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The code in Listing 2 is straightforward and shouldn't require further explanation.
Get and use an iterator
An iterator is an object instantiated from a specially designed class that implements the Iterator interface. The design of the class makes it possible for client code to gain sequential access to each element encapsulatedin an associated collection object without a requirement to know anything about how the collection is structured.
Required syntax
The first statement in Listing 3 shows the syntax required to get and save a reference to a generic iterator for the ArrayList object instantiated earlier in Listing 1 .
| Listing 3 - An iterator. |
|---|
//Get an iterator
Iterator<Date>iter = var1.iterator();//Perform the iteration
while(iter.hasNext()){System.out.println(iter.next().getTime());
}//end while loopSystem.out.println();//blank line |
Note the requirement to qualify the declaration of the local variable named iter with the type of data stored in the collection using angle-bracket notation similar to that used earlier.. You might think of this as a variable capable of holding areference to an iterator object, which is capable of iterating on an ArrayList object, which in turn is capable of storing references to objects of type Date only.
Perform the iteration
The remaining code in Listing 3 uses the iterator to sequentially access and display information encapsulated in each of the three Date objects whose references are stored in the ArrayList object. This is standard code for the use of an iterator and should not require further explanation.This code produces the first three lines of text (plus the blank line) shown in Figure 5 .
| Figure 5 - Iterator output. |
|---|
1378070280877
13781566808771378243080877
13780702808771378156680877
1378243080877 |
The program output
The output produced by this program depends on when you run it. The output produced for one particular run is shown in Figure 5 .
The output will be different each time you run the program depending on the current date and time.
The enhanced for loop
The code in Listing 4 performs the same iteration using the new enhanced for loop that was released in Java version 1.5.
| Listing 4 - Enhanced for loop. |
|---|
//Now perform the same iteration using
// the new for-each construct.for(Date element : var1){
System.out.println(element.getTime());}//end for-each
}//end runIt |
You might think of this syntax as meaning:
For each element of type Date contained in the collection referred to by var1 , get the value of the element and save it in the variable named element . Then use the contents of that variable to perform the operations specified within the body of the loop.
More compact syntax
As you can see, this approach does not require you to get an iterator and to explicitly use that iterator to sequentially access the elements in thecollection. Thus, the syntax is more compact than the syntax shown in Listing 3 . Further, by eliminating the requirement to get the iterator, this construct also eliminates the requirement for you to qualify the code usingthe angle-bracket syntax. All of those details are handled automatically behind the scenes.
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