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.Entry object which contains both key and value object.
When you want to retrieve the object, you call get() method and again pass key object. This time again key object generate same hash code (it's mandatory for it to do so to retrieve object and that's why HashMap keys are immutable e.g. String) and we end up at same bucket location. If there is only one object then it is returned and that's your value object which you have stored earlier.
Since internal array of HashMap is of fixed size, and if you keep storing objects, at some point of time hash function will return same bucket location for two different keys, this is called collision in HashMap. In this case, a linked list is formed at that bucket location and new entry is stored as next node.
If we try to retrieve object from this linked list, we need an extra check to search correct value, this is done by equals() method. Since each node contains an entry, HashMap keep comparing entry's key object with passed key using equals() and when it return true, Map returns corresponding value. Since searching in lined list is O(n) operation, in worst case hash collision reduce a map to linked list. This issue is recently addressed in Java 8 by replacing linked list to tree to search in O(logN) time.
How to Reset Arraylist In Java
How HashMap Work in Java
Why wait (), notify () and notifyAll () must be called from synchronized block or method in Java
XPath to locate Information in XML
Internals of Garbage Collector
Reference Type in Java
Different Ways to Create ObjectClass Loaders in Java
Producer Consumer Problem
Why String is Final in Java
Singleton Class using Enum
JSON tutorial
Exceptional Handling in Java
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.Entry object which contains both key and value object.
When you want to retrieve the object, you call get() method and again pass key object. This time again key object generate same hash code (it's mandatory for it to do so to retrieve object and that's why HashMap keys are immutable e.g. String) and we end up at same bucket location. If there is only one object then it is returned and that's your value object which you have stored earlier.
Since internal array of HashMap is of fixed size, and if you keep storing objects, at some point of time hash function will return same bucket location for two different keys, this is called collision in HashMap. In this case, a linked list is formed at that bucket location and new entry is stored as next node.
If we try to retrieve object from this linked list, we need an extra check to search correct value, this is done by equals() method. Since each node contains an entry, HashMap keep comparing entry's key object with passed key using equals() and when it return true, Map returns corresponding value. Since searching in lined list is O(n) operation, in worst case hash collision reduce a map to linked list. This issue is recently addressed in Java 8 by replacing linked list to tree to search in O(logN) time.
Read More
Java 8 FeaturesHow to Reset Arraylist In Java
How HashMap Work in Java
Why wait (), notify () and notifyAll () must be called from synchronized block or method in Java
XPath to locate Information in XML
Internals of Garbage Collector
Reference Type in Java
Different Ways to Create ObjectClass Loaders in Java
Producer Consumer Problem
Why String is Final in Java
Singleton Class using Enum
JSON tutorial
Exceptional Handling in Java
