How
hashmap works in java
HashMap is most favorite topic for discussion in interviews now-a-days.I am sharing with you as much i know about it.
May be it'll help you.
Lets start...
This is for those who has some basic Understanding about hashmap.if you do not have as much understanding about then click on the link:- Hashmap Basics
Sections in this post
Answer in just a Line
Understanding Hashing
Some taste of Entry class
How put() method works
How get() method works
Key Notes to remember
Single statement answer
Answer in just a Line:- “On principle of Hashing“. As simple as it is. Now before answering it, one must be very sure to know at least basics of Hashing. Right??
What is Hashing
Hashing in its simplest form, is a way to assigning a unique code for any variable/object after applying any formula/algorithm on its properties. A true Hashing function must follow this rule:
Hash function should return the same hash code each and every time, when function is applied on same or equal objects. In other words, two equal objects must produce same hash code consistently.
Note: All objects in java inherit a default implementation of hashCode() function defined in Object class. This function produce hash code by typically converting the internal address of the object into an integer, thus producing different hash codes for all different objects.
Read more here : Working with hashcode() and equals()
Understanding Hashing
Some taste of Entry class
How put() method works
How get() method works
Key Notes to remember
Single statement answer
Answer in just a Line:- “On principle of Hashing“. As simple as it is. Now before answering it, one must be very sure to know at least basics of Hashing. Right??
What is Hashing
Hashing in its simplest form, is a way to assigning a unique code for any variable/object after applying any formula/algorithm on its properties. A true Hashing function must follow this rule:
Hash function should return the same hash code each and every time, when function is applied on same or equal objects. In other words, two equal objects must produce same hash code consistently.
Note: All objects in java inherit a default implementation of hashCode() function defined in Object class. This function produce hash code by typically converting the internal address of the object into an integer, thus producing different hash codes for all different objects.
Read more here : Working with hashcode() and equals()
Some taste of Entry class:-
Remember map by definition is : “An object that maps keys to values”. Very easy.. right?
So, there must be some mechanism in HashMap to store this key value pair. Answer is YES.
HashMap has a static class named Entry which implements Map.Entry interface. The Entry class looks like:
static class Entry implements Map.Entry {
final Object key;
Object value;
final int hash;
Entry next;
Entry(int i, Object obj, Object obj1, Entry entry) {
value = obj1;
next = entry;
key = obj;
hash = i;
}
// Other methods
}
Every time we insert a <key,value> into hashmap using .put() method, a new Entry object is created (not true is some cases. if key already exists, then it just replace the value). Map internally used two data structures to manage/store data:
Array
Link List
Surely Entry class has key and value mapping stored as attributes. Key has been marked as final and two more fields are there: next and hash. We will try to understand the need of these fields as we go forward.
What put() method actually does?
Before going into put() method’s implementation, it is very important to learn that instances of Entry class are stored in an array. HashMap class defines this variable as:
/**
* The table, resized as necessary. Length MUST Always be a power of two.
*/
transient Entry[] table;
Now look at code implementation of put() method:Direct from google docs
view sourceprint?
01./**
02.* Associates the specified value with the specified key in this map. If the
03.* map previously contained a mapping for the key, the old value is
04.* replaced.
05.*
06.* @param key
07.* key with which the specified value is to be associated
08.* @param value
09.* value to be associated with the specified key
10.* @return the previous value associated with <tt>key</tt>, or <tt>null</tt>
11.* if there was no mapping for <tt>key</tt>. (A <tt>null</tt> return
12.* can also indicate that the map previously associated
13.* <tt>null</tt> with <tt>key</tt>.)
14.*/
15.public V put(K key, V value) {
16.if (key == null)
17.return putForNullKey(value);
18.int hash = hash(key.hashCode());
19.int i = indexFor(hash, table.length);
20.for (Entry<k , V> e = table[i]; e != null; e = e.next) {
21.Object k;
22.if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
23.V oldValue = e.value;
24.e.value = value;
25.e.recordAccess(this);
26.return oldValue;
27.}
28.}
29.
