Sunday, May 20, 2018

Java Basic Interview Questions for Beginners

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Java Basic Interview Questions for Beginners

Can you help me debug my program?

If the problem you are having is not directly related to our software, we suggest that you use a Java development tool that helps you with debugging, and that you invest in some books or training to help you learn Java. There are many ways to build debugging into your program, and getting good training in Java programming is a good start to understanding how to do this.

Where can I get help learning Java?

There are a lot of books and online references to Java. A good starting place is at the JavaSoft website documentation index, which has links to white papers and the Java Tutorial. You can find books on Java at any of the major online book shopping sites.

Where do I get the JDK?

WebLogic 6.1 bundles JDK131. See our Platform support page for information about specific JDKs that we have tested and certified for use with WebLogic software.

Once you determine which version of the JDK you will be using, go to the JavaSoft website. Many platform vendors provide an optimized version of the JDK for their computers.

How do I set up my CLASSPATH?

Setting up your CLASSPATH correctly depends on what you are trying to do. The most common tasks are described below:

Starting WebLogic Server. See Setting the Classpath Option in the Starting and Stopping WebLogic Servers section of the Administration Guide. In addition, your WebLogic distribution includes shell scripts that you can use to start the server. These scripts, which are located in the domain directories under the config directory of your WebLogic Server distribution, automatically set up the CLASSPATH variable in the shell before starting the server.

Compiling Application Classes or Using WebLogic Server Utilities. See Setting the Classpath for Compiling in the Developing WebLogic Server Components section of Developing WebLogic Server Applications.

Working With WebLogic Server Code Examples. See the WebLogic Server Examples Guide located at samples/examples/examples.html in your WebLogic Server distribution.

Why won’t the examples work?

Usually problems with examples are related to your environment. Here are some troubleshooting hints:

1. If you are using a database, make sure you have run the utility utils.dbping to verify that your JDBC driver is correctly installed and configured.
2. Run the setEnv script to make sure your CLASSPATH is correctly set in the shell or DOS window in which you are running the examples. For more information, see Setting your development environment.
3. Check the instructions for the examples to make sure you have changed any user-specific variables in the code before compiling.
4. Verify that you are compiling with the -d option to direct the class files into the proper directory, as defined in the example instructions.
If the example is an applet, check the CODE and CODEBASE, and make sure WebLogic Server is running.

Where can I get help with Java error messages?

Many questions we receive at BEA are related to generic Java error messages and are not specific to WebLogic Server. Here are some links that contain helpful information about Java error messages.

Source Description

Sun’s Java Developer Connection
This forum includes Questions and Answers on a wide variety of Java topics, including error messages. Use the Search box for fast results. For example, type “classpath error” in the Search box.

Compiler Error Messages
An extensive list of compiler error messages, including the infamous NoClassDefFoundError.
Sun’s Java APIs:-
Check the Java API to see if there is an exception description for the class you are using.

Why did a client-server message generate a StackOverflowException?

If you are sending a particularly large data structure using, you may exceed the per-thread size limit for either the Java or native stack. You can increase the stack size by using the following command line options:

-ss Stacksize to increase the native stack size or
-oss Stacksize to increase the Java stack size,
where Stacksize is expressed as an integer followed by “k” or “m” for kbytes or mbytes. 

For example

$java -ss156k (native)
$java -oss600k (Java)

The default native stack size is 128k, with a minimum value of 1000 bytes. The default java stack size is 400k, with a minimum value of 1000 bytes.

Will a JIT make my Java application run faster?

A Just-In-Time compiler will make some Java applications run faster. A JIT works by storing generated machine code in memory and reusing it when possible. For example, if you execute the same operation 1000 times in a loop, a JIT will improve performance of this operation since the code will only be generated once. Applications with a lot of native methods will not see as much performance improvement as pure-Java applications.

If you use a JIT, you may want to turn off the JIT during debugging to facilitate stack tracing. If you are doing performance testing with a JIT, make sure that you execute the same test several times in the same invocation and then throw away the first result to get an idea of how long the transaction will take when your application is running in a steady state. The first time the code is executed, your test will take longer (the “code generation hit”).

Can I redistribute the JDK that is bundled with WebLogic Server?

BEA Systems has the non-exclusive right to grant a third party, such as an independent software vendor (ISV), the right to redistribute the JDK that is bundled with WebLogic Server without any modifications of any kind. The following are caveats to this general statement:

The ISV cannot remove or alter any proprietary legends or notices contained in the JDK. The ISV shall not decompile, disassemble, decrypt, extract, or otherwise reverse engineer or modify the JDK. The JDK may not be leased or assigned in whole or in part.

The ISV must enter into a signed agreement with its distributors on terms substantially similar to those contained here in this redistribution policy explanation.

The ISV requires an end user license agreement with the product within which it embeds WebLogic Server.

The embedding of the JDK in ISV products does not include maintenance and support of the JDK by the JDK Provider. BEA Systems shall be solely responsible for providing maintenance and support for its ISVs and distributors. The ISV shall be solely responsible for providing maintenance and support for the end users of its products.

If an ISV wants to ship a JDK that is different from the JDK that BEA ships with WebLogic Server, that ISV needs to get those bundling rights directly from Sun or HP. 

For example:

Assume that BEA ships WebLogic Server 6.0 with JDK 1.3 only, that BEA ships WebLogic Server 5.1 with JDK 1.1 only, and that an ISV wants to ship JDK 1.1 with WebLogic Server 6.0 in an integrated product offered by that ISV. Unless BEA, for BEA business reasons, elects to make WebLogic Server 6.0 with JDK 1.1 generally available, the ISV couldn’t ship JDK 1.1 with WebLogic Server 6.0 in an integrated product offered by that ISV under BEA’s agreement with that ISV or under BEA’s agreement with Sun. The ISV could, however, obtain its own binary distribution agreement for the JDK from Sun and under that agreement bundle JDK 1.1 with its value added software solution consisting of the ISV applications and WebLogic Server 6.0 integrated.
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