How to use ConcurrentHashMap in Java - Example Tutorial and Working

ConcurrentHashMap in Java is introduced as an alternative of Hashtable in Java 1.5 as part of 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. ConcurrentHashMap performs better than earlier two because it only locks a portion of Map, instead of whole Map, which is the case with Hashtable and synchronized Map. CHM allows concurred read operations and the same time maintains integrity by synchronizing write operations. We have seen basics of ConcurrentHashMap on Top 5 Java Concurrent Collections from JDK 5 and 6 and in this Java tutorial, we will learn:

Ø       How ConcurrentHashMap works in Java or how it is implemented in Java.
Ø       When to use ConcurrentHashMap in Java
Ø       ConcurrentHashMap examples in Java
Ø       And some important properties of CHM.

How to get Key from Value in Hashtable, HashMap or Map in Java

It's not easy to get key from 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 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 Hashtable 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. We can still do this, but it highly depends data in your Map because Hashtable and HashMap both allows duplicate values mapped to the different key. In this Java tutorial, we will see an example of how to get key from value in Hashtable in Java.

2 ways to combine Arrays in Java – Integer, String Array Copy Example

There are multiple ways to combine or join two arrays in Java, both for primitive like int array and Object e.g. String array. You can even write your own combine() method which can use System.arrayCopy() to copy both those array into the third array. But being a Java developer, I first looked in JDK to find any method which concatenates two arrays in Java. I looked at java.util.Arrays class, which I have used earlier to compare two arrays and print arrays in Java, but didn't find a direct way to combine two arrays. Then I looked into Apache Commons, ArrayUtils class, and bingo, it has several overloaded method to combine int, long, float, double or any Object array. Later I also found that Guava ,earlier known as Google collections also has a class ObjectArrays in the package, which can concatenate two arrays in Java. It's always good to add Apache commons and Guava, as supporting library in Java project, they have lots of supporting classes, utility and method, which complements rich Java API. In this Java programming tutorial, we will see an example of these 2 ways to combine, join or concatenate two arrays in Java and will write a method in core Java to concatenate two int arrays in Java.

How to Swap Two Numbers without Temp or Third variable in Java - Interview Question

How to swap two numbers without using temp or third variable is common interview question not just on Java interviews but also on C and C++ interviews. It is also a good programming questions for freshers. This question was asked to me long back and didn't had any idea about how to approach this question without using temp or third variable, may be lack of knowledge on bitwise operators in Java or may be it didn't click at that time. Given some time and trial error, I eventually come out with a solution with just arithmetic operator but interviewer was keep asking about other approaches of swapping two variables without using temp or third variable. Personally, I liked this question and included in list of my programming interview question because of its simplicity and some logical work, it force you to do. When learned bit-wise operation in Java I eventually find another way of swapping two variables without third variable, which I am going to share with you guys.

How to install JDK 7 on Windows 8 - Java Programming Tutorial

Installing JDK is first step in learning Java Programming. If you are using Windows 8 or Windows 7 Operating System, than installing JDK is quite easy as you just need to follow instruction given by Java SE Installation wizard. Only thing which requires some attention is, choosing correct JDK installer based upon, whether you are running with 32-bit or 64-bit Windows 8 or Windows 7 OS. JDK 7 is latest Java version but JDK 6 is still most popular in software and programming world. You can choose to install JDK 7 or JDK 6 based upon your course material. In this Java tutorial, we will learn how to install JDK 7 in Windows 8 operating system by following step by step guide. Another thing, which is part of JDK installation is setting PATH for Java in Windows 8, this will enable to run javac and java command from any directory in Windows 8. See that link for step by step guide on setting PATH in Windows 8. Though, I will use Windows 8 operating system to install JDK 7, You can still install JDK 6 or 7 in Windows 7 by following those steps. In order to choose correct JDK installer, we first need to check if Windows is running with 32-bit or 64-bit Windows 8. For that you can go to Control Panel-->System and Security and check System Type, if it shows 32-bit operating system means you need Windows x86 Java Installer, if it shows 64-bit operating system than you need Windows x64 Java Installer. Let's see steps involved to install JDK 6 or JDK 7 in Windows 8 operating system.

