visual basic 6 barcode generator javautil Part 1: The Collections Framework in Java

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javautil Part 1: The Collections Framework
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The following is the output produced by the program; notice how the exception handler for EmptyStackException is caught so that you can gracefully handle a stack underflow: stack: [ ] push(42) stack: [42] push(66) stack: [42, 66] push(99) stack: [42, 66, 99] pop -> 99 stack: [42, 66] pop -> 66 stack: [42] pop -> 42 stack: [ ] pop -> empty stack One other point: Although Stack is not deprecated, with the release of Java SE 6, ArrayDeque is a better choice
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Dictionary
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Dictionary is an abstract class that represents a key/value storage repository and operates much like Map Given a key and value, you can store the value in a Dictionary object Once the value is stored, you can retrieve it by using its key Thus, like a map, a dictionary can be thought of as a list of key/value pairs Although not currently deprecated, Dictionary is classified as obsolete, because it is fully superseded by Map However, Dictionary is still in use and thus is fully discussed here With the advent of JDK 5, Dictionary was made generic It is declared as shown here: class Dictionary<K, V> Here, K specifies the type of keys, and V specifies the type of values The abstract methods defined by Dictionary are listed in Table 17-17
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Method V get(Object key) boolean isEmpty( ) Enumeration<K> keys( ) V put(K key, V value) Purpose Returns the object that contains the value associated with key If key is not in the dictionary, a null object is returned Returns true if the dictionary is empty, and returns false if it contains at least one key Returns an enumeration of the keys contained in the dictionary Inserts a key and its value into the dictionary Returns null if key is not already in the dictionary; returns the previous value associated with key if key is already in the dictionary Removes key and its value Returns the value associated with key If key is not in the dictionary, a null is returned Returns the number of entries in the dictionary
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The Abstract Methods Defined by Dictionary
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Enumeration<V> elements( ) Returns an enumeration of the values contained in the dictionary
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V remove(Object key) int size( ) TABLE 17-17
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To add a key and a value, use the put( ) method Use get( ) to retrieve the value of a given key The keys and values can each be returned as an Enumeration by the keys( ) and elements( ) methods, respectively The size( ) method returns the number of key/value pairs stored in a dictionary, and isEmpty( ) returns true when the dictionary is empty You can use the remove( ) method to delete a key/value pair
REMEMBER The Dictionary class is obsolete You should implement the Map interface to obtain
key/value storage functionality
Hashtable
Hashtable was part of the original javautil and is a concrete implementation of a Dictionary However, with the advent of collections, Hashtable was reengineered to also implement the Map interface Thus, Hashtable is now integrated into the Collections Framework It is similar to HashMap, but is synchronized Like HashMap, Hashtable stores key/value pairs in a hash table However, neither keys nor values can be null When using a Hashtable, you specify an object that is used as a key, and the value that you want linked to that key The key is then hashed, and the resulting hash code is used as the index at which the value is stored within the table Hashtable was made generic by JDK 5 It is declared like this: class Hashtable<K, V> Here, K specifies the type of keys, and V specifies the type of values A hash table can only store objects that override the hashCode( ) and equals( ) methods that are defined by Object The hashCode( ) method must compute and return the hash code for the object Of course, equals( ) compares two objects Fortunately, many of Java s built-in classes already implement the hashCode( ) method For example, the most common type of Hashtable uses a String object as the key String implements both hashCode( ) and equals( ) The Hashtable constructors are shown here: Hashtable( ) Hashtable(int size) Hashtable(int size, float fillRatio) Hashtable(Map< extends K, extends V> m) The first version is the default constructor The second version creates a hash table that has an initial size specified by size (The default size is 11) The third version creates a hash table that has an initial size specified by size and a fill ratio specified by fillRatio This ratio must be between 00 and 10, and it determines how full the hash table can be before it is resized upward Specifically, when the number of elements is greater than the capacity of the hash table multiplied by its fill ratio, the hash table is expanded If you do not specify a fill ratio, then 075 is used Finally, the fourth version creates a hash table that is initialized with the elements in m The capacity of the hash table is set to twice the number of elements in m The default load factor of 075 is used In addition to the methods defined by the Map interface, which Hashtable now implements, Hashtable defines the legacy methods listed in Table 17-18 Several methods throw NullPointerException if an attempt is made to use a null key or value
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