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Instantiate and populate an ArrayList object

One of the capabilities of the Collection Framework is to create a new Collection object and populate it with the contents of an existing Collection object of a different (or the same) actual type.

The code in Listing 3 instantiates an ArrayList object and populatesit with the contents of the existing TreeSet object. As a result, we then have two different Collection objects of different actual types containing the same elements.

Listing 3 . Instantiate and populate an ArrayList object.
Collection aList = new ArrayList(aTree);

Identify a target element

The objective of this program is to compare the times required to search for and to find an element in each of the collections. Thus, we need a target elementto search for.

The code in Listing 4 extracts a value near the center of the ArrayList object using an index to find and extract the value. This is a very fast operation on a List object. This value is saved in testVal to be used later for test purposes.

Note that the reference to the ArrayList object was saved as type Collection (and not as type ArrayList ) in Listing 3 above.

Note also that it was necessary to cast that reference to type List in Listing 4 in order to call the get method on the reference. This is because the Collection interface does not declare a method named get . Rather, the get method is added to the List interface to define a more specialized form of collection.

(Author's note: This program was originally written before the introduction of Generics. The above requirement may not be true if the programwere to be rewritten making proper use of Generics.)
Listing 4 . Identify a target element.
Object testVal = ((List)aList).get(size/2);

Search for the test value in each collection

The code in Listing 5 calls the contains method to search for the test value in each of the collections. It uses the system clock to measurethe time required to find the element in each case. (I will assume that you understand how to use the Date class for this purpose, and won't provide a detailed explanation.)

Listing 5 . Search for the test value in each collection.
long start = new Date().getTime(); boolean found = aList.contains(testVal);long stop = new Date().getTime(); System.out.println(found + " " + (stop - start));start = new Date().getTime(); for(int x = 0; x<100000; x++){ found = aTree.contains(testVal);}//end for loop stop = new Date().getTime();System.out.println(found + " " + (stop - start)/100000.0); }//end doIt()

Program output

Running the program several times produced the following range of output values:

  • First output value ranged from "true 93" to "true 109"
  • Second output value ranged from "true 0.00031" to "true 0.00046"

The first output value applies to the ArrayList object, and the second output value applies to the TreeSet object.

As we would expect, the test value was successfully found in both cases; hence the display of true in both cases.

Time required to search the ArrayList collection

The output indicates that approximately 100 milliseconds were required to find the test value in the ArrayList object.

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