30.modCount++;
31.addEntry(hash, key, value, i);
32.return null;
33.}
Lets note down the steps one by one:
- First of all, key object is checked for null. If key is null, value is stored in table[0] position. Because hash code for null is always 0.
- Then on next step, a hash value is calculated using key’s hash code by calling its hashCode() method. This hash value is used to calculate index in array for storing Entry object. JDK designers well assumed that there might be some poorly written hashCode() functions that can return very high or low hash code value. To solve this issue, they introduced another hash() function, and passed the object’s hash code to this hash() function to bring hash value in range of array index size.
- Now indexFor(hash, table.length) function is called to calculate exact index position for storing the Entry object.
- Here comes the main part. Now, as we know that two unequal objects can have same hash code value, how two different objects will be stored in same array location [called bucket].
Answer is LinkedList. If you remember, Entry class had an attribute “next”. This attribute always points to next object in chain. This is exactly the behavior of LinkedList.
So, in case of collision, Entry objects are stored in LinkedList form. When an Entry object needs to be stored in particular index, HashMap checks whether there is already an entry?? If there is no entry already present, Entry object is stored in this location.
If there is already an object sitting on calculated index, its next attribute is checked. If it is null, and current Entry object becomes next node in LinkedList. If next variable is not null, procedure is followed until next is evaluated as null.
What if we add the another value object with same key as entered before. Logically, it should replace the old value. How it is done? Well, after determining the index position of Entry object, while iterating over LinkedList on calculated index, HashMap calls equals method on key object for each Entry object. All these Entry objects in LinkedList will have similar hash code but equals() method will test for true equality. If key.equals(k) will be true then both keys are treated as same key object. This will cause the replacing of value object inside Entry object only.
In this way, HashMap ensure the uniqueness of keys.
How get() methods works internally:-
Now we have got the idea, how key-value pairs are stored in HashMap. Next big question is : what happens when an object is passed in get method of HashMap? How the value object is determined?
Answer we already should know that the way key uniqueness is determined in put() method , same logic is applied in get() method also. The moment HashMap identify exact match for the key object passed as argument, it simply returns the value object stored in current Entry object.
If no match is found, get() method returns null.
Let have a look at code:
view sourceprint?
01./**
02.* Returns the value to which the specified key is mapped, or {@code null}
03.* if this map contains no mapping for the key.
04.*
05.* <p>
06.* More formally, if this map contains a mapping from a key {@code k} to a
07.* value {@code v} such that {@code (key==null ? k==null :
08.* key.equals(k))}, then this method returns {@code v}; otherwise it returns
09.* {@code null}. (There can be at most one such mapping.)
10.*
11.* </p><p>
12.* A return value of {@code null} does not <i>necessarily</i> indicate that
13.* the map contains no mapping for the key; it's also possible that the map
14.* explicitly maps the key to {@code null}. The {@link #containsKey
15.* containsKey} operation may be used to distinguish these two cases.
16.*
17.* @see #put(Object, Object)
18.*/
19.public V get(Object key) {
20.if (key == null)
21.return getForNullKey();
22.int hash = hash(key.hashCode());
23.for (Entry<k , V> e = table[indexFor(hash, table.length)]; e != null; e = e.next) {
24.Object k;
25.if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
26.return e.value;
27.}
28.return null;
29.}
Above code is same as put() method till if (e.hash == hash && ((k = e.key) == key || key.equals(k))), after this simply value object is returned.
Key Notes to remember:-
1.Data structure to store Entry objects is an array named table of type Entry.
2.A particular index location in array is referred as bucket, because it can hold the first element of a LinkedList of Entry objects.
3.Key object’s hashCode() is required to calculate the index location of Entry object.
4.Key object’s equals() method is used to maintain uniqueness of Keys in map.
5.Value object’s hashCode() and equals() method are not used in HashMap’s get() and put() methods.
6.Hash code for null keys is always zero, and such Entry object is always stored in zero index in Entry[].
I hope, this article gives you some understanding about hashmap. If you find any difference or need any help in any point, please drop a comment.
