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The code in Listing 6 shows how to use the simple version of the toArray method to create an array of type Object that contains a copy of each element in the LinkedList collection.
| Listing 6 . Copy collection elements into an array. |
|---|
Object[] array = ref.toArray(); |
Here is some of what Oracle has to say about this version of the toArray method:
"Returns an array containing all of the elements in this collection. If the collection makes any guarantees as to what order its elements arereturned by its iterator, this method must return the elements in the same order.
The returned array will be "safe" in that no references to it are maintained by this collection. ... The caller is thus free to modify thereturned array."
I will have some more to say about the safe aspects of the array shortly.
The code in Listing 7 calls a method named showArray to cause the current contents of the array to be displayed.
| Listing 7 . Display the array contents. |
|---|
System.out.println("New array contents");
showArray(array); |
The entire showArray method is shown in Listing 8 .
| Listing 8 . The showArray method. |
|---|
void showArray(Object[] array){for(int i = 0; i<array.length;i++){
if(array[i]== null){
System.out.print("null ");}else{
System.out.print(((JComponent)array[i]).
getToolTipText() + " ");}//end else
}//end for loopSystem.out.println();
}//end showArray() |
The behavior of the ShowArray method is straightforward. The method uses a for loop to access each of the elements stored in the array in increasing index order.
A test is made to determine if the element contains a null reference. If so, then the word null is displayed for that element. If not, the getToolTipText method is used to access and display the value of the toolTipText property for each element in the array.
The output produced by the code in Listing 8 is shown below:
New array contents
B0 B1 L2 B3 B4 L5 As you can see, (except for the String that identifies the type of output) this is an exact match to the output produced when the contents of thecollection were displayed.
While it is "safe" to modify the contents of the array as explained in the quotation from Oracle earlier, there is still some danger here that you need tobe aware of.
Java collections do not store objects. Rather, Java collections store references to objects. In Java, it is entirely possible to have two or morereferences to the same object.
Each element in the array is a copy of an element in the collection.
Therefore, at this point, for each object being managed by the collection, at least two references exist that refer to that object. One copy is contained inthe collection. The other copy is contained in the array.
If you use a reference stored in the array to modify the state of one of those objects, that modification is made to the object that is also referencedby an element in the collection. This may or may not be what you intend. It's not necessarily a problem as long as you understand what is going on and becareful how you use the references stored in the array.
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