CODEWITHSUNDEEP
java
Akash
Akash

Reputation: 631

Create an array of references

If I have a class say A, and I declare an array of 10 elements of this class as,

A [] arr=new A[10];

Then 10 new objects of A are created and stored in the array.

However, i'd like to be able to do something on the lines of A arr[10]; , where the array just holds references to null objects.

The reason I need this is because I only need the array to hold instances I fill in later in the code. So the objects the above statement creates are lost anyway and as I understand object creation is expensive.

So, is there any way to just have an array of references that I can point to the objects I desire later? Or is this not possible and I should resort to using an ArrayList?

Upvotes: 5

Views: 17835

Answers (3)

SaleemKhair
SaleemKhair

Reputation: 531

This is a common OOP problem and can be solved using the Null Object design pattern.

In order to not have to initialize n objects, you create a single static object of the same type for example:

public class A {
  public static final A NULL = new A();
.
.
.
}

Now this object can act as your null reference by calling:


A[] arr = new A[10];
Arrays.fill(arr, A.NULL);

Which also conveniently leverages the use of .equals and . hashCode if you want to check for nulls.

Upvotes: 0

Peter
Peter

Reputation: 5798

if you do A [] arr=new A[10]; then no objects are created except your array, each field will be null until you initialize it.

A [] arr=new A[10]; only create place to store reference of A Class object. Although array arr is created but its not referencing any object and you can't do like arr[i].someMethod().

To correct it, allocate object at individual memory in array do like this: 

A [] arr=new A[10];
arr[0] = new A();
arr[1] = new A();
:
:

or in a loop like:

  for(i=0; i<10; i++){
      arr[i] = new A();
  }

After there arr that is an array of reference, refers to a valid A class object. And after this expression arr[i].someMethod() will not cause an error.

Upvotes: 4

NPE
NPE

Reputation: 500367

If I have a class say A, and I declare an array of 10 elements of this class as,

A [] arr=new A[10];

Then 10 new objects of A are created and stored in the array.

That's not correct. Here, an array of ten references gets created, and each reference gets set to null. There are no instances of A created by this code.

In other words, the code already does exactly what you'd like it to do.

Upvotes: 13

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