Understanding List.java's `<T> T[] toArray(T[] a)` method

Have a Java list as List<int[]> lst = new ArrayList<>();

I have the following code snippet's that I am trying to decipher. They all relate to understanding of generics and the the method toArray()

1. lst.toArray(new int[0][]); - The size 0 does not matter here. Trying to understand why? I tried looking at the implementation of the toArray method inside ArrayList as well.

2. lst.toArray(new int[lst.size()][]) - Similar to (1), I can init use lst.size(). Does it mean there is no conceptual difference between (1) and (2)

3. Given (1) and (2) both work correctly, I tried this lst.toArray(new int[][]) - This does not compile and gives Array initializer expected. Is it related to the grammar of Java where if you are instantiating an array, size needs to be mandatory and hence the error?

Can someone help me understand this?

> lst.toArray(new int[0][]); - The size 0 does not matter here. Trying to understand why? I tried looking at the implementation of the toArray method inside ArrayList as well.

From the documentation:
> ...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.

Any size less than lst.size() will result in a new array being allocated.

> lst.toArray(new int[lst.size()][]) - Similar to (1), I can init use lst.size(). Does it mean there is no conceptual difference between (1) and (2)

Pretty much. The only difference is that the array object you passed in here will be used, not a newly allocated array.

> Given (1) and (2) both work correctly, I tried this lst.toArray(new int[][]) - This does not compile and gives Array initializer expected. Is it related to the grammar of Java where if you are instantiating an array, size needs to be mandatory and hence the error?

This is correct.

According to toArray(T[] a) javadoc:

> If the list 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 list.

So size of passed array affects would a new array be allocated or not. Thus in practice you are either pass zero sized array (new int[0][] in your case), either array of size >= to lst.size(). Passing an array of positive size, but less then list size makes no sense as it would be thrown to create a new array that fits the list. Usually programmers prefer to pass zero sized array as it easy and short.

Relating to 3) - you are absolutely right, Java requires array size to be defined when you create an array.

Technically, you should be using a List&lt;Integer&gt;, and not an int[].
Although, in Java, an array of int is recognized as an object.&nbsp; Thus, providing it as the type will work.

The Collections Framework was made for use with objects, and not primitive data-types.

> "... The size 0 does not matter here. Trying to understand why? ..."

Essentially, the compiler needs to know what the object is&mdash;so new int[0] will suffice.

> "... Does it mean there is no conceptual difference between (1) and (2) ..."

More or less, yes.

> "... I tried this lst.toArray(new int[][]) ...
> ... Is it related to the grammar of Java where if you are instantiating an array, size needs to be mandatory and hence the error?"

Exactly, the syntax must comply.

I recommend the following book, which covers this topic well&mdash;the interoperability of the Collections Framework, objects, and arrays.
Java: The Complete Reference, Twelfth Edition, 12th Edition.