René's URL Explorer Experiment

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{"@context":"http:\/\/schema.org","@type":"Article","dateCreated":"2014-02-07T22:19:52+02:00","datePublished":"2014-02-07T22:19:52+02:00","dateModified":"2014-02-07T22:30:07+02:00","headline":"Java Dictionary example","name":"Java Dictionary example","keywords":"hashmap,hashtable,linkedhashmap","url":"https:\/\/examples.javacodegeeks.com\/java-development\/java-basics\/data-types\/java-dictionary-example\/","description":"In this tutorial we will discuss about dictionaries in Java. A Dictionary is an abstract class that maps keys to values. Every key is associated with a unique value and key are unique. Any non-null ob","copyrightYear":"2014","articleSection":"data types","articleBody":"In this tutorial we will discuss about dictionaries in Java. A Dictionary is an abstract class that maps keys to values. Every key is associated with a unique value and key are unique. Any non-null object can be used for either a key or a value. An attempt to insert either a null key or a null value to a dictionary will result to a NullPointerException.\r\n\r\nHowever, the original Dictionary class is now deprecated and instead, all new implementations should implement the Map interface. The Map interface provides the functionality of a dictionary, using exactly the same semantics. A Map can provide three views, which allow the contents of the map to be viewed as a set of keys, collection of values, or set of key-value mappings. Finally, some implementations of the Map interface maintain an order among its values.\r\n\r\nThe Map Interface\r\n\r\nA map has the form Map <K, V> where:\r\n\r\n\tK: specifies the type of keys maintained in this map.\r\n\tV: defines the type of mapped values.\r\n\r\n\r\nFurthermore, the\u00a0Map\u00a0interface provides a set of methods that must be implemented. In this section, we will present some of the most fundamental methods of a map:\r\n\r\n\tcontainsKey: Returns true if the map contains the requested key.\r\n\tcontainsValue: Returns true if the map contains the requested value.\r\n\tget: Retrieve the value of the requested key.\r\n\tkeySet: Returns a\u00a0Set\u00a0that contains all keys of the map.\r\n\tput: Adds the requested key-value pair in the map.\r\n\r\n\r\nThe\u00a0Map\u00a0interface is implemented by different Java classes, such as\u00a0HashMap, Hashtable and\u00a0LinkedHashMap. These classes are able to provide the full functionality of a dictionary. However, these classes differ in some key aspects, as presented below:\r\n\r\n\r\n\r\n\r\n\r\nNull Keys\r\n\r\n\r\nNull Values\r\n\r\n\r\nOrder\r\n\r\n\r\nSynchronized\r\n\r\n\r\n\r\nHashMap\r\n\r\nPermitted\r\n\r\n\r\nPermitted\r\n\r\n\r\nNo\r\n\r\n\r\nNo\r\n\r\n\r\n\r\nHashTable\r\n\r\nProhibited\r\n\r\n\r\nProhibited\r\n\r\n\r\nNo\r\n\r\n\r\nYes\r\n\r\n\r\n\r\nLinkedHashMap\r\n\r\nPermitted\r\n\r\n\r\nPermitted\r\n\r\n\r\nYes\r\n\r\n\r\nNo\r\n\r\n\r\n\r\n\r\n\r\nHashTable - HashMap\r\nThe Hashtable class implements a hash table and maps keys to values. A\u00a0HashMap\u00a0is a hash table based implementation of the Map interface. They both contain two fundamental parameters: initial capacity and performance. The capacity is defined as the number of buckets in the hash table, while the load factor is a measure that indicates the maximum value the hash table can reach, before being automatically increased.\r\n\r\nAn example that uses a HashMap\u00a0as a dictionary is shown below. The program can also be executed properly if we change the type of our map to a Hashtable:\r\n\r\nCountWords_v1.java:\r\n\r\nimport java.io.BufferedReader;\r\nimport java.io.File;\r\nimport java.io.FileReader;\r\nimport java.io.IOException;\r\nimport java.util.HashMap;\r\nimport java.util.Hashtable;\r\nimport java.util.Map;\r\n\r\npublic class CountWords_v1 {\r\n\t\r\n\tpublic static void main(String[] args) throws IOException {\r\n\t\tBufferedReader reader = new BufferedReader(new FileReader(new File(\"input.txt\")));\r\n\t\tString inputLine = null;\r\n\t\tMap dictionary = new HashMap();\r\n\t\t\/\/Map dictionary = new Hashtable();\r\n\t\t\r\n\t\twhile((inputLine = reader.readLine()) != null) {\r\n\t\t\t\/\/ Split the input line.