 Arraysint[] intArray;             // Gooddouble[] doubleArray;// One Dimensional Arraysbyte byteArray[];           // Ugly!long longArray[];// Two Dimensional Arraysint[][] int2DArray;         // Gooddouble[][] double2DArray;// Two Dimensional Arraysbyte[] byte2DArray[];       // Uglylong[] 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