Can Anybody explain Internal code of HashMap?

HashMap works on the principle of Hashing . To understand Hashing , we should understand the three terms first i.e Hash Function , Hash Value and Bucket . What is Hash Function , Hash Value and Bucket ? hashCode() function which returns an integer value is the Hash function. The important point to note that , this method is present in Object class (... more» read less

HashMap is based on storing key and value pairs, so first lets try and understand the hashcode function right, hashcode function in the object class returns an integer value, so lets say we have an employee class and we want to use the object of this class as a key in the map, so now when we try and store this object,the map.put api will call the hashcode of the employee class and if not overridden it will use the default implementation from object class which is not advisable at all, so now we have a problem statement of implementing a hashcode method.A simple way to implement hashcode method is call the hashcode of all(all even boolean) the attributes in the Employee class,sum it up and use this integer,this will ensure our hashcodes are diffrent for every object (because only equal objects will have equal hashcode but vice versa may not be true)and actually use the state of the object to calculate the hashcode and inturn when the hashmap put method calls hashcode for employee objects it will work seamlessly without any problems and we want to write when we call the put function. So the map.put function will then use this hashcode to identify if this hashcode exists or not if it does then it returns the earlier object associated is removed and yes the return type of put method if you look up is Object and if there is not object associated to the hashcode then the hashcode and the object are linked in the map. The Get method uses the hashcode method of the key object, again the implementation of the hashcode is important here The remove method works in a similar way using the hashcode to check. The removeAll method simply removes all the associations. Hence all the operations revolve around the hashcode and understanding its implementation is the key to all this. For more read Effective Java by Joshua Blosch. read less

Hashmap will work based on hashing technique,HashMap in Java works on hashing principle. It is a data structure which allows us to store object and retrieve it in constant time O(1) provided we know the key. In hashing, hash functions are used to link key and value in HashMap. Objects are stored by calling put(key, value) method of HashMap and retrieved by calling get(key) method. When we call put method, hashcode() method of key object is called so that hash function of map can find a bucket location to store value object, which is actually index of internal array, known as table. HashMap internally store mapping in form of Map. read less

Hash table based implementation of the Map interface. This implementation provides all of the optional map operations, and permits null values and the null key. (The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.) This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time. This implementation provides constant-time performance for the basic operations (get and put), assuming the hash function disperses the elements properly among the buckets. Iteration over collection views requires time proportional to the "capacity" of the HashMap instance (the number of buckets) plus its size (the number of key-value mappings). Thus, it's very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important. An instance of HashMap has two parameters that affect its performance: initial capacity and load factor. The capacity is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created. The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the hash table is rehashed (that is, internal data structures are rebuilt) so that the hash table has approximately twice the number of buckets. read less

Note : HashMap and LinkedHashMap stores the elements according to Hashtable algorithm only. In Map interface, elements will be stored using keys. Keys are always unique(duplicates not allowed). Keys are also objects. Duplicate elements are allowed with unique keys. "null" key is allowed in HashMap and LinkedHashMap. "null" object is also allowed. It is allowed to store the elements with heterogenous keys(but unique). Duplicates will be replaced but not rejected. HashMap : 1) available in util package 2) since from jdk 1.2 3) not ordered 4) duplicates allowed with unique keys 5) default capacity is 16 6) default load factor is 0.75 7) HashMap is not synchronized by default. but can be synchronized explicitly as follows. Collections.synchronizedMap(new HashMap(...)); read less

TreeMap : 1) TreeMap maintains ascending order of keys. 2) TreeMap does it through Balanced binary trees. 3) null key is not allowed 4) only unique data type keys(homogenous) are allowed. 5) Using what type of key, we are storing the first element into map Object, remaining elements also must be inserted with the same type of Key. read less