In this lesson, I would teach you all you need to be able to use Hashtables in Java.<\/p>\n
A hashtable in Java is an implementation of the dictionary data structure. Therefore you can use it for storage of key-values pairs. This is similar to hashmaps. But unlike hashmaps, hashtables are thread-safe (or synchronized).<\/p>\n
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We cover the following:<\/p>\n
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To store an item in a hashtable, you need to specify the value you want to store. Also you must specify a key along with this value. Then this\u00a0 key is then hashed. This means passing it through a hash function. The output of the hashing is a hash code which represents the index where the value is to be stored.<\/p>\n
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How to Create a Hashtable<\/strong><\/p>\n You can create a hashtable using the new keyword. An example is given below:<\/p>\n <\/p>\n The first line creates a new hashtable while the second line creates a new hashtable with a size of 10. You can also use any of the constructors below.<\/p>\n <\/p>\n The default constructor of the hash table. It creates a new hashtable.<\/td>\n<\/tr>\n Takes the initial size of the table as parameter and then creates a hash table that with an initial size given by the value size.<\/td>\n<\/tr>\n This creates a hash table with an initial size given by size parameter and also a fill ratio. The fill ratio must be between 0.0 and 1.0. It then determines how full the hash table can be before it is resized.<\/td>\n<\/tr>\n This constructor creates a Hashtable with the given mappings.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n Table 1.1. Hashtable Constructors<\/p>\n <\/p>\n <\/p>\n You can use the following methods the perform various operations on a hashtable. Similar to arraylist, you can add and delete items from a hashtable.<\/p>\n This is used to clear (or empty) the hash table.<\/td>\n<\/tr>\n Used to return a duplicate of the invoking hashtable. Create another copy<\/td>\n<\/tr>\n You use this to check if the hashtable contains the specified value. It true if\u00a0 the object exists within the hash table. Otherwise, it returns false<\/td>\n<\/tr>\n You use this to check if the hashtable contains the specified key. It true if\u00a0 the key exists within the hash table. Otherwise, it returns false<\/td>\n<\/tr>\n You use this to check if the hashtable contains the specified value. It true if\u00a0 the value exists within the hashtable. Otherwise, it returns false<\/td>\n<\/tr>\n Used to return an enumeration of all the values contained in the hashtable.<\/td>\n<\/tr>\n Used to return the object that contains the value associated with the key. If the key is not in the hash table, a null object is returned.<\/td>\n<\/tr>\n Used to check if the hashtable is empty. Return true if the hash table is empty. Otherwise returns false.<\/td>\n<\/tr>\n List of the keys contained in the hash table.<\/td>\n<\/tr>\n You use this to add a key and a value pair into the hash table. It returns null if the key is not already existing in the hashtable; returns the previous value associated with the key if the key is already in the hashtable.<\/td>\n<\/tr>\n It Increases the size of the hashtable and then rehashes all of its keys.<\/td>\n<\/tr>\n Removes the specified key along with its value. Returns the value associated with the specified key. If the key is not found in the hashtable, a null object is returned.<\/td>\n<\/tr>\n Returns an integer value of the number of entries in the hashtable.<\/td>\n<\/tr>\n Creates the string equivalent of a hashtable.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n Table 1.1: Hashtable Methods<\/p>\n <\/p>\n In the example below, we create a hashtable and insert some items. Then remove an item. We also demonstrate the use of some other methods of the hash table. I therefor suggest you try this code on your own.