Java Programming Interview Questions

Java Programming Interview Questions

Recently I am taking a lot of interview for my organization. So I was in the search of some java programming test questions that are little bit tricky also.

Here I am providing five of the tricky questions I found interesting and need a closer look to understand. The explanation will be provided after the questions. Test your knowledge of java by trying to provide the answer of the below test questions.

Java Classloader

Java ClassLoader

The Java ClassLoader is one of the crucial but rarely used components of Java in Project Development. Personally I have never extended ClassLoader in any of my projects but the idea of having my own ClassLoader that can customize the Java Class Loading thrills me.

This article will provide an overview of Java Class Loading and then move forward to create a custom ClassLoader and use it.
What is a ClassLoader?

We know that Java Program runs on Java Virtual Machine (JVM). When we compile a Java Class, it transforms it in the form of bytecode that is platform and machine independent compiled program and store it as a .class file. After that when we try to use a Class, Java ClassLoader loads that class into memory.

There are three types of built-in Class Loaders in Java:

1. Bootstrap Class Loader – It loads JDK internal classes, typically loads rt.jar and other core classes for example java.lang.* package classes
2. Extensions Class Loader – It loads classes from the JDK extensions directory, usually $JAVA_HOME/lib/ext directory.
3. System Class Loader – It loads classes from the current classpath that can be set while invoking a program using -cp or -classpath command line options.

Java Class Loaders are hierarchical and whenever a request is raised to load a class, it delegates it to its parent and in this way uniqueness is maintained in the runtime environment. If the parent class loader doesn’t find the class then the class loader itself tries to load the class.

java read file, read file in java

Java Read File

In my last post, I was going through with different options we have for SimpleDateFormat. Today I am providing a program for reading file in Java.

I am using BufferedReader for reading the file because it buffers the input from the specified file. Without buffering, each invocation of read() or readLine() could cause bytes to be read from the file, converted into characters, and then returned, which can be very inefficient.

Here I am providing three functions.

1. Read complete file as String
2. Read file line by line and return list of String
3. Count the occurrence of a String in the given file.

SimpleDateFormat Example with Formats, Patterns and Date Conversion

SimpleDateFormat is the Java class for formatting and parsing dates with Locale support. You can format date to text and vice-versa using SimpleDateFormat class.

This class is very useful in formatting date to display in UI in specific pattern. SimpleDateFormat is the concrete class that extends DateFormat abstract class.

Date and Time formats are specified by patterns, which are nothing but a String. In pattern every letter has its specific meaning. For example, y is for year, M is for month in year and m is for minutes in Hour.

In the below attached code, we will see the different usage of SimpleDateFormat class with different string patterns.

AXIS2 Web Services Tutorial

Recently I was trying to upgrade my Apache Axis2 skills from 1.3 to latest version 1.5.4 and I was not able to find out any tutorial that is self-explanatory and covering the latest release. So it forms the basis of my post for Axis2 Web Services Tutorial.
Who Should Use This Tutorial

This tutorial is intended for Java programmers who are interested in developing and deploying Web Services using Apache Axis2.

The scope of this tutorial is to use Axis2 for creating web services and invoking the web service using Java client program and testing web service using Soap UI tool. Basic understanding of Java, Web Services, XML, Ant and application server (Tomcat) is required to understand the tutorial with ease.

Jsch Java Api

Java JSch Example to run shell commands on SSH Unix

Earlier I wrote a program to connect to remote database on SSH server. Today I am presenting a program that can be used to connect to SSH enabled server and execute shell commands. I am using JSch to connect to remote ssh server from java program.

java generate CSR, Java CSR

Java Generate CSR

Recently I had to write a program to generate Certificate Signing Request (CSR) using Java API. Here I am providing the steps I followed with Java Program to generate CSR. After that we will also make sure that its valid by validating it with verisign CSR validator tool.

How to install Oracle WebLogic on Mac and Creating WebLogic domain

Oracle Weblogic is one of the most widely used application server. Recently I have installed it on Mac OS X (10.6.4) system and created a domain. This article is intended to provide the steps for installation and creating domain and then starting and stopping domain with Terminal. I have setup Oracle Weblogic on my system but I am sure that the setup instructions will work for other versions also.
Prerequisite for Oracle Weblogic Server Installation

1. Oracle Weblogic setup – You can download it from Oracle website for development purpose.
2. JDK 6 – Oracle Weblogic 10.3 requires Java 6 and it comes with the default Mac OS X system. You can check whether its installed or not. It will be in “/System/Library/Frameworks/JavaVM.framework/Versions/1.6/Home” directory.

Java Read Url To String

Java Read URL to String

In my current project, I had a requirement to read the WSDL file from the URL and store it into database as Clob. There was no validation required, so it was kind of reading URL as String and then storing it into database table.

Here is the program I wrote to read URL to String.

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