Java Dictionary example

https://examples.javacodegeeks.com/java-basics/data-types/java-dictionary-example/

Java Dictionary example

Posted by: Sotirios-Efstathios Maneas in data types February 7th, 2014

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.

However, the original Dictionary class is now deprecated and instead, all new implementations should implement the Map interface. The Mapinterface 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.

The Map Interface

A map has the form Map <K, V> where:

• K: specifies the type of keys maintained in this map.
• V: defines the type of mapped values.

Furthermore, the Map interface provides a set of methods that must be implemented. In this section, we will present some of the most fundamental methods of a map:

• containsKey: Returns true if the map contains the requested key.
• containsValue: Returns true if the map contains the requested value.
• get: Retrieve the value of the requested key.
• keySet: Returns a Set that contains all keys of the map.
• put: Adds the requested key-value pair in the map.

The Map interface is implemented by different Java classes, such as HashMap, Hashtable and LinkedHashMap. These classes are able to provide the full functionality of a dictionary. However, these classes differ in some key aspects, as presented below:

	Null Keys	Null Values	Order	Synchronized
HashMap	Permitted	Permitted	No	No
HashTable	Prohibited	Prohibited	No	Yes
LinkedHashMap	Permitted	Permitted	Yes	No

HashTable – HashMap

The Hashtable class implements a hash table and maps keys to values. A HashMap is 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.

An example that uses a HashMap as a dictionary is shown below. The program can also be executed properly if we change the type of our map to a Hashtable:

CountWords_v1.java:

01	import java.io.BufferedReader;

02	import java.io.File;

03	import java.io.FileReader;

04	import java.io.IOException;

05	import java.util.HashMap;

06	import java.util.Hashtable;

07	import java.util.Map;

08

09	public class CountWords_v1 {

10

11	public static void main(String[] args) throws IOException {

12	BufferedReader reader = new BufferedReader(new FileReader(new File("input.txt")));

13	String inputLine = null;

14	Map dictionary = new HashMap();

15	//Map dictionary = new Hashtable();

16

17	while((inputLine = reader.readLine()) != null) {

18	// Split the input line.

19	String[] words = inputLine.split("\\s+");

20

21	// Ignore empty lines.

22	if(inputLine.equals(""))

23

continue;

24

25	for(String word: words) {

26	// Remove any commas and dots.

27	word = word.replace(".", "");

28	word = word.replace(",", "");

29

30	if(dictionary.containsKey(word)) {

31	Integer val = dictionary.get(word);

32	dictionary.put(word, val + 1);

33

}

34

else

35	dictionary.put(word, 1);

36

}

37

}

38

39	// Printing all words stored in the map.

40	for(String key: dictionary.keySet())

41	System.out.println(key + ": " + dictionary.get(key));

42

43

44	reader.close();

45

}

46

}

In this example we used a HashMap to store the words of a file and how many times each word appears in that file.

A sample execution is shown below:

to: 2
Geeks: 1
HashMaps: 1
is: 2
text: 1
a: 1
Also: 1
Hashtables: 1
from: 1
LinkedHashMaps: 1
the: 2
namely: 1
Maps: 1
used: 1
Code: 1
This: 1
Java: 2
and: 1
hello: 1
that: 1
present: 1
of: 2
power: 2
everybody: 1
sample: 1

LinkedHashMap

The LinkedHashMap class provides an implementation of a map that has a predictable iteration order.

The same example that counts the references of a word in a file and stores the key-value pairs in a LinkedHashMap is shown below:

CountWords_v2.java:

01	import java.io.BufferedReader;

02	import java.io.File;

03	import java.io.FileReader;

04	import java.io.IOException;

05	import java.util.Iterator;

06	import java.util.LinkedHashMap;

07	import java.util.Map.Entry;

08	import java.util.Set;

09

10	public class CountWords_v2 {

11

12	public static void main(String[] args) throws IOException {

13	BufferedReader reader = new BufferedReader(new FileReader(new File("input.txt")));

14	String inputLine = null;

15	LinkedHashMap dictionary = new LinkedHashMap();

16

17	while((inputLine = reader.readLine()) != null) {

18	// Split the input line.

19	String[] words = inputLine.split("\\s+");

20

21	// Ignore empty lines.

22	if(inputLine.equals(""))

23

continue;

24

25	for(String word: words) {

26	// Remove any commas and dots.

27	word = word.replace(".", "");

28	word = word.replace(",", "");

29

30	if(dictionary.containsKey(word)) {

31	Integer val = dictionary.get(word);

32	dictionary.put(word, val + 1);

33

}

34

else

35	dictionary.put(word, 1);

36

}

37

}

38

39	// Printing all words stored in the map.

40	Set<Entry> entries = dictionary.entrySet();

41	Iterator<Entry> iter = entries.iterator();

42

43	while(iter.hasNext()) {

44	Entry entry = iter.next();

45	System.out.println(entry.getKey() + ": " + entry.getValue());

46

}

47

48	reader.close();

49

}

50

}

A sample execution is shown below:

This: 1
is: 2
a: 1
sample: 1
text: 1
that: 1
used: 1
to: 2
present: 1
the: 2
power: 2
of: 2
Java: 2
Maps: 1
namely: 1
HashMaps: 1
Hashtables: 1
and: 1
LinkedHashMaps: 1
Also: 1
hello: 1
everybody: 1
from: 1
Code: 1
Geeks: 1

Notice 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.