Top Most Difference between EnumMap and HashMap in Java

HashMap vs EnumMap in Java

What is difference between EnumMap and HashMap in Java is the latest Java collection interview question which has been asked to couple of my friends. This is one of the tricky Java question, specially if you are not very much familiar with EnumMap in Java, which is not uncommon, given you can use it with only Enum keys. Main difference between EnumMap and HashMap is that EnumMap is a specialized Map implementation exclusively for Enum as key. Using Enum as key, allows to do some implementation level optimization for high performance which is generally not possible with other object's as key. We have seen lot of interview questions on HashMap in our article How HashMap works in Java but what we missed there is this question which is recently asked to some of my friend. Unlike HashMap, EnumMap is not applicable for every case but its best suited when you have Enum as key. We have already covered basics of EnumMap and some EnumMap example in my last article What is EnumMap in Java and In this post we will focus on key differences between HashMap and EnumMap in Java.

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Top Most What is static import in Java 5 with Example

Static import in Java allows to import static members of class and use them, as they are declared in the same class. Static import is introduced in Java 5 along with other features like Generics, Enum, Autoboxing and Unboxing and variable argument methods. Many programmer think that using static import can reduce code size and allow you to freely use static field of external class without prefixing class name on that. For example without static import you will access static constant MAX_VALUE of Integer class as Integer.MAX_VALUE but by using static import you can import Integer.MAX_VALUE and refer it as MAX_VALUE. Similar to regular importstatements, static import also allows wildcard * to import all static members of a class. In next section we will see Java program to demonstrate How to use static import statements to import static fields.

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Top Most What is CountDownLatch in Java - Concurrency Example Tutorial

What is CountDownLatch in Java

CountDownLatch in Java is a kind of synchronizer which allows one Thread  to wait for one or more Threads before starts processing. This is very crucial requirement and often needed in server side core Java application and having this functionality built-in as CountDownLatch greatly simplifies the development. CountDownLatch in Java is introduced on Java 5 along with other concurrent utilities like CyclicBarrier, Semaphore, ConcurrentHashMap and BlockingQueue in java.util.concurrent package. In this Java concurrency tutorial we will  what is CountDownLatch in Java, How CountDownLatch works in Java, an example of CountDownLatch in Java and finally some worth noting points about this concurrent utility. You can also implement same functionality using  wait and notify mechanism in Java but it requires lot of code and getting it write in first attempt is tricky,  With CountDownLatch it can  be done in just few lines. CountDownLatch also allows flexibility on number of thread for which main thread should wait, It can wait for one thread or n number of thread, there is not much change on code.  Key point is that you need to figure out where to use CountDownLatch in Java application which is not difficult if you understand What is CountDownLatch in Java, What does CountDownLatch do and How CountDownLatch works in Java.

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Top Most What is CyclicBarrier Example in Java 5 – Concurrency Tutorial

What is CyclicBarrier in Java

CyclicBarrier in Java is a synchronizer introduced in JDK 5 on java.util.Concurrent package along with other concurrent utility like Counting Semaphore, BlockingQueue, ConcurrentHashMap etc. CyclicBarrier is similar to CountDownLatch which we have seen in last article  What is CountDownLatch in Java and allows multiple threads to wait for each other (barrier) before proceeding. Difference between CountDownLatch and CyclicBarrier is a also very popular multi-threading interview question in Java. CyclicBarrier is a natural requirement for concurrent program because it can be used to perform final part of task once individual tasks  are completed. All threads which wait for each other to reach barrier are called parties, CyclicBarrier is initialized with number of parties to be wait and threads wait for each other by calling CyclicBarrier.await() method which is a blocking method in Java and  blocks until all Thread or parties call await(). In general calling await() is shout out that Thread is waiting on barrier. await() is a blocking call but can be timed out or Interrupted by other thread. In this Java concurrency tutorial we will see What is CyclicBarrier in Java  and  an example of CyclicBarrier on which three Threads will wait for each other before proceeding further.

