How HashMap works in Java?

Hello guys, if you are looking for an answer of popular Java interview question, how HashMap works in Java? or How HashMap works internally in Java then you have come to the right place. In this article, I will not just answer those question but also many related questions like How get() and put() method works in Java? what role equals() and hashcode() play in HashMap, How HashMap resize itself, and why key object of HashMap should be Immutable in Java. Let's start with the basics first. HashMap in Java works on hashing principles. It is a data structure that allows us to store object and retrieve it in constant time O(1) provided we know the key. Also this is one of my most popular article with more than 6 million views and I have now updated it to include latest information like how HashMap now uses a balanced tree instead of linked list for storing collision objects and new diagrams on HashMap working which provide visual explanations. 

Difference between fail-safe vs fail-fast Iterator in Java? Example

The difference between fail-safe and fail-fast Iterator is becoming favorite core java interview questions day by day, the reason it touches concurrency a bit, and the interviewee can go deep on it to ask how fail-safe or fail-fast behavior is implemented. In this article, we will see what are fail-safe and fail-fast iterators in java and the differences between fail-fast and fail-safe iterators. The concept of the fail-safe iterator is relatively new in Java and was first introduced with Concurrent Collections in Java 5 like ConcurrentHashMap and CopyOnWriteArrayList.

Difference between List and Set in Java Collection? Example

What is the difference between List and Set in Java is a very popular Java collection interview question and an important fundamental concept to remember while using the Collections class in Java. Both List and Set are two of the most important Collection classes Java Program use along with various Map implementations. The basic feature of List and Set are abstracted in the List and Set interface in Java and then various implementations of List and Set adds specific features on top of that e.g. ArrayList in Java is a List implementation backed by Array while LinkedList is another List implementation that works like linked list data-structure.

Difference between ArrayList and Vector in Java

ArrayList and Vector are two of the most used classes on the java collection package and the difference between Vector and ArrayList is one of the most frequently asked java interview questions on first-round or phone interviews. Though it’s quite a simple question in my opinion but knowledge of when to use Vector over ArrayList or does matter if you are working on a project. In this article, we will some point-based differences between Vector and ArrayList in Java and trying to understand the concept behind those differences. 

What is difference between HashMap and Hashtable in Java?

HashMap vs Hashtable in Java
Though both Hashtable and HashMap are data-structure based upon hashing and implementation of Map interface, the main difference between them is that HashMap is not thread-safe but Hashtable is thread-safe. This means you cannot use HashMap in a multi-threaded Java application without external synchronization. Another difference is HashMap allows one null key and null values but Hashtable doesn't allow null key or values. Also, the thread-safety of the hash table is achieved using internal synchronization, which makes it slower than HashMap.

Difference between LinkedList and ArrayList in Java

LinkedList and ArrayList both implement List Interface but how they work internally is where the differences lie. The main difference between ArrayList and LinkedList is that ArrayList is implemented using a resizable array while LinkedList is implemented using doubly LinkedList. ArrayList is more popular among Java programmers than LinkedList as there are few scenarios on which LinkedList is a suitable collection than ArrayList. In this article, we will see some differences between LinkedList and ArrayList and try to find out when and where to use LinkedList over ArrayList.

Difference between TreeSet, LinkedHashSet and HashSet in Java with Example

TreeSet, LinkedHashSet, and HashSet all are implementation of the Set interface and by virtue of that, they follow the contract of Set interface i.e. they do not allow duplicate elements. Despite being from the same type of hierarchy,  there are a lot of differences between them; which is important to understand, so that you can choose the most appropriate Set implementation based upon your requirement. By the way difference between TreeSet and HashSet or LinkedHashSet is also one of the popular Java Collection interview questions, not as popular as Hashtable vs HashMap or ArrayList vs Vector but still appears in various Java interviews.

Difference between PriorityQueue and TreeSet in Java? Example

The PriorityQueue and TreeSet collection classes have a lot of similarities e.g. both provide O(log(N)) time complexity for adding, removing, and searching elements, both are non-synchronized and you can get elements from both PriorityQueue and TreeSet in sorted order, but there is a fundamental difference between them, TreeSet is a Set and doesn't allow a duplicate element, while PriorityQueue is a queue and doesn't have such restriction. It can contain multiple elements with equal values and in that case head of the queue will be arbitrarily chosen from them.

