What is the difference between abstract classes and interfaces?

Programming techniques

• How do I add two numbers in Java?
• How should I round a double up to an int?
• What is a recursive method?

Handling exceptions

• Can you give an example of catching an exception?
• How can I throw a NumberFormatException back to the main() method?

Java applications

• How can I make spelling checker with a LinkedList dictionary?
• How can I use SAX to parse a Web page via JTidy?

Network programming

• How can I download a Web page and write it to a file?
• How do I get a domain name from an IP address?
• My Java Web client gets the wrong site!
• How can I set a code name identifier for my feed reader?

Static context questions

• What is special about static variables?
• What is the difference between static and final keywords?
• I get "non-static variable this cannot be referenced from a static context"!

Language features

• Is Java a fully object oriented language?
• What are Java types are available?
• What's the difference between checked and un-checked exceptions?
• What is dynamic method dispatch?

Design concepts

• What's the difference between abstraction and encapsulation?
• What are keywords and reserved words?
• Is println overloading or overriding?

Design patterns

• What is a singleton?
• How do I format my price correctly?
• What is a factory method?
• How should I create an immutable class?

Compiler concepts

• Which class should be compiled first?

Compiler commands

• What are the steps in compiling a class?
• Where is my compiled class file?
• How can I ensure my compiler will locate the SAX package?

Compiler diagnostics

• What does this deprecation message mean?
• What are undefined and undeclared variables?

Development environment

• How do I set the compiler path?
• How do I set environment variables on Windows XP?
• How do I configure EditPlus to compile Java and capture output?

The static main method

• How can I write a program that takes command line input?
• What does public static void main(String[]) mean?

Main method arguments

• How many arguments can be passed on the command line?
• How can I print the main method arguments in reverse order?

Object methods

• How does an object call an inner class method?

Object design

• How can I count the number of instances of an object?
• Can objects be used in place of arrays?
• What's the difference between equals and ==?

Understanding constructors

• How do I invoke a constructor?
• How can I call a constructor from a constructor?

Java inheritance principles

• Why doesn't Java support multiple inheritance?
• Can you give an example of multiple inheritance with interfaces?

Extending classes

• What's the difference between importing and extending a class?

Overridden methods

• How can I find the inherited attributes of a class?

Visibility modifiers

• Can constructors have visibility modifiers?

Abstract classes and methods

• How can I call a concrete instance method on an abstract class?
• Can you give an example of an abstract class?

Interface concepts

• What is the difference between abstract classes and interfaces?

Interface usage

• What are the rules for passing subclasses for method arguments?
• When should I use abstract classes rather than interfaces?
• This code seems to be instantiating an interface!

The Java API

• When is hashCode() used?
• What is a NullPointerException in Java?
• How can I create a constructor to equate two other objects?
• How do I draw a Rectangle on a Graphics instance?

Java jargon explained

• What is the difference between the JVM, JRE and JDK?
• What is the difference between heavyweight and lightweight components?

Class loading

• How can I get a class reference without using new operator?
• What's the difference between the Class.forName() method and the new operator?
• How can I implement the Class.forName() scheme?
• How can I cast an unknown sub-class returned by newInstance()?
• Is Class.forName() runtime polymorphism?
• How does Class.forName(dbDriver) work with DriverManager.getConnection?

The String class

• Why is the String class declared final?

String methods and operators

• The == operator doesn't match strings correctly!
• How can I reverse the characters in a string?

String buffers and tokenizers

• Why don't two StringBuffers match?
• How can I pad a StringBuffer?
• What is a StringTokenizer for?

Regular expressions

• How can I check a string has no numbers?

Storage structures

• What is the difference between Hashtable and HashMap?

Working with dates and calendars

• How do I insert slashes in a date?
• How can I convert a string into a Date object?
• How can I get the difference between two dates in days?
• Why isn't my custom Calendar class called?

Read and write files

• How can I divert console output to a file?
• How can I generate an array from a list?

File system navigation

• How can I get the full path of Explorer.exe?
• How can I get the path to an installed program?
• How do I limit the scope of a file chooser?

Getting started with JDBC

• How do I use JDBC with Microsoft SQLServer?

Java SQL API

• What's the difference between Statement and PreparedStatement?
• How can I insert multiple records at a time?
• Why it is better to use PreparedStatement?

JDBC techniques

• How do you make a database insert using JDBC?
• How can I use system variables to make a database connection?

JDBC problems

• I get "cannot find symbol" for my executeUpdate(String) call!

SQL queries

• How can I select records between two dates?
• What is a database view?

Concurrent programming features

• Which class is the wait() method defined in?
• Why are there separate wait and sleep methods?

Threads and runnable types

• How does the run() method in Runnable work?
• A Thread is runnable, how does that work?

Multi-threaded design questions

• Do I need to use synchronized on setValue(int)?
• How do I create a Runnable with inheritance?

Servlet concepts

• How does the JVM execute a servlet compared with a regular Java class?

Servlet start up

• How do I compile a servlet?
• Why won't my servlet compile?
• Can I use a constructor in my servlet?
• Can I use a normal class to handle my requests?
• What is the difference between JAR and WAR files?

Servlet techniques

• What is URL-rewriting?
• What's the difference between forward, include and redirection?

General container questions

• Is the session ID stored in the servlet or container?

Web application configuration

• What is a servlet URL mapping?
• How do I load a servlet at start-up?
• How can I create database connections for all servlets?