javarevisited |
- How to create auto incremented identity column in SQL Server, MySQL, Sybase and Oracle ?
- How to convert String to long in Java - 4 Examples
- How to fix java.io.NotSerializableException: org.apache.log4j.Logger Error in Java
- How to initialize List with Array in Java - One Liner Example
- How to create and evaluate XPath Expression in Java:
| How to create auto incremented identity column in SQL Server, MySQL, Sybase and Oracle ? Posted: 25 Dec 2012 09:22 PM PST Automatic incremented ID, Sequence or Identity columns are those columns in any table whose value is automatically incremented by database based upon predefined rule. Almost all databases e.g. Microsoft SQL Server, MySQL, Oracle or Sybase supports auto incremented identity columns but in different ways like Oracle provides SEQUENCE object which can be used to generate automatic numbers, Microsoft SQL Server upto 2008 version provides IDENTITY() functions for similar purpose. Sybase also has IDENTITY function but little different than SQL Server and MySQL uses auto_incremented keyword to make any numeric column auto incremented. As first normal form advised about primary keys which is used to uniquely identity row and if there is no natural column or combination of column exists to act as primary key, mostly database developer use auto incremented surrogate keys which is used to uniquely identify each row. In this SQL tutorial we will see how to generate auto incremented ID column in Microsoft SQL Server, Oracle 11g, MySQL and Sybase ASE Server. By the way this SQL article is continuation of my earlier post on SQL and database like difference between truncate and delete in SQL and Finding second highest salary in MySQL and SQL Server. If you haven't got chance to read them than I suggest they are worth looking.         |
| How to convert String to long in Java - 4 Examples Posted: 25 Dec 2012 07:39 PM PST How to convert string to long in Java is one of those frequently asked questions by beginner who has started learning Java programming language and not aware of how to convert from one data type to another. Converting String to long is similar to converting String to Integer in Java, in-fact if you know how to convert String to Integer than you can convert String to Long by following same procedure. Though you need to remember few things while dealing with long and String first of all long is primitive type which is wrapped by Long wrapper class and String is an Object in Java backed by character array. Before Java 5 you need to take an extra step to convert Long object into long primitive but after introduction of autoboxing and unboxing in Java 5, Java language will perform that conversion for you. This article is in continuation of our previous conversion tutorial like how to convert String to Double in Java or how to convert String to Enum in Java. In this Java tutorial we will learn 3 different ways to convert String to long by code example and we will also analyze pros and cons of each approach.     |
| How to fix java.io.NotSerializableException: org.apache.log4j.Logger Error in Java Posted: 25 Dec 2012 07:39 AM PST java.io.NotSerializableException: org.apache.log4j.Logger error says that instance of org.apache.lo4j.Logger is not Serializable. This error comes when we use log4j for logging in Java and create Logger in a Serializable class e.g. any domain class or POJO which we want to store in HttpSession or want to serialize it. As we know from 10 Java Serialization interview question that, if you have a non serializable class as member in a Serializable class, it will throw java.io.NotSerializableException Exception. Look at the below code : public class Customer implements Serializable{ private Logger logger = Logger.getLogger(Customer.class) ...... } If instance of Customer will be stored in HttpSession or Serialized externally it will throw "java.io.NotSerializableException: org.apache.log4j.Logger" because here logger instance is neither static or transient and it doesn't implement Serializable or Externalzable interface. How to solve java.io.NotSerializableException: org.apache.log4j.Logger Solving java.io.NotSerializableException: org.apache.log4j.Logger is simple, you have to prevent logger instance from default serializabtion process, either make it transient or static. Making it static final is preferred option due to many reason because if you make it transient than after deserialization logger instance will be null and any logger.debug() call will result in NullPointerException in Java because neither constructor not instance initializer block is called during deserialization. By making it static and final you ensure that its thread-safe and all instance of Customer class can share same logger instance, By the way this error is also one of the reason Why Logger should be declared static and final in Java program. Just make following code change to fix java.io.NotSerializableException: org.apache.log4j.Logger in Java. public class Customer implements Serializable{ private static final Logger logger = Logger.getLogger(Customer.class) ...... } That's all on how to fix java.io.NotSerializableException: org.apache.log4j.Logger in Java. We have seen what cause java.io.NotSerializableException: org.apache.log4j.Logger, it's because Logger class is not Serializable but we also learned that there is no point serializing Logger instance and better to make Logger static and final. Other Serialization and troubleshooting articles from Javarevisited Blog     |
| How to initialize List with Array in Java - One Liner Example Posted: 25 Dec 2012 04:59 AM PST Initializing list while declaring it is very convenient for quick use. If you have been using Java programming language for quite some time then you must be familiar with syntax of array in Java and how to initialize an array in the same line while declaring it as show below: String[] oldValues = new String[] {"list" , "set" , "map"}; or even shorter : String[] values = {"abc","bcd", "def"}; Similarly we can also create List and initialize it at same line, popularly known as initializing List in one line example. Arrays.asList() is used for that purpose which returns a fixed size List backed by Array. By the way don't confuse between Immutable or read only List which doesn't allow any modification operation including set(index) which is permitted in fixed length List.Here is an example of creating and initializing List in one line :     |
| How to create and evaluate XPath Expression in Java: Posted: 25 Dec 2012 12:26 AM PST You can use XPathExpression from javax.xml.xpath package to create and execute XPATH expression in Java. Java API provides javax.xml.xpath package, which contains classes like Xpath, XpathFactory to work with XPATH and XML documents. By the way this is third article on Xpath, In last couple of tutorials e.g. XPATH examples to select values using attributes and my XPATH notes for Java programmer, we have seen basics of Xpath expression from XML and Java point of view. Anyone who has work with XML technologies in Java knows importance of XPATH, it is one the most useful technology to retrieve selected data from XML files. XPATH is like SQL which is used to retrieve data from relational database. Many back-office and middle-office Java application which transfers data in form of XML documents makes extensive use of XPATH expression to retrieve value for printing or for decision making. Though there are lot of open source XML libraries are available to assist with Java and XML development e.g. XML Beans. I personally prefer to use standard Java API if functionality is available there. In this Java tutorial we will learn how to create XPath expression in Java program and retrieving values from XPATH e.g. text, numbers and boolean values, which is critical for programming flow.     |
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