Difference between Synchronized and Concurrent Collections in Java? Answer

Synchronized vs Concurrent Collections
Though both Synchronized and Concurrent Collection classes provide thread-safety, the differences between them come 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 like ConcurrentHashMap, CopyOnWriteArrayList, and CopyOnWriteHashSet. The main reason for this slowness is locking; synchronized collections lock the whole collection e.g. whole Map or List while concurrent collection never locks the whole Map or List.

How to choose the Right Collection Class in Java? List, Set, Map, and Queue Example

The Java collection framework offers implementation of different data structure like an array, list, set, map, queue, tree, etc and the choice really depends upon the situation and properties of the different data structure. For example, if your requirement is fast search with index then you can use ArrayList and if you want to store key-value pairs then you would consider using hash table data structure and there are a couple of implementation of hash table data structure in Java, like HashMap, Hashtable, LinkedHashMap, TreeMap, and ConcurrentHashMap. Now, which one will you choose? If you don't know or confused don't worry, I will give you a set of rules and use cases which will help you choose the right Collection type in Java depending upon scenario. 

Difference between HashMap, LinkedHashMap and TreeMap in Java with Example

Hello guys, if you are wondering what is difference between HashMap, TreeMap, and LinkedHashMap in Java and when to use them then you are at the right place. In past, I have shared frequently asked HashMap Interview questions and ConcurrentHashMAp Interview questions and today, I Will answer this question in detail. The java.util.Map is one of the most important interfaces from the Java Collection Framework.  It provides hash table data structure functionality by its implementations like HashMap, Hashtable, LinkedHashMap, and a little bit of sorting with the TreeMap. So if you are looking to store key-value pairs in the Java program,  you have a wide range of choices available depending upon your requirement. The main difference between LinkedHashMap, TreeMap, and HashMap comes in their internal implementation and specific features, which a Java programmer should know to use them effectively. 

How to get Key From Value in Hashtable, HashMap in Java? Example

It's not easy to get key from the value in Hashtable or HashMap, as compared to getting value from key because HashMap or Hashtable doesn't enforce one to one mapping between key and value inside Map in Java. in fact, Map allows the same value to be mapped to multiple keys inside HashMap, Hashtable, or any other Map implementation. What you have in your kitty is Hashtable.containsValue(String value) or Hashtable.containsKey(String key) to check whether key or value exists in HashMap or not, but sometimes we want to retrieve a value from Map corresponding to any key and there is no API method to do in Map.

Difference between HashMap and IdentityHashMap in Java? Example

Hello guys, if you are wondering what is differnece between an HashMap and IdentiyHashMap in Java then you are at right place. IdentityHashMap in Java was added in Java 1.4 but still, it's one of those lesser-known classes in Java. The main difference between IdentityHashMap and HashMap in Java is that IdentityHashMap is a special implementation of Map interface which doesn't use equals() and hashCode() method for comparing objects unlike other implementations of Map e.g. HashMap. Instead, IdentityHashMap uses the equality operator "=="  to compare keys and values in Java which makes it faster compared to HashMap and suitable where you need reference equality check and instead of logical equality.

What is difference between ArrayList and ArrayList<?> in Java?- Raw Type vs Wildcard Example Tutorial

One of my readers asked me about the difference between ArrayList vs ArrayList< in Java?>, which was actually asked to him in a recent Java development interview. The key difference between them is that ArrayList is not using Generics while ArrayList is a generic ArrayList but they look very similar. If a method accepts ArrayList or ArrayList<?> as a parameter then it can accept any type of ArrayList like ArrayList of String, Integer, Date, or Object, but if you look closely you will find that one is raw type while the other is using an unbounded wildcard. What difference that could make? Well, that makes a significant difference because ArrayList with raw type is not type-safe but ArrayList<?> with the unbounded wildcard is type-safe.

How to Filter Stream and Collections in Java 8? Example Tutorial

Java 8 provides excellent features to support the filtering of elements in Java Collections. Prior to Java 8, the only better way to filter elements is by using a foreach loop or iterating over Collection using the Iterator and selecting the required object, leaving out rest. Though that approach work, it was very difficult to run them in parallel and take advantage of multiple CPUs available in modern-day servers. Java 8 provides Streams, which not only makes it easy to run any operation parallel but also supports lazy loading and lazy evaluation, which means as soon as the filtering condition is satisfied, it stooped doing work, doesn't matter how many objects the collection contains.