In this chapter you will learn
What is HashMap Class in Java?
HashMap is the same as HashTable class and only difference is it allows inserting null values and it is unsynchronized. HashMap doesn’t maintain insertion order and the elements can be stored at any places and iteration result might be different than the insertion order. It is also based on Map Interface and inherits all the functionality of Map Interface. More about HashMap class in java are listed below in the list.
- It contains value based on Key.
- It allows inserting null values and null keys.
- There may be only one null key in the list but can have multiple null values.
- It doesn’t maintain insertion order.
- It uses HashTable to implement the Map Interface.
Constructors and Methods
| Constructor | Description |
|---|---|
| HashMap() | It is used to construct a default HashMap. |
| HashMap(Map m) | It is used to initializes the hash map by using the elements of the given Map object m. |
| HashMap(int capacity) | It is used to initializes the capacity of the hash map to the given integer value, capacity. |
| HashMap(int capacity, float fillRatio) | It is used to initialize both the capacity and fill ratio of the hash map by using its arguments |
Methods
| Methods | Description |
|---|---|
| void clear() | It is used to remove all of the mappings from this map. |
| boolean containsKey(Object key) | It is used to return true if this map contains a mapping for the specified key. |
| boolean containsValue(Object value) | It is used to return true if this map maps one or more keys to the specified value. |
| boolean isEmpty() | It is used to return true if this map contains no key-value mappings. |
| Object clone() | It is used to return a shallow copy of this HashMap instance: the keys and values themselves are not cloned. |
| Set entrySet() | It is used to return a collection view of the mappings contained in this map. |
| Set keySet() | It is used to return a set view of the keys contained in this map. |
| Object put(Object key, Object value) | It is used to associate the specified value with the specified key in this map. |
| int size() | It is used to return the number of key-value mappings in this map. |
| Collection values() | It is used to return a collection view of the values contained in this map. |
Programming Example
import java.util.*;
public class HashMap_Example
{
public static void main(String[] args)
{
HashMap<Integer,String> Hm=new HashMap<Integer,String>();
Hm.put(1,"Steven");
Hm.put(null,null);
Hm.put(6,"Clark");
Hm.put(3,"Jack");
//Direct Printing
System.out.println(Hm);
//Method 1
PrintMethod1(Hm);
//Method 2
PrintMethod2(Hm);
//Remove an item()
System.out.println("Item Removed : "+Hm.remove(1));
}
//Iteration Method 1
public static void PrintMethod1(HashMap<Integer,String> hm)
{
Set set=hm.entrySet();
Iterator itr=set.iterator();
while(itr.hasNext())
{
Map.Entry me=(Map.Entry)itr.next();
System.out.println("Key is : "+me.getKey()+" and Value is : "+me.getValue());
}
}
//Iteration Method 2
public static void PrintMethod2(HashMap<Integer,String> hm)
{
for(Map.Entry m:hm.entrySet())
{
System.out.println("Key : "+m.getKey() + " | Value = "+m.getValue());
}
}
}
Output
{null=null, 1=Steven, 3=Jack, 6=Clark}
Key is : null and Value is : null
Key is : 1 and Value is : Steven
Key is : 3 and Value is : Jack
Key is : 6 and Value is : Clark
Key : null | Value = null
Key : 1 | Value = Steven
Key : 3 | Value = Jack
Key : 6 | Value = Clark
Item Removed : Steven
_
Key is : null and Value is : null
Key is : 1 and Value is : Steven
Key is : 3 and Value is : Jack
Key is : 6 and Value is : Clark
Key : null | Value = null
Key : 1 | Value = Steven
Key : 3 | Value = Jack
Key : 6 | Value = Clark
Item Removed : Steven
_
Summary
The HashMap class is another implementaion of Map Interface that uses HashTable. In this chapter you learn all about HashMap class in Java with complete programming example. In the next chapter you will learn LinkedHashMap class in Java.