<\/p>\n <\/p>\n Listing 1.1: Demo of hashtables<\/p>\n <\/p>\n If you run the code, you would have the output below:<\/p>\n <\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":" In this lesson, I would teach you all you need to be able to use Hashtables in Java. A hashtable in Java is an implementation … <\/p>\n","protected":false},"author":395,"featured_media":170,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[2],"tags":[],"class_list":["post-169","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-java-tutorial"],"_links":{"self":[{"href":"https:\/\/kindsonthegenius.com\/java\/wp-json\/wp\/v2\/posts\/169","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/kindsonthegenius.com\/java\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/kindsonthegenius.com\/java\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/kindsonthegenius.com\/java\/wp-json\/wp\/v2\/users\/395"}],"replies":[{"embeddable":true,"href":"https:\/\/kindsonthegenius.com\/java\/wp-json\/wp\/v2\/comments?post=169"}],"version-history":[{"count":1,"href":"https:\/\/kindsonthegenius.com\/java\/wp-json\/wp\/v2\/posts\/169\/revisions"}],"predecessor-version":[{"id":171,"href":"https:\/\/kindsonthegenius.com\/java\/wp-json\/wp\/v2\/posts\/169\/revisions\/171"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/kindsonthegenius.com\/java\/wp-json\/wp\/v2\/media\/170"}],"wp:attachment":[{"href":"https:\/\/kindsonthegenius.com\/java\/wp-json\/wp\/v2\/media?parent=169"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/kindsonthegenius.com\/java\/wp-json\/wp\/v2\/categories?post=169"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/kindsonthegenius.com\/java\/wp-json\/wp\/v2\/tags?post=169"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}\tHashtable ht =<\/span> new<\/span> Hashtable();<\/span>\r\n\tHashtable ht2 =<\/span> new<\/span> Hashtable(<\/span>10<\/span>);<\/span>\r\n<\/pre>\n\n\n
\n SN<\/th>\n Hashtable constructors<\/th>\n<\/tr>\n \n 1<\/td>\n Hashtable( )<\/b><\/p>\n \n 2<\/td>\n Hashtable(int size)<\/b><\/p>\n \n 3<\/td>\n Hashtable(int size, float fillRatio)<\/b><\/p>\n \n 4<\/td>\n Hashtable(Map < ? extends K, ? extends V > t)<\/b><\/p>\n 2. Hashtable Methods<\/strong><\/h4>\n
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\n Sr.No<\/th>\n Method & Description<\/th>\n<\/tr>\n \n 1<\/td>\n clear( )<\/b><\/p>\n \n 2<\/td>\n clone( )<\/b><\/p>\n \n 3<\/td>\n contains(Object value)<\/b><\/p>\n \n 4<\/td>\n containsKey(Object key)<\/b><\/p>\n \n 5<\/td>\n containsValue(Object value)<\/b><\/p>\n \n 6<\/td>\n elements( )<\/b><\/p>\n \n 7<\/td>\n get(Object key)<\/b><\/p>\n \n 8<\/td>\n isEmpty( )<\/b><\/p>\n \n 9<\/td>\n keys( )<\/b><\/p>\n \n 10<\/td>\n put(Object key, Object value)<\/b><\/p>\n \n 11<\/td>\n rehash( )<\/b><\/p>\n \n 12<\/td>\n remove(Object key)<\/b><\/p>\n \n 13<\/td>\n size( )<\/b><\/p>\n \n 14<\/td>\n String toString( )<\/b><\/p>\n 3. Hashtable Example<\/strong><\/h4>\n
import<\/span> java.util.Hashtable<\/span>;<\/span>\r\n\r\npublic<\/span> class<\/span> HashTableDemo<\/span> {<\/span>\r\n\r\n\tpublic<\/span> static<\/span> void<\/span> main<\/span>(<\/span>String[]<\/span> args)<\/span> {<\/span>\r\n\t\t\r\n\tHashtable ht =<\/span> new<\/span> Hashtable();<\/span>\r\n\tht.<\/span>put<\/span>(<\/span>1<\/span>,<\/span> \"Monday\"<\/span>);<\/span>\r\n\tht.<\/span>put<\/span>(<\/span>2<\/span>,<\/span> \"Tuesday\"<\/span>);<\/span>\t\r\n\tht.<\/span>put<\/span>(<\/span>3<\/span>,<\/span> \"Wednesday\"<\/span>);<\/span>\t\r\n\tht.<\/span>put<\/span>(<\/span>4<\/span>,<\/span> \"Thursday\"<\/span>);<\/span>\t\r\n\tht.<\/span>put<\/span>(<\/span>5<\/span>,<\/span> \"Friday\"<\/span>);<\/span>\t\r\n\tht.<\/span>put<\/span>(<\/span>6<\/span>,<\/span> \"Saturday\"<\/span>);<\/span>\t\r\n\tht.<\/span>put<\/span>(<\/span>7<\/span>,<\/span> \"Sunday\"<\/span>);<\/span>\t\r\n\t\r\n\tSystem.<\/span>out<\/span>.<\/span>println<\/span>(<\/span>\"Is Empty? \"<\/span> +<\/span> ht.<\/span>isEmpty<\/span>());<\/span>\r\n\tSystem.<\/span>out<\/span>.<\/span>println<\/span>(<\/span>\"Contains Tuesday? \"<\/span> +<\/span> ht.<\/span>contains<\/span>(<\/span>\"Tuesday\"<\/span>));<\/span>\r\n\tSystem.<\/span>out<\/span>.<\/span>println<\/span>(<\/span>\"Value of 5: \"<\/span> +<\/span> ht.<\/span>get<\/span>(<\/span>5<\/span>));<\/span>\r\n\tSystem.<\/span>out<\/span>.<\/span>println<\/span>(<\/span>\"Number of entries: \"<\/span> +<\/span> ht.<\/span>size<\/span>());<\/span>\r\n\t\r\n\tSystem.<\/span>out<\/span>.<\/span>println<\/span>(<\/span>ht.<\/span>toString<\/span>());<\/span>\r\n\t}<\/span>\r\n}<\/span>\r\n<\/pre>\nIs Empty?<\/span> false<\/span>\r\nContains Tuesday?<\/span> true<\/span>\r\nValue of 5<\/span>:<\/span> Friday\r\nNumber of entries:<\/span> 7<\/span>\r\n{<\/span>7<\/span>=<\/span>Sunday,<\/span> 6<\/span>=<\/span>Saturday,<\/span> 5<\/span>=<\/span>Friday,<\/span> 4<\/span>=<\/span>Thursday,<\/span> 3<\/span>=<\/span>Wednesday,<\/span> 2<\/span>=<\/span>Tuesday,<\/span> 1<\/span>=<\/span>Monday}<\/span>\r\n<\/pre>\n