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Top Most 3 Example to print array values in Java - toString and deepToString from Arrays

Printing array values in Java or values of array element in Java would have been much easier if  arrays are allowed to directly prints its values whenever used inside System.out.println() or format and printf method, Similar to various classes in Java do this by overriding toString() method. Despite being an object, array in Java doesn't print any meaningful representation of its content when passed to System.out.println() or any other print methods. If you are using array in method argument or any other prominent place in code and actually interested in values of array then you don't have much choice than for loop until Java 1.4. Things has been changed since Java 5 because it introduced two extremely convenient methods for printing values of both primitive and object arrays in Java. Arrays.toString(array) and Arrays.deepToString(twoDimensionArray) can print values of any array. Main difference between Arrays.toString() and Arrays.deepToString  is that deepToString is used to print values of multidimensional array which is far more convenient than nesting of multiple for loops. In this Java tutorial we will see 3 different ways of printing array values in Java or value of element from Array in Java.

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Top Most How to write parametrized Generic class and method in Java Example

Parameterized Generic class and method in Java

Writing Generic parametrized class and method in Java is easy and should be used as much as possible. Generic in Java was introduced in version 1.5  along with Autoboxing, Enum, varargs and static import. Most of the new code development in Java uses type-safe Generic collection i.e. HashSet in place of HashSet but still Generic is underused in terms of writing own parametrized classes and method. I agree that most Java programmers has started using Generic while working with the Java collection framework but they are still not sure how Generic can allow you to write Template kind of classes which can work with any Type just like the parametrized ArrayList in Java which can store any Type of element. In the last couple of article  about Generics we have seen How Generic works in Java and  explored wild cards of Generic in Java and In this part of Java Generic example we will see How to write parametrized Generic Class and method in Java.

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Top Most Java Generics Tutorial: How Generics in Java works with Example

Java Generics Tutorial

Generics in Java is one of important feature added in Java 5 along with Enum, autoboxing and varargs , to provide compile time type-safety. Generics is also considered to be one of tough concept to understand in Java and somewhat it’s true as well. I have read many articles on generics in Java, some of them are quite good and detailed but I still felt that those are either too much technical or exhaustively detailed, so I thought to write a simple yet informative article on Java generics to give a head start to beginners without bothering there head too much. In this Java generics tutorial I will cover How Generics works in Java, Mysterious wild-cards in Generics and some important points about Generic in Java. I will try explaining generics concept in simple words and simple examples.

By the way I thought about writing on Java Generics when I completed my post on Advanced Example of Enum in Java. Since Enum and Generics are introduced at same time in JDK 5. If you like to read about generics than you can also check my other tutorials on generics e.g. 10 generics interview question in Java and Difference between bounded and unbounded wildcards in Generics.

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Top Most How to add leading zeros to Integers in Java – String left padding Example Program

From Java 5 onwards it's easy to left pad Integers with leading zeros when printing number as String. In fact, You can use same technique to left and right pad Java String with any characters including zeros, space. Java 5 provides String format methods to display String in various format. By using format() method we can add leading zeros to any Integeror String number. By the way this is also known as left padding Integerswith zero in Java. For those who are not familiar with left and right padding of String, here is a quick recap; When we add characters like zero or space on left of a String, its known as left padding and when we do the same on right side of String, it's known as right padding of String. This is useful when you are displaying currency amounts or numbers, which has fixed width, and you want to add leading zeros instead of space in front of Integers.

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Top Most Java Enum Tutorial: 10 Examples of Enum in Java

What is Enum in Java
Enum in Java is a keyword, a feature which is used to represent fixed number of well known values in Java, For example Number of days in Week, Number of planets in Solar system etc. Enumeration (Enum) in Java was introduced in JDK 1.5 and it is one of my favorite features of J2SE 5 among Autoboxing and unboxing , Generics, varargs and static import. Java Enum as type is more suitable on certain cases for example representing state of Order as NEW, PARTIAL FILL, FILL or CLOSED. Enumeration(Enum) was not originally available in Java though it was available in other language like C and C++ but eventually Java realized and introduced Enum on JDK 5 (Tiger) by keyword Enum. In this Java Enum tutorial we will see different Enum example in Java and learn using Enum in Java. Focus of this Java Enum tutorial will be on different features provided by Enum in Java and how to use them. If you have used Enumeration before in C or C++ than you will not be uncomfortable with Java Enum but in my opinion Enum in Java is more rich and versatile than in any other language. One of the common use of Enum which emerges is Using Enum to write Singleton in Java, which is by far easiest way to implement Singleton and handles several issues related to thread-safety, Serialization automatically.

