visual basic 2010 barcode generator The MapEntry Interface in Java

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The MapEntry Interface
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The MapEntry interface enables you to work with a map entry Recall that the entrySet( ) method declared by the Map interface returns a Set containing the map entries Each of these set elements is a MapEntry object Table 15-8 summarizes the methods declared by this interface
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Table 15-8 The Methods Defined by MapEntry
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boolean equals(Object obj) Object getKey( ) Object getValue( ) int hashCode( )
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Returns true if obj is a MapEntry whose key and value are equal to that of the invoking object Returns the key for this map entry Returns the value for this map entry Returns the hash code for this map entry
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Object setValue(Object v) Sets the value for this map entry to v A ClassCastException is thrown if v is not the correct type for the map An IllegalArgumentException is thrown if there is a problem with v A NullPointerException is thrown if v is null and the map does not permit null keys An UnsupportedOperationException is thrown if the map cannot be changed
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The Map Classes
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Several classes provide implementations of the map interfaces The classes that can be used for maps are summarized here: Class AbstractMap HashMap TreeMap WeakHashMap Description Implements most of the Map interface Extends AbstractMap to use a hash table Extends AbstractMap to use a tree Extends AbstractMap to use a hash table with weak keys
Notice that AbstractMap is a superclass for the three concrete map implementations WeakHashMap implements a map that uses "weak keys," which allows an element in a map to be garbage-collected when its key is unused This class is not discussed further here The others are described next
The HashMap Class
The HashMap class uses a hash table to implement the Map interface This allows the execution time of basic operations, such as get( ) and put( ), to remain constant even for large sets The following constructors are defined:
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HashMap( ) HashMap(Map m) HashMap(int capacity) HashMap(int capacity, float fillRatio) The first form constructs a default hash map The second form initializes the hash map by using the elements of m The third form initializes the capacity of the hash map to capacity The fourth form initializes both the capacity and fill ratio of the hash map by using its arguments The meaning of capacity and fill ratio is the same as for HashSet, described earlier HashMap implements Map and extends AbstractMap It does not add any methods of its own You should note that a hash map does not guarantee the order of its elements Therefore, the order in which elements are added to a hash map is not necessarily the order in which they are read by an iterator The following program illustrates HashMap It maps names to account balances Notice how a set-view is obtained and used import javautil*; class HashMapDemo { public static void main(String args[]) { // Create a hash map HashMap hm = new HashMap(); // Put elements to the map hmput("John Doe", new Double(343434)); hmput("Tom Smith", new Double(12322)); hmput("Jane Baker", new Double(137800)); hmput("Todd Hall", new Double(9922)); hmput("Ralph Smith", new Double(-1908)); // Get a set of the entries Set set = hmentrySet(); // Get an iterator Iterator i = setiterator(); // Display elements while(ihasNext()) { MapEntry me = (MapEntry)inext(); Systemoutprint(megetKey() + ": "); Systemoutprintln(megetValue()); } Systemoutprintln(); // Deposit 1000 into John Doe's account double balance = ((Double)hmget("John Doe"))doubleValue(); hmput("John Doe", new Double(balance + 1000)); Systemoutprintln("John Doe's new balance: " + hmget("John Doe"));
Output from this program is shown here:
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Ralph Smith: -1908 Tom Smith: 12322 John Doe: 343434 Todd Hall: 9922 Jane Baker: 13780 John Doe's current balance: 443434 The program begins by creating a hash map and then adds the mapping of names to balances Next, the contents of the map are displayed by using a set-view, obtained by calling entrySet( ) The keys and values are displayed by calling the getKey( ) and getValue( ) methods that are defined by MapEntry Pay close attention to how the deposit is made into John Doe's account The put( ) method automatically replaces any preexisting value that is associated with the specified key with the new value Thus, after John Doe's account is updated, the hash map will still contain just one "John Doe" account
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