In terms of usage, Java HashMap is very versatile and I have mostly used HashMap as a cache in an electronic trading application I have worked on. Since each node contains an entry, HashMap keeps comparing the entry's key object with the passed key using equals() and when it returns true, Map returns the corresponding value. If we try to retrieve an object from this linked list, we need an extra check to search for the correct value, this is done by the equals() method. In this case, a linked list is formed at that bucket location and a new entry is stored as the next node. Since the internal array of HashMap is of fixed size, and if you keep storing objects, at some point in time hash function will return the same bucket location for two different keys, this is called collision in HashMap. If you know how the hash table data structure works then this is a piece of cake. It's easy to answer this question if you have read a good book or course on data structure and algorithms like these data structure courses and books. Things get a little tricky when collisions occur. If there is only one object then it is returned and that's your value object which you have stored earlier. String) and we end up at the same bucket location.
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This time again key objects generate the same hash code (it's mandatory for it to do so to retrieve the object and that's why HashMap keys are immutable e.g. When you want to retrieve the object, you call the get() method and again pass the key object. HashMap internally stores mapping in the form of Map.Entry object which contains both key and value object. When we call the put method, the hashcode() method of the key object is called so that the hash function of the map can find a bucket location to store the value object, which is actually an index of the internal array, known as the table. Objects are stored by the calling put(key, value) method of HashMap and retrieved by calling the get(key) method. In hashing, hash functions are used to link keys and values in HashMap. It is a data structure that allows us to store object and retrieve it in constant time O(1) provided we know the key. HashMap in Java works on hashing principles.
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