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Top Most How to format String in Java – String format Example

String format and printf Example

How to format String in Java is most common problem developer encounter because classic System.out.println() doesn’t support formatting of String while printing on console. For those who doesn’t  know What is formatted String ? here is a simple definition,  Formatted String is a String which not only display contents but also display it in a format which is widely accepted like including comma while displaying large numbers e.g. 100,000,000 etc. Displaying formatted String is one of need for modern GUI application and thankfully Java has good support for formatting String and all other types like Integers, Double and Date. How to format a String in Java is never as easy as it has been since Java 1.5 which along-with front line features like Generics, Enum, Autoboxing and Varargs also introduces several utility method to support rich formatting of String in Java. prior to Java 5 java programmer relies java.text API for all there formatting need but with Java 5 we have now two more convenient way to format String in Java. JDK 1.5 has added format() method in java.lang.String class and provided a printf() method in PrintStream class for printing formatted output in console. printf() method is similar to C programming language printf() method and allows programmer to print formatting string directly to console, which makes System.out.printf() better alternative of System.out.println() method. Both format() and printf()  are overloaded method to support Locale specific formatting.

By the way this is the third article about formatting in Java , earlier we have seen Decimal Format examples and DateFormat examples for formatting numbers and dates in Java.

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Top Most How to Convert String to Integer to String in Java with Example

Converting String to integer and Integer to String is one of the basic tasks of Java and most people learned about it when they learn Java programming. Even though String to integer and Integer to String conversion is basic stuff but same time its most useful also because of its frequent need given that String and Integer are two most widely used type in all sort of program and you often gets data between any of these format. One of the common task of programming is converting one data type another e.g. Converting Enum to String or Converting Double to String, Which are similar to converting String to Integer. Some programmer asked question that why not Autoboxing can be used to Convert String to int primitive or Integer Object? Remember autoboxing only converts primitive to Object it doesn't convert one data type to other. Few days back I had to convert a binary String into integer number and then I thought about this post to document all the way I know to convert Integer to String Object and String to Integer object. Here is my way of converting String to Integer in Java with example :

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Top Most How Volatile in Java works ? Example of volatile keyword in Java

How to use Volatile keyword in Java

What  is Volatile variable in Java  and when to use Volatile variable in Java is famous multi-threading interview question in Java interviews. Though many programmer knows what is a volatile variable but they fail on second part i.e. where to use volatile variable in Java as its not common to have clear understanding and hands-on on volatile in Java. In this tutorial we will address this gap by providing simple example of volatile variable in Java and discussing some when to use Volatile variable in Java. Any way  Volatile keyword in Java is used as an indicator to Java compiler and  Thread that do not cache value of this variable and always read it from main memory. So if you want to share any variable in which read and write operation is atomic by implementation e.g. read and write in int or boolean variable you can declare them as volatile variable. From Java 5 along with major changes like Autoboxing, Enum, Generics and Variable arguments ,  Java introduces some change in Java Memory Model (JMM),  Which  guarantees visibility of changes made by one thread to another also as "happens-before" which solves the problem of memory writes that happen in one thread can "leak through" and be seen by another thread. Java volatile keyword cannot be used with method or class and it can only be used with variable. Java volatile keyword also guarantees visibility and ordering , after Java 5 write to any volatile variable happens before any read into volatile variable. By the way use of volatile keyword also prevents compiler or JVM from reordering of code or moving away them from synchronization barrier.

This Java tutorial on Volatile keyword is in continuation of my article How HashMap works in Java  and difference between HashMap and Hashtable in Java  , How Garbage collection works in Java and How Synchronization works in Java if you haven’t read already you may find some useful information based on my experience in Java .

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