How to create and call stored procedure in MySQL with IN and OUT parameters

It's hard to remember exact syntax of, how to create stored procedure in MySQL, until you are creating and working on stored procedure frequently, simply because syntax is not a one liner. You need to remember exact syntax, if you are using MySQL database from command line. What help in this case is, quick examples. In last couple of MySQL tutorial we have seen How to find second highest salary and How to join 3 tables in one query; In this MySQL tutorial we will see couple of examples of creating stored procedure and calling stored procedure using IN and OUT parameters. All these MySQL examples are simple and help you to understand syntax of creating stored procedure with parameters in MySQL. These examples are tested in MySQL 5.5 database. We will also use following employee table to create and test these stored procedures :

How to set Java Path and Classpath in Windows 8 and Windows 7 - Tutorial

So, you just bought a new PC or Laptop with Windows 8 operating system, and wondering how to set PATH and Classpath on Windows 8; Or, you might have just upgraded your windows 7 laptop to professional edition of Windows 8 and looking to set JDK Path to compile Java programs. Not to worry, this is the second step for anyone who wants to learn Java programming. Of course, the first step is to install JDK.  In this Java tutorial, we will see step by step guide to set Java PATH and CLASSPATH in Windows 8 operating system. By the way, if you are learning Java in Windows 7 operating system, You should still be able to set Java path and classpath in Windows 7 by following steps given here, because navigation steps for modifying environment variables on Windows 7 and Windows 8 are almost same.

How to convert XMLGregorianCalendar to Date to XMLGregorianCalendar in Java - Example Tutorial

There are several ways to convert XMLGregorianCalendar to Date in Java. You can convert XMLGregorianCalendar to either java.util.Date or java.sql.Date based upon your need. JAXB (Java API/Architecture for XML Bindings) is a popular framework to create XML documents from Java Objects and Java objects from XML files. JAXB also helps to create Java classes from XML Schema file (.XSD file). By default JAXB maps XSD data type xs:date, xs:time and xs:dateTime to XMLGregorianCalendar in Java, but you can configure XJC to create java.util.Date objects instead of javax.xml.datatype.XMLGregorianCalendar. Since java.util.Date is most popular way of dealing with date and time in Java, we often need to convert XMLGregorianCalendar instance to Date instance in Java. Thankfully by using Java API, we can easily do this conversion of XMLGregorianCalendar to Date and Date to XMLGregorianCalendar in Java. By the way, It's good to remember that XML Schema has three different types which can represent either date, time or both, while java.util.Date contains date and time information together. In this Java and XML tutorial, we will see example of converting XMLGregorianCalendar to Date and inverse in Java.

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 the 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 Integer or String number. By the way, this is also known as left padding Integers with 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 the 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.

5 JSTL Core IF Tag Examples in JSP - Tutorial

<c:if>  or if tag of JSTL core tag library in JSP is one of the most versatile and useful tag. JSTL if tag allows you
to test for a condition, like checking for a particular parameter in requestScope, sessionScope or pageScope. You can also  check any parameter in request parameters and headers or can check for a variable in JSP page using <c:if> tag. JSTL if tag helps a lot to reduce amount of Java code from JSP  page and if used, along with expression language JSTL core tag library, can remove almost all Java code from JSP files. Earlier we have seen examples of JSTL foreach tag and JSTL core set tag and this JSP JSTL tutorial is based on if tag of JSTL core tag library. We will, see how to use <core:if> tag inside JSP files and different example of <core:if> tag to get ourselves familiar with functionality and power offered by JSTL <c:if> tag. After seeing these examples of <core:if> tag along with expression language, You will be amazed to see, how clean your JSP looks like.

How to convert JSON String to Java object - Jackson Example

JSON stands for JavaScript object notation, is a lightweight text or string representation of an object and quickly becoming a popular data exchange format. Though it's pretty early to say that JSON is going to replace XML as popular data interchange format, It is certainly providing an alternative. JSON represent data in two format either an object or an array. JSON object is an unordered collection of key and value, similar to String representation of hash table. On the other hand, JSON Array is an ordered collection of values. The main difference between  JSON Object and  JSON array is there representation. JSON object is started with left brace { and ends with right brace } and key values are separated using a colon (:). On the other hand, JSON Array starts with left bracket [ and ends with right bracket ] and each value is separated by comma. By looking at structure, You can write  your JSON parser to parse JSON object or array to Java object, but you don't need to. 

How to Fix Connection refused: connect in Java Connection refused: connect is one of the most common networking exceptions in Java. This error comes when you are working with client-server architecture and trying to make TCP connection from the client to the server. Though this is not as cryptic as java.lang.OutOfMemoryError: Java heap Space or java.lang.UnsupportedClassVersionError, it’s still a frequent problem in distributed Java applications. Connection refused: connect also comes in the case of RMI (Remote Method Invocation) because RMI also uses TCP-IP protocol underneath. While writing client socket code in Java, You should always provide proper handling of this exception

Top 5 Concurrent Collections from JDK 5 and 6 Java Programmer Should Know

Several new Collection classes are added in Java 5 and Java 6 specially concurrent alternatives of standard synchronized ArrayList, Hashtable and  synchronized HashMap collection classes. Many Java programmer still not familiar with these new collection classes from java.util.concurrent package and misses a whole new set of functionality which can be utilized to build more scalable and high performance Java application. In this Java tutorial we will some of useful collection classes e.g. ConcurrentHashMap, BlockingQueue which provides some of the very useful functionalities to build concurrent Java application. By the way this is not a comprehensive article explaining each feature of all these concurrent collections, Instead I will just try to list out why they are there, which Collection class they replace or provides alternative for. Idea is to keep it short and simple while highlighting key points of those useful java.util.concurrent collections.