\r\n\t\t\tString[] words = inputLine.split(\"\\\\s+\");\r\n\t\t\t\r\n\t\t\t\/\/ Ignore empty lines.\r\n\t\t\tif(inputLine.equals(\"\"))\r\n\t\t\t\tcontinue;\r\n\t\t\t\r\n\t\t\tfor(String word: words) {\r\n\t\t\t\t\/\/ Remove any commas and dots.\r\n\t\t\t\tword = word.replace(\".\", \"\");\r\n\t\t\t\tword = word.replace(\",\", \"\");\r\n\t\t\t\t\r\n\t\t\t\tif(dictionary.containsKey(word)) {\r\n\t\t\t\t\tInteger val = dictionary.get(word);\r\n\t\t\t\t\tdictionary.put(word, val + 1);\r\n\t\t\t\t}\r\n\t\t\t\telse\r\n\t\t\t\t\tdictionary.put(word, 1);\r\n\t\t\t}\r\n\t\t}\r\n\t\t\r\n\t\t\/\/ Printing all words stored in the map.\r\n\t\tfor(String key: dictionary.keySet())\r\n\t\t\tSystem.out.println(key + \": \" + dictionary.get(key));\r\n\t\t\r\n\t\t\r\n\t\treader.close();\r\n\t}\r\n}\r\n\r\n\r\nIn this example we used a HashMap\u00a0to store the words of a file and how many times each word appears in that file.\r\n\r\nA sample execution is shown below:\r\nto: 2\r\nGeeks: 1\r\nHashMaps: 1\r\nis: 2\r\ntext: 1\r\na: 1\r\nAlso: 1\r\nHashtables: 1\r\nfrom: 1\r\nLinkedHashMaps: 1\r\nthe: 2\r\nnamely: 1\r\nMaps: 1\r\nused: 1\r\nCode: 1\r\nThis: 1\r\nJava: 2\r\nand: 1\r\nhello: 1\r\nthat: 1\r\npresent: 1\r\nof: 2\r\npower: 2\r\neverybody: 1\r\nsample: 1\r\n\r\nLinkedHashMap\r\nThe LinkedHashMap class provides an implementation of a map that has a predictable iteration order.\r\n\r\nThe same example that counts the references of a word in a file and stores the key-value pairs in a LinkedHashMap is shown below:\r\n\r\nCountWords_v2.java:\r\n\r\nimport java.io.BufferedReader;\r\nimport java.io.File;\r\nimport java.io.FileReader;\r\nimport java.io.IOException;\r\nimport java.util.Iterator;\r\nimport java.util.LinkedHashMap;\r\nimport java.util.Map.Entry;\r\nimport java.util.Set;\r\n\r\npublic class CountWords_v2 {\r\n\t\r\n\tpublic static void main(String[] args) throws IOException {\r\n\t\tBufferedReader reader = new BufferedReader(new FileReader(new File(\"input.txt\")));\r\n\t\tString inputLine = null;\r\n\t\tLinkedHashMap dictionary = new LinkedHashMap();\r\n\t\t\r\n\t\twhile((inputLine = reader.readLine()) != null) {\r\n\t\t\t\/\/ Split the input line.\r\n\t\t\tString[] words = inputLine.split(\"\\\\s+\");\r\n\t\t\t\r\n\t\t\t\/\/ Ignore empty lines.\r\n\t\t\tif(inputLine.equals(\"\"))\r\n\t\t\t\tcontinue;\r\n\t\t\t\r\n\t\t\tfor(String word: words) {\r\n\t\t\t\t\/\/ Remove any commas and dots.\r\n\t\t\t\tword = word.replace(\".\", \"\");\r\n\t\t\t\tword = word.replace(\",\", \"\");\r\n\t\t\t\t\r\n\t\t\t\tif(dictionary.containsKey(word)) {\r\n\t\t\t\t\tInteger val = dictionary.get(word);\r\n\t\t\t\t\tdictionary.put(word, val + 1);\r\n\t\t\t\t}\r\n\t\t\t\telse\r\n\t\t\t\t\tdictionary.put(word, 1);\r\n\t\t\t}\r\n\t\t}\r\n\t\t\r\n\t\t\/\/ Printing all words stored in the map.\r\n\t\tSet<Entry> entries = dictionary.entrySet();\r\n\t\tIterator<Entry> iter = entries.iterator();\r\n\t\t\r\n\t\twhile(iter.hasNext()) {\r\n\t\t\tEntry entry = iter.next();\r\n\t\t\tSystem.out.println(entry.getKey() + \": \" + entry.getValue());\r\n\t\t}\r\n\t\t\r\n\t\treader.close();\r\n\t}\r\n}\r\n\r\n\r\nA sample execution is shown below:\r\nThis: 1\r\nis: 2\r\na: 1\r\nsample: 1\r\ntext: 1\r\nthat: 1\r\nused: 1\r\nto: 2\r\npresent: 1\r\nthe: 2\r\npower: 2\r\nof: 2\r\nJava: 2\r\nMaps: 1\r\nnamely: 1\r\nHashMaps: 1\r\nHashtables: 1\r\nand: 1\r\nLinkedHashMaps: 1\r\nAlso: 1\r\nhello: 1\r\neverybody: 1\r\nfrom: 1\r\nCode: 1\r\nGeeks: 1\r\n\r\nNotice that the usage of a LinkedHashMap enables us to print the stored keys in the way in which the words were read and stored in the map.\r\n\r\nDownload the Eclipse Project\r\nThe Eclipse project of this example: CountWords.zip\r\n\r\n \r\nThis was a tutorial about dictionaries in Java.","publisher":{"@id":"#Publisher","@type":"Organization","name":"Examples Java Code 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