How to Iterate through ConcurrentHashMap and print all keys and values in Java

Suppose you have a ConcurrentHashMap of String and Integer and you want to print all keys and values, how do you that? This is a common, day to day programming task for Java programmer and there are many ways to do it. The Map interface provides several view methods e.g. keySet(), values(), and entrySet() to retrieve all keys, values, and all key and value pairs as entries. You can use respective methods to print all keys, all values, or all key values pairs. For printing, you also have multiple choice e.g. you can either use enhanced for loop or Iterator, though later also provide you the facility to remove key value pairs while printing if needed. Though you should remember that these views are backed by Map, so when you remove a key-value pair from entry set it will also be removed by the ConcurrentHashMap.

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What is difference between Synchronized and Concurrent Collections in Java?

Synchronized vs Concurrent Collections
Though both Synchronized and Concurrent Collection classes provide thread-safety, the differences between them comes  in performance, scalability and how they achieve thread-safety. Synchronized collections like synchronized HashMap, Hashtable, HashSet, Vector, and synchronized ArrayList are much slower than their concurrent counterparts e.g. ConcurrentHashMap, CopyOnWriteArrayList, and CopyOnWriteHashSet. Main reason for this slowness is locking; synchronized collections locks the whole collection e.g. whole Map or List while concurrent collection never locks the whole Map or List. They achieve thread safety by using advanced and sophisticated techniques like lock stripping. For example, the ConcurrentHashMap divides the whole map into several segments and locks only the relevant segments, which allows multiple threads to access other segments of same ConcurrentHashMap without locking.

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9 difference between Array vs ArrayList in Java

Both array and ArrayList are two important data structure in Java and frequently used in Java programs. Even though ArrayList is internally backed by an array, knowing the difference between an array and an ArrayList in Java is critical for becoming a good Java developer. If you know the similarity and differences, you can judiciously decide when to use an array over an AraryList or vice-versa. In this article, I'll help you understand the difference between ArrayList and array in Java. If you are coming from C or C++ background then you already know that array is one of the most useful data structure in the programming world. It offers O(1) performance for index-based search and one of the fundamental way to store data.

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How does Java HashMap or LinkedHahsMap handles collisions?

Prior to Java 8, HashMap and all other hash table based Map implementation classes in Java handle collision by chaining, i.e. they use linked list to store map entries which ended in the same bucket due to a collision. If a key end up in same bucket location where an entry is already stored then this entry is just added at the head of the linked list there. In the worst case this degrades the performance of the get() method of HashMap to O(n) from O(1). In order to address this issue in the case of frequent HashMap collisions, Java8 has started using a balanced tree instead of linked list for storing collided entries. This also means that in the worst case you will get a performance boost from O(n) to O(log n).

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