creating and initializing List at the same time Sometimes we want to create and initialize a List like ArrayList or LinkedList in one line much like creating an array and initializing it on the same line. If you look at The array on Java programming language you can create and initialize both primitive and object arrays e.g. String array very easily in just one line but in order to create a List equivalent of that array, you need to type a lot of code. This is also one of the tricky Java question sometimes appears in Interview as Write Java code to create and initialize ArrayList in the same line. --- Java Interview questions and tutorials

Read-only List, Map and Set in Java A read-only List means a List where you can not perform modification operations like add, remove or set. You can only read from the List by using the get method or by using the Iterator of List, This kind of List is good for a certain requirement where parameters are final and can not be changed. In Java, you can use Collections.unModifiableList() method to create read-only List, Collections.unmodifiableSet() for creating read-only Set like read-only HashSet and similarly creating a read-only Map in Java, as shown in below example. Any modification in the read-only List will result in java.lang.UnSupportedOperationException in Java.   --- Java, Unix, Tibco RV and FIX Protocol Tutorial

Sometimes, we need to merge multiple lists into one before performing any operation, say Iteration or transformation. It's quite common to merge two lists, or combine them into a bigger list and there are multiple ways to do it. In this article, we will take a look at two simple way to join two lists in Java, you can further extend that idea to join any number of List or it's implementation e.g. ArrayList or LinkedList in Java. One way to merge multiple lists is by using addAll() method of java.util.Collection class, which allows you to add the content of one List into another List. By using the addAll() method you can add contents from as many List as you want, it's the best way to combine multiple List. --- Java, Unix, Tibco RV and FIX Protocol Tutorial

CopyOnWriteArraySet is little brother of CopyOnWriteArrayList class. These are special purpose collection classes which was added on JDK 1.5, along with their most popular cousin ConcurrentHashMap. They are part of concurrent collection framework and reside in java.util.concurrent package. CopyOnWriteArraySet is best suited as a read-only collection whose size is small enough to copy if some mutative operation happens, for example, you can use CopyOnWriteArraySet to store objects at the start-up of the application and let multiple application thread access them during application life time. If an new condition or object comes up during that time, it can also be added into this Set, with incurring cost of creating a new array. --- Java, Unix, Tibco RV and FIX Protocol Tutorial

What is EnumMap in Java EnumMap in Java is added on JDK  5 release along with other important features like  Autoboxing, varargs, and Generics. EnumMap is a specialized Map implementation designed and optimized for using Java Enum as key. Since enum can represent a type (like class or interface)  in Java and it can also override equals() and hashCode(), It can be used inside HashMap or any other collection but using  EnumMap brings implementation-specific benefits which are done for enum keys, In short EnumMap is optimized Map implementation exclusively for enum keys. As per Javadoc Enum is implemented using Arrays and common operations result in constant time. --- Java, Unix, Tibco RV and FIX Protocol Tutorial

PriorityQueue is an unbounded Queue implementation in Java, which is based on a priority heap. PriorityQueue allows you to keep elements in a particular order, according to their natural order or custom order defined by the Comparator interface in Java. Head of priority queue data structure will always contain the least element with respect to specified ordering. For example, in this post, we will create a PriorityQueue of Items, which are ordered based upon their price, this will allow us to process Items, starting from the lowest price. A priority queue is also very useful in implementing the Dijkstra algorithm in Java. You can use to PriorityQueue to keep unsettled nodes for processing. --- Java, Unix, Tibco RV and FIX Protocol Tutorial

The ConcurrentHashMap class part of concurrent collections package added on JDK 1.5 which contains utility classes like BlockingQueue, CopyOnWriteArrayList, CopyOnWriteArraySet etc. It is a replacement of synchronized hash-based map implementations e.g. Hashtable and synchronized HashMap. It implements Map and ConcurrentMap (a sub-interface of Map) interface which allows you to store key-value pairs. The class is similar to HashMap or Hashtable but it's more scalable and the right fit for concurrent Java application. Unlike Hashtable which achieves its thread-safety by compromising the scalability, ConcurrentHashMap uses advanced techniques e.g. dividing the map into segments to remain thread-safe and scalable at the same time. --- Java, Unix, Tibco RV and FIX Protocol Tutorial

EnumSet is one of the specialized implementations of the Set interface for an enumeration type, introduced in Java 1.5 along with the enumeration type itself. Programmer often stores Enum into common collection classes e.g. HashSet or ArrayList, mostly because they are unaware of this little gem. Even I wasn't aware of this class a few years ago until I come across one of the finest books for Java programmers, Effective Java. It has an Item on EnumSet, which highlights some typical use-cases for this collection class instead of using int variable and bitwise operator. Since Enum constants are unique and have pre-defined lengths, as you can not define a new enum constant at runtime; it allows Java API designers to highly optimize EnumSet. --- Java, Unix, Tibco RV and FIX Protocol Tutorial

ConcurrentHashMap in Java is introduced as an alternative of Hashtable in Java 1.5 as part of the Java concurrency package. Prior to Java 1.5 if you need a Map implementation, which can be safely used in a concurrent and multi-threaded Java program, then, you only have Hashtable or synchronized Map because HashMap is not thread-safe. With ConcurrentHashMap, now you have a better choice; because not only it can be safely used in the concurrent multi-threaded environment but also provides better performance over Hashtable and synchronizedMap. --- Java, Unix, Tibco RV and FIX Protocol Tutorial

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. --- Java, Unix, Tibco RV and FIX Protocol Tutorial

There are two ways to search an element in a List class, by using contains() method or by using Collections.binarySearch() method. There are two versions of binarySearch() method, one which takes a List and Comparator and other which takes a List and Comparable. This method searches the specified list for the specified object using the binary search algorithm. The list must be sorted into ascending order according to the natural ordering of its elements (as by the sort(List) method) prior to making this call. If List is not sorted, then results are undefined. If the List contains multiple elements equal to the specified object, there is no guarantee which one will be returned. This method runs in log(n) time for a "random access" list (which provides near-constant-time positional access). --- Java, Unix, Tibco RV and FIX Protocol Tutorial

From Java 5 onwards, we have a for-each loop for iterating over collection and array in Java. For each loop allows you to traverse over collection without keeping track of index like traditional for loop, or calling hasNext() method in while loop using Iterator or ListIterator. For-each loop indeed simplified iteration over any Collection or array in Java, but not every Java programmer is aware of some useful details of the for-each loop, which we will see in this tutorial. Unlike other popular items from Java 5 release alias Generics, Autoboxing, and variable arguments, Java developers tend to use for-each loop more often than any other feature, but when asked about how does advanced foreach loop works or what is a basic requirement of using a Collection in the for-each loop, not everyone can answer. --- Java, Unix, Tibco RV and FIX Protocol Tutorial

It's easy to sort an array on increasing order by using Arrays.sort() method, which by default sort the array on increasing order, but if you have to sort array into decreasing order then how do you it? For example, if you have an array of Strings {"a", "b", "c"} how do you sort them into descending order? Well, if you remember, Arrays.sort() is overloaded and there is another version, which accepts a Comparator. To sort the array in decreasing order, you can simply providing a comparator which compares element in the reverse order. Thankfully there is one such Comparator already exists in JDK API, Collections.reverseOrder().  --- Java, Unix, Tibco RV and FIX Protocol Tutorial