Finally, we need to add a method, kidsInClub, to KidData to return this new filtered Enumeration in Java

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Finally, we need to add a method, kidsInClub, to KidData to return this new filtered Enumeration
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public Enumeration kidsInClub(String club) { return new kidClub(this, club); }
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This method passes the instance of KidClub to the Enumeration class KidClub along with the club's initials Figure 191 shows a simple program The program displays all of the kids on the left side, fills a combo box with a list of the clubs, and then allows the user to select a club Finally, it fills the right-hand list box with the names of those kids who belong to a single club Figure 191 A simple program illustrating filtered Enumeration
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The class diagram is shown in Figure 192 Figure 192 The classes used in the filtered Enumeration
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Note that the elements method in KidData supplies an Enumeration and the kidClub class is, in fact, itself an Enumeration class
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Consequences of the Iterator Pattern
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Use of the Iterator pattern has the following consequences: 1 Data modification The most significant question resulting from the use ofiterators concerns iterating through data while it is being changed If your code is wide ranging and only occasionally moves to the next element, an element might be added or deleted from the underlying collection whileyou are moving through it It is also possible that another thread could change the collection There are no simple solutions to this problem You can make an Enumeration thread safe by declaring the loop to be synchronized, but if you want to move through a loop using an Enumeration and delete certain items, you must be careful of the consequences Deleting or adding an element might mean that a particular element isskipped or accessed twice, depending on the storage mechanism that you are using 2 Privileged access Enumeration classes might need to have some sort of privileged access to the underlying data structures of the original container class so that they can move through the data If the data are stored in a Vector or Hashtable, accomplishing this is fairly easy But if it is some other collection structure contained in a class, you will probably have to make that structure available through a get operation Alternatively, you could make the Iterator a derived class of the containment class and access the data directly The friend class solution available in C++ does not apply in Java However, classes defined in the same module as the containing class do have access to the containing class's variables
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3 External versus internal Iterators Design Patterns describes two types of Iterators: external and internal Thus far, we have described only external iterators Internal Iterators are methods that move through the entire collection, performing some operation on each element directly, without any specific requests from the user Internal Iterators are less common inJava, but methods that normalize a collection of data values to lie between 0 and 1 or that convert all of the strings to a particular case are possible In general, external Iterators give you more control because the calling program accesses each element directly and can decide whether to perform an operation on it
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Composites and Iterators
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Iterators, or in the current case Enumerations, are also an excellent way to movethrough Composite structures In the Composite of an employee hierarchy developed in 18, each Employee contains a Vector whose elements method allows you to continue to enumerate down that chain If that Employee has no subordinates, the hasMoreElements method correctly returns false
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Iterators in Java 12
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Java 12 introduces the following Iterator interface that is quite similar to the Enumeration interface:
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public interface Iterator { public boolean hasNext(); public Object next(); public void remove(); }
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The Iterator and the Enumeration differ only in the Iterator's simpler names and remove method The remove method removes the last object returned from the next method and can be called only once per next method In Java 12, the Vector class now has an iterator method that returns an instance of the Iterator class The Hashtable class, however, has not been updated rather, it has been supplanted by a series of Collection and Map classes The HashMap class is a hash table implementation based on a Map interface that has an elements method This elements method returns an instance of the Collection class, which in turn returns an Iterator Thus, to iterate through a HashMap, you obtain an iterator as follows:
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private HashMap clubs; // fill hash map Iterator enum = clubsvalues()iterator();
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