How to Join Multiple Threads in Java - Thread Join Example

Join method from Thread class is an important method and used to impose order on execution of multiple Threads. The concept of joining multiple threads is very popular on a mutithreading interview question. Here is one of such question, “You have three threads T1, T2, and T3, How do you ensure that they finish in order T1, T2, T3 ?. This question illustrates power of join method on multithreaded programming. Unlike classical thread questions like difference between wait and sleep method or solving producer consumer problem in Java, This one is a bit tricky. You can do this by using join method, by calling T1.join() from T2 and T2.join() from T3. In this case thread, T1 will finish first, followed by T2 and T3. In this Java multithreading tutorial, we will have a closer look on join method with a simple example. Idea is to illustrate how join method works in simple words. By the way from Java 5 onwards you can also use CountDownLatch and CyclicBarrier classes to implement scenarios like one thread is waiting for other threads to finish their task.

How to disable submit button in HTML JavaScript to prevent multiple form submission

Avoiding multiple submission of HTML form or POST data is common requirement in Java web application. Thankfully, You can prevent multiple submission by disabling submit button in HTML and JavaScript itself, rather than handling it on server side. This is very common requirement while developing web application using Servlet and JSP or any other web technology. At same time there are lot of information and noise available in web and its very hard to find simple approach to disable submit button. When I search for simple way to disable submit button in HTML and JavaScript, I was overwhelmed by responses on forums and various online community. Those are good for intermediate HTML and JavaScript developer but a beginner might just get overwhelmed by amount of information presented and rather confused to use which approach will work, how exactly should I disable submit button to prevent multiple form submissions etc. That drives me to write this post and summarize the simplest possible approach to disable submit button using HTML and JavaScript. I see there could be issues related to browser compatibility which we can talk once we know the simplest approach and my intention is to add those issues and there remedy as and when I know about it to keep this article update, relevant and simple. By the way How to avoid multiple submission of HTML form is also a popular JSP Interview question and worth preparing.

What is Timer and TimerTask in Java – Tutorial Example

Timer in Java is a utility class which is used to schedule tasks for both one time and repeated execution. Timer is similar to alarm facility many people use in mobile phone. Just like you can have one time alarm or repeated alarm, You can use java.util.Timer to schedule one time task or repeated task. In fact we can implement a Reminder utility using Timer in Java and that's what we are going to see in this example of Timer in Java. Two classes java.util.Timer and java.util.TimerTask is used to schedule jobs in Java and forms Timer API. TimerTask is actual task which is executed by Timer. Similar to Thread in Java, TimerTask also implements Runnable interface and overrides run method to specify task details. This Java tutorial will also highlight difference between Timer and TimerTask class and explains how Timer works in Java. By the way difference between Timer and Thread is also a popular Java questions on fresher level interviews.

How to Fix Must Override a Superclass Method Error Eclipse IDE Java | @Override annotation With interface methods

One of annoying error while overriding Java method in Eclipse IDE is must override a superclass method error , This error is quite frequent when importing Java  projects and still I haven't seen any permanent solution of it. Must override a super class method error comes in Eclipse, if you have Java Class that implements interface and overrides method from interface and uses @Override annotation . Unfortunately Eclipse defaults to Java 1.5, which only recognize @Override annotation for overriding method from super class and not from interface. This creates lots of confusion between Java programmers as not every one is aware of this subtle difference of using @Override annotation with class and interface methods and they start looking classpath and path suspiciously. @Override annotation for interface methods are only available from Java 1.6 and if you use @Override while overriding interface method than you will get must override a superclass method error. In this Java and Eclipse tutorial we will see how to fix must override a superclass method error in eclipse but before that we will reproduce this error in a manner which will clear that Eclipse will complain only for methods which is overridden from interface.

5 ways to check if String is empty in Java - examples

String in Java is considered empty if its not null and it’s length is zero. By the way before checking length you should verify that String is not null because calling length() method on null String will result in java.lang.NullPointerException. Empty String is represented by String literal “”. Definition of empty String may be extended to those String as well which only contains white space but its an specific requirement and in general String with white space are not considered as empty String in Java. Since String is one of the most frequently used class and commonly used in method arguments, we often needs to check if String is empty or not. Thankfully there are multiple ways to find if String is empty in Java or not. You can also count number of characters in String, as String is represented as character array and decide if String is empty or not. If count of characters is zero than its an empty String. In this Java String tutorial we going to see 5 ways to find if any String in Java is empty or not. Here are our five ways to check empty String :