vb.net print barcode zebra Figure 4.4 Methods specified by the java.util.Iterator interface in Java

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Figure 4.4 Methods specified by the java.util.Iterator interface
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The next() method returns the current element of the collection to which the iterator is bound. Each time the next() method is invoked, it advances the iterator to the next element in the collection. The hasNext() method returns true unless the iterator has reached the end of its traversal. This predicate is used to determine whether the next() method can be called again. The remove() method deletes the last element returned by the next() method. This means that next() must be called before each call to remove(), designating the element to be deleted. Normally, iterators are obtained by means of a call to the collection s iterator() method that is required by the Collection interface. (See Figure 4.2 on page 71.) It returns a new iterator, bound to the collection and initialized to its first element.
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THE JAVA COLLECTIONS FRAMEWORK
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EXAMPLE 4.6 Using an Iterator
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public class TestIterators { public static void main(String[] args) { Set<String> port = new HashSet<String>(); Collections.addAll(port, "AO", "BR", "CV", "GW", "MO", "MZ", "PT"); System.out.println(port); Iterator it1 = port.iterator(); System.out.printf("it1.next(): %s%n", it1.next()); System.out.printf("it1.next(): %s%n", it1.next()); System.out.printf("it1.next(): %s%n", it1.next()); System.out.printf("it1.next(): %s%n", it1.next()); it1.remove(); System.out.println(port); System.out.printf("it1.next(): %s%n", it1.next()); it1.remove(); System.out.println(port); Iterator it2 = port.iterator(); while(it2.hasNext()) { System.out.printf("%s ", it2.next()); } System.out.println(""); System.out.printf("it1.next(): %s%n", it1.next()); } }
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The output is:
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[MZ, BR, PT, MO, GW, CV, AO] it1.next(): MZ it1.next(): BR it1.next(): PT it1.next(): MO [MZ, BR, PT, GW, CV, AO] it1.next(): GW [MZ, BR, PT, CV, AO] MZ BR PT CV AO it1.next(): CV The Iterator object it1 is instantiated at line 6 and returned by the iterator() method. Bound to the port collection, it visits the elements in the same order as they were printed at line 5. Note that this
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order is not the same as the order in which the elements were added to the collection. Since the collection is a set, its elements have no intrinsic order. It is the iterator itself that computes the traversal sequence. You can see from the output from line 5 that the toString() method (invoked implicitly by the println() method) uses the same traversal sequence, obviously using its own iterator to build the string. The it1 iterator s remove() method is invoked at lines 11 and 14. This causes it to delete the current element, which in each case is the last element returned by a call to next(). At line 11, the last next() call was at line 10, which returned the MO element; so MO is deleted by remove() at line 11. Similarly, at line 14, the last next() call was at line 13, which returned the GW element; so GW is deleted by remove() at line 14. These deletions are evident from the output produced by the while loop at lines 17 19. Notice that that loop uses the independent iterator it2 to traverse the collection. Note that the code at lines 16 19 make a complete traversal of the collection. This is guaranteed by the instantiation of the new iterator at line 16 and the use of the hasNext() method to control the loop. The last output, generated at line 21, confirms that the two iterators it1 and it2 are independent. The action of it2 had no effect on it1, which returns the CV element at line 21.
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THE JAVA COLLECTIONS FRAMEWORK
The iterator it2 in Example 4.6 used a while loop to traverse the collection. Iterators can be used in a for loop the same way:
for (Iterator it = countries.iterator(); it.hasNext(); ) { System.out.printf("%s ", it.next()); } This is analogous to using a for loop to traverse an array: for (int i = 0; i < countries.length; i++) { System.out.printf("%s ", countries[i]); } Java 5.0 introduced the for-each construct for simplifying for loops: for (String country : countries) { System.out.printf("%s ", country); } Here, the String variable country takes the place of the indexed expression countries[i].
This same code also replaces the iterator loop! Thus, the program in Example 4.6 works the same way if we replace lines 16 19 with this simpler for-each loop:
for (String country : port) { System.out.printf("%s ", country); } This code implicitly invokes the port collection s iterator() method, generating an implicit iterator, which then implicitly invokes the hasNext() and next() methods within the loop to traverse the collection. The declared variable country takes the place of the expression it.next()
inside the loop. Figure 4.5 illustrates the iterator it2 that traverses the port collection in Example 4.6. The essential feature is that the iterator locates one element at a time in the collection. It is up to the collection s iterator() method to determine the algorithm by which its iterator traverses the collection. THE TreeSet CLASS
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