Tuesday, July 31, 2012

How to create read only List, Map and Set in Java – unmodifiable example

Read only List, Map and Set in Java
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 get method or by using Iterator of List, This kind of List is good for 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 read only List will result in java.lang.UnSupportedOperationException in Java.  This read-only List example is based on Java 5 generics but also applicable  to other Java version like JDK 1.4 or JDK 1.3, just remove Generics code i.e. angle bracket which is not supported prior to Java 5. One common mistake programmer makes is that assuming fixed size List and read only List as same. As shown in our 3 example of converting Array to Array List , we can use Arrays.asList() method to create and initialized List at same line. List implementation returned by this method is fixed size and it doesn’t allow adding or removal of element but it is not read only because you can update objects by calling set(index) method. How to make a collection read only is also a popular Java collection interview question, which makes this Java collection tutorial even more important.

Saturday, July 28, 2012

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.

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.

Wednesday, July 25, 2012

When a class is loaded and initialized in JVM - Java

Classloading and initialization in Java
Understanding of when a class is loaded and initialized in JVM is one of the fundamental concept of Java programming language. Thanks to Java language specification we have everything clearly documented and explained, but many Java programmer still doesn't know when a class is loaded or when a class is initialized in Java. Class loading and initialization seems confusing and complex to many beginners and its true until having some experience in belt its not always easy to get into subtle details of How JVM works in Java. In this Java tutorial we will see when class loading occurs in Java and when and how class and interface are initialized in Java. I will not go into detail of ClasLoader or How ClassLoader works in  Java, that is subject of another post I am planning. just to keep this article focused and concise. There are several articles on Java fundamentals in Javarevisited like How HashMap works in Java and How Garbage collection works in Java. If you are interested you can also check those.

Monday, July 23, 2012

How to read file line by line in Java - BufferedReader Scanner Example Tutorial

Line by Line reading in Java using BufferedReader and Scanner
There are multiple ways to read file line by line in Java. Most simple example of reading file line by line is using BufferedReader which provides method readLine() for reading file. Apart from generics, enum and varargs Java 1.5  has also introduced several new class in Java API one of the  utility class is Scanner, which can also be used to read any file line by line in Java. Though BufferedReader is available in Java from JDK 1.1,  java.util.Scanner provides more utility methods compared to BufferedReader. Scanner has method like hasNextLine() and nextLine() to facilitate line by line reading of file's contents. nextLine() returns String similar to  readLine() but scanner has more utility methods like nextInt(), nextLong() which can be used to directly read numbers from file instead of converting String to Integer or other Number classes. By the way Scanner is rather new approach for reading file and BufferedReader is the standard one. This is my 6th tutorial on Java IO,  In last couple of post we have seen creating file and directory in Java, parsing XML files using DOM and how to read properties file in Java. In this post we will focus on reading file line by line in Java using both BufferedReader and Scanner.

Saturday, July 21, 2012

Difference between get and load in Hibernate

get vs load in Hibernate
Difference between get and load method in Hibernate is a one of the most popular question asked in Hibernate and spring interviews. Hibernate Session  class provides two method to access object e.g. session.get() and session.load() both looked quite similar to each other but there are subtle difference between load and get method which can affect performance of application. Main difference between get() vs load method is that get() involves database hit if object doesn't exists in Session Cache and returns a fully initialized object which may involve several database call while load method can return proxy in place and only initialize the object or hit the database if any method other than getId() is called on persistent or entity object. This lazy initialization can save couple of database round-trip which result in better performance. By the way there are many articles on interview questions in Java, you can use search button on top left to find them. Some of them like 20 design pattern interview questions and 10 Singleton pattern questions are my favorites, you may also like. Coming back to article, you can find more difference between load and get in rest of this article in point format but this is the one which really makes difference while comparing both of them. If you look at how get and load gets called its pretty identical.

Wednesday, July 18, 2012

What is Autoboxing and Unboxing in Java – Example Tutorial and Corner cases

What is Autoboxing in Java
Autoboxing and unboxing is introduced in Java 1.5 to automatically convert primitive type into boxed primitive( Object or Wrapper class). autoboxing allows you to use primitive and object type interchangeably in Java on many places like assignment, method invocation etc. If you have been using Collections like HashMap or ArrayList before Java 1.5 then you are familiar with the issues like you can not directly put primitives into Collections, instead you first need to convert them into Object only then only you can put them into Collections. Wrapper class like Integer, Double and Boolean helps for converting primitive to Object but that clutter the code. With the introduction of autoboxing and unboxing in Java this primitive to object conversion happens automatically by Java compiler which makes code more readable. But autoboxing and unboxing comes with certain caveats which needs to be understood before using them in production code and it becomes even more important because they are automatic and can create subtle bugs if you are not sure when autoboxing  in Java code occurs and when unboxing happens. This is my fifth article on features introduced in Java 5 after my post on Java EnumHow Generics works in Java and varargs example. In this Java tutorial we will see: What is autoboxing and unboxing in Java ?  When autoboxing and unboxing occurs in Java? and things to remember while dealing with primitives and objects in Java with code examples.

Friday, July 13, 2012

Why Enum Singleton are better in Java

Enum Singletons are new way to implement Singleton pattern in Java by using Enum with just one instance. Though Singleton pattern in Java exists from long time Enum Singletons are relatively new concept and in practice from Java 5 onwards after introduction of Enum as keyword and feature. This article is somewhat related to my earlier post on Singleton, 10 interview questions on Singleton pattern in Java where we have discussed common questions asked on interviews about Singleton pattern and 10 Java enum examples, where we have seen how versatile enum can be. This post is about why should we use Enum as Singleton in Java, What benefit it offers compared to conventional singleton methods etc.

Saturday, July 7, 2012

SubQuery Example in SQL – Correlated vs Noncorrelated

SubQuery in SQL is a query inside another query. Some time to get a particular information from database you may need to fire two separate sql queries, subQuery is a way to combine or join them in single query. SQL query which is on inner part of main query is called inner query while outer part of main query is called outer query. for example in below sql query

SELECT name FROM City WHERE pincode IN (SELECT pincode FROM pin WHERE zone='west')

section not highlighted is OUTER query while section highlighted with grey is INNER query. In this SQL tutorial we will see both Correlated and non correlated sub-query and there examples, some differences between correlated and noncorrelated subqueries and finally subquery vs join which is classic debatable topic in SQL. By the way this SQL tutorial is next in series of SQL and database articles in Javarevisited like truncate vs delete and 10 examples of  SELECT queries. If you are new here then you may find those examples interesting.