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http://www.linkedin.com/in/vinaykumar2 Read more from this authorA map is an interface in the java.util package which stores the association between key and its corresponding value. A map cannot contain duplicate keys; each key can map to at most one value Map differs from array in a way that we can store a value at a particular index in arrays but a Map determines the index itself and does this based on the value on the key.
HASHMAP is a class which implements the map interface.
Hash map is a data structure which uses a hash function to map identifying values, known as keys (e.g., a person’s name) to their associated values (e.g., their telephone number).
The hash function transforms the key into the index (the hash) of an array element where the corresponding value is to be sought.
Take for instance the case of a phonebook. you can have a map where the keys are phone numbers and the value is the name of the person..
Given a key you can find its value.
Ex.[“key”, “value”]=[“88028”,”vinay”]
With the given key value i.e 88028 one can find its value….. which is vinay.
Take another example…..
import java.util.Map;
import java.util.HashMap;
public class Map_example {
Map<Integer, String> addPhoneNum = new HashMap<Integer, String>();
public void addPhoneNum(Integer phonenumber, String name) {
addPhoneNum.put(phonenumber, name);
}
public String getName(Integer phonenumber) {
return addPhoneNum.get(phonenumber);
}
public static void main(String[] args) {
Map_example m1 = new Map_example();
m1.addPhoneNum(9911538992 "vinay");
System.out.println(
m1.getName(9911538992));
}
Main features of HASHMAP are.
• HashMap is a library class in Java.
• It is already implemented, hence can be used immediately HashMap is a library class in Java.
• HashMap stores only object references. That’s why, it’s impossible to use primitive data types like double or int. Use wrapper class (like Integer or Double) instead.
• For multi-theaded(synchronized) array class use Hashtable (java.lang.Hashtable)
The two most important HashMap’s methods are:
• get( Object key ) –this method returns the value associated with specified key in this hash map, or null if there is no value for this key
• put(K key, V value) – this method associates the specified value with the specified key in the map.
Internally a HashMap maintains an array. To have the HashMap work efficiently, the array must be large enough so that the key/value pairs are well-distributed and the performance is not affected. Therefore, the HashMap maintains two (customizable) variables :
1. Capacity
2. LoadFactor.
The capacity is the length of the internal array whereas the LoadFactor controls when the capacity should be increased.
Peter Lawrey
Hi, A very useful post for those new to Maps. However, one correction.
For multi-theaded(synchronized) array class use Hashtable (java.lang.Hashtable)
HashMap and Hashtable are Maps, not arrays.
Hashtable is in the java.util package like HashMap.
IMHO Hashtable has been a legacy class since JDK 1.2. If you Have JDK 1.2, 1.3 or 1.4 Use Collections.synchronizedMap(new HashMap());
However if you have JDK 5.0 or 6 (available since ’04) I suggest trying ConcurrentHashMap which is much improved.
Rick Ryker
When using HashSet, HashMap, or any of the implementation class that uses a hashCode for internal lookups, you must ensure that the classes of objects you place into the HashMap have a deterministic hash code, one that will not change once an instance has been added to the set or to the key set of the map.
Venkat
Hi Rick, that is a good idea which you have mentioed. If my understanding is correct, what you mean is to override the hashCode() method in the class which is going to be used as key.
Kefaleas Stavros
Nice and clear description!
Well Done!
Javin Paul
Hi,
HashMap is indeed very useful class and proper knowledge of it along with its counterpart hashtable is very important. here is some point worth looking.
1. The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesn’t allow nulls).
2. HashMap does not guarantee that the order of the map will remain constant over time.
3. HashMap is non synchronized whereas Hashtable is synchronized.
4. Iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn’t.
to read more about Hashtable vs HashMap see here.
Javin @ Tibco RV Tutorial
Hi,
Thanks for this Nice artilce just to add while discussing about HashMap its worth mentioning following questions which frequently asked in Java interviews now days like How HashMap works in Java or How get() method of HashMap works in JAVA very often. on concept point of view these questions are great and expose the candidate if doesn’t know deep details.
Javin
FIX Protocol tutorial
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