yes this is common core java interview question but when you ask cross question then half of developer fails. This same question was asked to me during my interview with a Investment bank for Java FIX developer position.
You can also join these best Data Structures and Algorithms courses to brush up on some of your data structure and algorithm skills. This book contains questions from all important Java topics. If you are preparing for Java interviews then I suggest you take a look at the Java Interview Guide: 200+ Java Interview questions, an excellent resource for all levels of Java programmers. This ensures performance or order O(log(n)) even in the worst case where a hash function is not distributing keys properly. which always return the location of the same bucket, can turn a HashMap into a linked list, like converting the get() method to perform in O(n) instead of O(1) and someone can take advantage of this fact, Java now internally replace linked list to a binary true once a certain threshold is breached. Also, From JDK 1.8 onwards HashMap has introduced an improved strategy to deal with a high collision rate. 16.Īfter some research, the Java team found that most of these Map are temporary and never use that many elements, and only end up wasting memory. Earlier, when you create HashMap like new HashMap() it automatically creates an array of default length e.g. Due to this empty Maps are lazily initialized and will cost you less memory. HashMap Changes in JDK 1.7 and JDK 1.8 There is some performance improvement done on HashMap and ArrayList from JDK 1.7, which reduces memory consumption. You can also check the following articles from Javarevisited to learn more about HashMap and Hashtable in Java: Since the finance domain used Java heavily and due to performance reasons we need caching HashMap and ConcurrentHashMap comes as very handy there. 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.
Compute method map code#
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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