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The size issue

Here is some of what the Oracle documentation for the LinkedList class has to say about the size issue for this version of the toArray method.

"If the list fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specifiedarray and the size of this list. If the list fits in the specified array with room to spare (i.e., the array has more elements than the list), theelement in the array immediately following the end of the collection is set to null. This is useful in determining the length of the list only if thecaller knows that the list does not contain any null elements."

So, what did I do?

Knowing all of this in advance, I purposely caused the size of the JComponent array to be larger (by two elements) than the number of elements in the collection. Therefore, the array that I passed to the toArray method was populated and a reference to that populated array was returned.

(Had my array been smaller than the number of elements in the collection, the toArray method would have created and populated a new array of typeJComponent and would have returned a reference to that new array object. In that case, my array would have been used by the toArray method only for the purposeof determining the runtime type of my array.)

More information from Oracle

Here is some additional information about the toArray method provided by the Oracle documentation for the Collection interface:

"If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in thesame order."

Because the iterator for a LinkedList object returns the elements in increasing index order, the toArray method, in this case, copies the element at each index position in the collection into the element at the sameindex position in the array. Thus, reference values are copied from each element in the collection into the first six elements in the array.

The output

The output produced by the code in Listing 5 is shown below:

New array contents B0 B1 L2 B3 B4 L5 null 17

You will note that the first six elements in the array match the six elements in the collection (the initial values placed in the array earlier when the array was instantiated have been overwritten).

You will also note that the value of the seventh element in the array (index value 6) has been overwritten with a null reference.

Demonstrates same array was used

Note finally that the last element in the array was not overwritten. It still contains the value placed there when the array object was instantiated. Thisdemonstrates that the array that I passed to the toArray method was populated with the collection data, and a reference to that array was returnedby the toArray method.

What if the array was too small?

Had my array been too small, it would have been discarded by the toArray method. The toArray method would have created and populated a new array object of the correct size and runtime type, and would have returned a referenceto that new array.

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Source:  OpenStax, Object-oriented programming (oop) with java. OpenStax CNX. Jun 29, 2016 Download for free at https://legacy.cnx.org/content/col11441/1.201
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