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Java Hashtable class implements a hashtable, which maps keys to values. It inherits Dictionary class and implements the Map interface.
The important points about Java Hashtable class are:
- A Hashtable is an array of list. Each list is known as a bucket. The position of bucket is identified by calling the hashcode() method. A Hashtable contains values based on the key.
- It contains only unique elements.
- It may have not have any null key or value.
- It is synchronized.
Hashtable class declaration
Let's see the declaration for java.util.Hashtable class.
public class Hashtable<K,V> extends Dictionary<K,V> implements Map<K,V>, Cloneable, Serializable click below button to copy the code. By - java tutorial - team
Hashtable class Parameters
Let's see the Parameters for java.util.Hashtable class.
- K: It is the type of keys maintained by this map.
- V: It is the type of mapped values.
Constructors of Java Hashtable class
| Constructor | Description |
|---|---|
| Hashtable() | It is the default constructor of hash table it instantiates the Hashtable class. |
| Hashtable(int size) | It is used to accept an integer parameter and creates a hash table that has an initial size specified by integer value size. |
| Hashtable(int size, float fillRatio) | It is used to create a hash table that has an initial size specified by size and a fill ratio specified by fillRatio. |
Methods of Java Hashtable class
| Method | Description |
|---|---|
| void clear() | It is used to reset the hash table. |
| boolean contains(Object value) | This method return true if some value equal to the value exist within the hash table, else return false. |
| boolean containsValue(Object value) | This method return true if some value equal to the value exists within the hash table, else return false. |
| boolean containsKey(Object key) | This method return true if some key equal to the key exists within the hash table, else return false. |
| boolean isEmpty() | This method return true if the hash table is empty; returns false if it contains at least one key. |
| void rehash() | It is used to increase the size of the hash table and rehashes all of its keys. |
| Object get(Object key) | This method return the object that contains the value associated with the key. |
| Object remove(Object key) | It is used to remove the key and its value. This method return the value associated with the key. |
| int size() | This method return the number of entries in the hash table. |
Java Hashtable Example
import java.util.*;
public class TestCollection16{
public static void main(String args[]){
Hashtable<Integer,String> hm=new Hashtable<Integer,String>();
hm.put(100,"Amit");
hm.put(102,"Ravi");
hm.put(101,"Vijay");
hm.put(103,"Rahul");
for(Map.Entry m:hm.entrySet()){
System.out.println(m.getKey()+" "+m.getValue());
}
}
} click below button to copy the code. By - java tutorial - team
Output:
103 Rahul
102 Ravi
101 Vijay
100 AmitJava Hashtable Example: remove()
import java.util.*;
public class HashtableExample {
public static void main(String args[]) {
// create and populate hash table
Hashtable<Integer, String> map = new Hashtable<Integer, String>();
map.put(102,"Let us C");
map.put(103, "Operating System");
map.put(101, "Data Communication and Networking");
System.out.println("Values before remove: "+ map);
// Remove value for key 102
map.remove(102);
System.out.println("Values after remove: "+ map);
}
} click below button to copy the code. By - java tutorial - team
Output:
Values before remove: {103=Operating System, 102=Let us C, 101=Data Communication and Networking}
Values after remove: {103=Operating System, 101=Data Communication and Networking}Java Hashtable Example: Book
import java.util.*;
public class Book {
int id;
String name,author,publisher;
int quantity;
public Book(int id, String name, String author, String publisher, int quantity) {
this.id = id;
this.name = name;
this.author = author;
this.publisher = publisher;
this.quantity = quantity;
}
}
public class HashtableExample {
public static void main(String[] args) {
//Creating map of Books
Map<Integer,Book> map=new Hashtable<Integer,Book>();
//Creating Books
Book b1=new Book(101,"Let us C","Yashwant Kanetkar","BPB",8);
Book b2=new Book(102,"Data Communications & Networking","Forouzan","Mc Graw Hill",4);
Book b3=new Book(103,"Operating System","Galvin","Wiley",6);
//Adding Books to map
map.put(1,b1);
map.put(2,b2);
map.put(3,b3);
//Traversing map
for(Map.Entry<Integer, Book> entry:map.entrySet()){
int key=entry.getKey();
Book b=entry.getValue();
System.out.println(key+" Details:");
System.out.println(b.id+" "+b.name+" "+b.author+" "+b.publisher+" "+b.quantity);
}
}
} click below button to copy the code. By - java tutorial - team
Output:
3 Details:
103 Operating System Galvin Wiley 6
2 Details:
102 Data Communications & Networking Forouzan Mc Graw Hill 4
1 Details:
101 Let us C Yashwant Kanetkar BPB 8