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THE JAVA COLLECTIONS FRAMEWORK
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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:
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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].
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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:
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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()
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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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Figure 4.5 The iterator it2 on the set port
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In addition to the Set interface, the TreeSet class also implements 6 methods specified by the SortedSet interface (Figure 4.6) and 13 methods specified by its NavigableSet extension (Figure 4.7). These additional 19 methods require implementing classes to maintain an ordering mechanism among their elements, allowing them to be compared for size. In Java, there are two ways for objects to be compared: either by means of their natural ordering or by the application of an external Comparator object. Classes whose objects enjoy a natural order implement the comparable interface. These include the wrapper classes (Integer, Double, etc.), the BigInteger and BigDecimal classes in the java.math package, and the String class. User-defined orderings can be defined for a class by implementing the Comparator interface, which is part of the JCF. It specifies a compare() method, which returns an int that indicates
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THE JAVA COLLECTIONS FRAMEWORK
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Figure 4.6 Specialized methods specified by the java.util.SortedSet interface
how its two arguments are ordered: compare(x,y) > 0 means that x is greater than y, compare(x,y) < 0 means that x is less than y, and compare(x,y) == 0 means that x equals y. A Comparator object can be passed to a SortedSet constructor to specify how the elements should be ordered. EXAMPLE 4.7 Constructing a TreeSet Collection with a Comparator Object
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public class TestTreeSetWithComparator { public static void main(String[] args) { SortedSet<String> ital = new TreeSet<String>(new RevStringComparator()); Collections.addAll(ital, "IT", "VA", "SM", "CH"); System.out.println(ital); } } class RevStringComparator implements Comparator<String> { public int compare(String s1, String s2) { StringBuilder sb1 = new StringBuilder(s1); StringBuilder sb2 = new StringBuilder(s2); String s1rev = sb1.reverse().toString(); String s2rev = sb2.reverse().toString(); return s1rev.compareTo(s2rev); } }
The output is:
[VA, CH, SM, IT]
The separate RevStringComparator class implements the Comparator interface for String objects. It defines its compare() method by applying the String class s CompareTo() method to the reversed
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THE JAVA COLLECTIONS FRAMEWORK
Figure 4.7 Specialized methods specified by the java.util.NavigableSet interface
THE JAVA COLLECTIONS FRAMEWORK
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strings. As a result, the SortedSet collection orders its elements by applying alphabetical ordering to their reversed strings. Since AV < HC < MS < TI (alphabetically), the order of the four inserted elements is as shown in the output from line 5.
If the TreeSet collection is instantiated without an explicit Comparator object, then it uses its elements natural ordering. EXAMPLE 4.8 Testing the TreeSet Class
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public class TestTreeSet { public static void main(String[] args) { NavigableSet<String> engl = new TreeSet<String>(); Collections.addAll(engl, "IN", "US", "PK", "NG", "PH", "GB", "ZA"); System.out.println(engl); engl.add("KE"); System.out.println(engl); SortedSet<String> head = engl.headSet("KE"); SortedSet<String> mid = engl.subSet("KE", "US"); SortedSet<String> tail = engl.tailSet("US"); System.out.printf("%s %s %s%n", head, mid, tail); System.out.printf("engl.first(): %s%n", engl.first()); System.out.printf("engl.last(): %s%n", engl.last()); } }
The output is:
[GB, IN, NG, PH, PK, US, ZA] [GB, IN, KE, NG, PH, PK, US, ZA] [GB, IN] [KE, NG, PH, PK] [US, ZA] engl.first(): GB engl.last(): ZA engl.lower("KE"): IN engl.higher("KE"): NG The set engl is instantiated as a TreeSet of String elements at line 3 and loaded with seven elements
at line 4. An eighth element is added at line 6. The outputs from lines 5 and 7 confirm that the TreeSet is maintaining its elements in their natural (alphabetical) order. Lines 8 13 illustrate some of the specialized methods implemented by the TreeSet class. (See Figure 4.6 on page 80.) The headSet() method returns the sorted subset of all elements that precede its argument. The subSet() method returns the sorted subset of elements that begin with its first argument and precede its second argument. The tailSet() method returns the sorted subset of all elements that do not precede its argument. Note that these three methods adhere to the Java s left-continuous policy that whenever a linear segment is to be specified by two bounding points a and b, the segment will include the lower element a and exclude the upper element b. For example, the subset [KE, NG, PH, PK] returned by the call engl.subSet("KE", "US") at line 9 includes the lower element KE and excludes the upper element US.
The intrinsic difference between the HashSet and TreeSet classes is their backing data structure. The HashSet class uses a hash table (outlined in 8), which uses each element s hashCode() to compute its location in the set. The TreeSet class uses a balanced binary search tree (outlined in 12) to store its elements. The advantages and disadvantages of these two data structures are summarized in Table 4.2. These relative advantages and disadvantages make the choice easy: If you want to preserve the natural order of the elements in your set, use the TreeSet class; otherwise, use the HashSet
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