vb.net print barcode zebra THE JAVA COLLECTIONS FRAMEWORK in Java

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THE JAVA COLLECTIONS FRAMEWORK
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The variable E used in lines 1 6 of Example 4.2 is called a type parameter. Like a method parameter, it stands for a type that can be substituted for it. The actual type that is substituted for the type parameter is called a type argument. For example:
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Queue<String> stringQueue = new Queue<String>(); Here, the collection stringQueue is instantiated by substituting the type argument String for the type parameter E.
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EXAMPLE 4.3 Using Type Arguments
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This program uses two special user-defined types at line 3: an enum Month type defined at line 11 and a generic Pair type defined at lines 13 32:
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
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public class TestPairClass { public static void main(String[] args) { Pair<Month, Integer> christmas = new Pair<Month,Integer>(Month.DEC, 25); System.out.println(christmas); Month month = christmas.getFirst(); int day = christmas.getSecond(); System.out.printf("%d %s%n", day, month); } } enum Month { JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC } class Pair<S, T> { private S first; private T second; public Pair(S first, T second) { this.first = first; this.second = second; } public S getFirst() { return first; } public T getSecond() { return second; } public String toString() { return "(" + first + ", " + second + ")"; } }
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The output is:
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(DEC, 25) 25 DEC
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The generic class is defined at line 13. It has two type parameters: S and T. In the code at lines 14, 15, 17, 22, and 26, these two type parameters are used as place holders for actual types. Note that the <S,T> expression is not used in the constructor definition at line 18. But the <> construct is required when the constructor is invoked, as at line 3.
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THE JAVA COLLECTIONS FRAMEWORK
[CHAP. 4
If you compile a program using nongeneric JCF classes (i.e., without specifying a type arguments for the element type) you re likely to get a compiler message like this:
uses unchecked or unsafe operations. Note: Recompile with -Xlint:unchecked for details. You can avoid that simply by specifying an element type; even Object itself, like this: List<Object> list = new ArrayList<Object>();
GENERIC METHODS In addition to generic types, type parameters can also be used to define generic methods, identified by the generic parameter specifier <T> placed in front of the return type. EXAMPLE 4.4 A Generic Method
1 2 3 4 5 6 7 8 9 10 11 12 13
public class TestPrint { public static void main(String[] args) { args = new String[]{"CA", "US", "MX", "HN", "GT"}; print(args); } static <E> void print(E[] a) { for (E ae : a) { System.out.printf("%s ", ae); } System.out.println(); } }
The output is:
CA US MX HN GT
The method is identified as generic by the <E> specifier at line 7. This allows the type parameter E to be used in place of an actual type in the method block at line 8.
GENERIC WILDCARDS The symbol can be used as a wildcard, in place of a generic variable. It stands for unknown type, and is called the wildcard type. EXAMPLE 4.5 A Universal print() Method
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static void print(Collection< > c) { for (Object o : c) { System.out.printf("%s ", o); } System.out.println(); }
This method can be used to print any type of collection, for example, a HashSet<String> or a
Queue<Date>.
Note that if we used Collection<Object> instead of Collection< > at line 1, then the method would only apply to collections whose element type is specified as Object. For example, it could not be used to print a HashSet<String> because that is not an extension of Collection<Object>.
CHAP. 4]
THE JAVA COLLECTIONS FRAMEWORK
The wildcard type can be used more selectively to limit the range of types that can be used in its place. For example, if you wanted to restrict the print() method in Example 4.5 to extensions of a Person class, then you would replace line 1 with:
static void print(Collection< extends Person> c) {
The expression extends Person is called a bounded wildcard type. ITERATORS An iterator is an object that provides access to the elements of a Collection object. It acts like a finger, pointing to one element at a time, much like the cursor of a text editor points to one character at a time. And like a cursor, an iterator has the capability of moving from one element to the next and of deleting its current element. The analogy between iterators and cursors is not complete. Most iterators cannot jump around the way cursors can. On the other hand, it is possible to have several independent iterators traversing the same collection. The algorithm that determines each next element in the iterator s path is an intrinsic property of the iterator itself. Moreover, it is possible to have several different iterator classes defined for the same collection class, each with its own traversal algorithms. The requirements for an iterator are specified in the java.util.Iterator interface, shown in Figure 4.4.
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