Explain DTD

A document type definition (DTD) describes the permissible tags of an XML document.

The DTD serves as a data template. It defines entities, elements, attributes, and notations, as well as the relationships between these.

For example, the DTD can state that a memo element consists of To, From, Subject, and Message elements.

You need to use a DTD if you want the XML processor to validate your XML documents.

DTDs can help you ensure that your XML documents are well-formed.

You need to indicate to the XML processor that a DTD should be used. This is done by adding a document type declaration before the document element.

The DTD can be internal or external to the document. If the DTD is external, you will need to specify its location or URL.


Is an XML schema an alternative to DTD

Schemas are becoming more popular and DTDs less so. A schema is an XML-based syntax for describing how the XML document is marked up or how it looks – very similar to a DTD, but a DTD has a lot of drawbacks.
A DTD doesn’t use anything like XML syntax to describe the definition. You can’t do data typing in a DTD, and it’s not extensible.

An XML schema allows you to specify elements as an integer, a float, a Boolean, or whatever, and has more extensibility to it. Plus, it’s in XML format. Your XML schema, the definition for your XML, is also in XML format.
Microsoft is pushing the XML schema – so you’re likely to see schemas more and more. That’s all we use. In fact, the new BizTalk Server from Microsoft actually uses schemas.

Explain XML Namespaces

An XML namespace is a collection of element names or attribute names to be defined within an XML document.
Let’s say you have an invoice XML document and an order XML document and you want to put them together. You usually have certain names that overlap each other – like the date. You might have an invoice date and an order date – both called “date”.

With a namespace, you have an invoice date and an order date, and you can reference them as such – “invoice:date” and then “order:date.” That way you can still use the same two tag names and you won’t get confused about which is which. That’s really what an XML namespace is supposed to do for you.

What are the advantages of XML

XML is free form which means that you can configure it any way you want. It’s also very easy to read – you don’t have to read cryptic code to figure it out.
It was designed specifically for internet protocols, and this makes it simple to transmit an XML document across, for example, a HTTP protocol.
Strong data typing is available for xml, and it is also compatible with the SGML standard.

XML is Application independent – you can transfer XML data from a C language program to a Visual Basic program. Or you could, for example, have XML data going from one server out across the internet and being picked up by another application running on a Unix box. XML is also platform independent – it works equally well on Windows, UNIX, or for example, a CICS on a mainframe.
XML is also language independent so it doesn’t matter what type of programming language you’re using – C, Visual Basic, ASP using JavaScript. It doesn’t matter – all have mechanisms to read an XML document.

XML is Unicode-based which makes it very flexible and very good for operating across languages like English, Spanish, or French. You can describe all these types of languages since it’s in Unicode.
A huge benefit of XML is that it’s license free –
it doesn’t cost anything to use XML.

Differences between DTD and Schema

  XML Schema DTD
Markup validation Any global element can be root. No ambiguous content support. Can specify only the root element in the instance document. No ambiguous content support.
Namespace support Yes. Declarations only where multiple namespaces are used. No.
Code reuse Can reuse definitions using named types. Poorly supported. Can use parameter entities.
Datatype Validation Provides flexible set of datatypes. Provides multi-field key cross references. No co-occurrence constraints. No real datatype support.

What are the disadvantages of DTD

  • They are not written in XML syntax, which means you have to learn a new syntax in order to write them
  • there is no support for namespaces
  • there are no constraints imposed on the kind of character data allowed, so datatyping is not possible
  • there is minimal support for code modularity and none for inheritance
  • large DTDs are hard to read and maintain
  • there are no default values for elements and attribute defaults must be specified when they are declared
  • its attribute value models and ID attribute mechanism are simplistic
  • there is limited ability to control whitespace
  • there is limited documentation support, as you cannot use the structured documentation features available for schema notation

Exception Drill

1. What is an Exception ?
An exception is an abnormal condition that arises in a code sequence at run time. In other words, an exception is a run-time error.

2. What is a Java Exception ?
A Java exception is an object that describes an exceptional condition i.e., an error condition that has occurred in a piece of code. When this type of condition arises, an object representing that exception is created and thrown in the method that caused the error by the Java Runtime. That method may choose to handle the exception itself, or pass it on. Either way, at some point, the exception is caught and processed.

3.Where does Exception stand in the Java tree hierarchy ?

4.What are checked exceptions ?
Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions.

5.What are runtime exceptions ?
Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time.

6.What is the difference between error and an exception ?
An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.).

7.How to create custom exceptions ?
Your class should extend class Exception, or some more specific type thereof.

8.If I want an object of my class to be thrown as an exception object, what should I do ?
The class should extend from Exception class. Or you can extend your class from some more precise exception type also.

9.If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object ?
One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well.

10.How does an exception permeate through the code ?
An unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates.

11.What are the different ways to handle exceptions ?
There are two ways to handle exceptions,
1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and
2. List the desired exceptions in the throws clause of the method and let the caller of the method hadle those exceptions.

12.What is the basic difference between the 2 approaches to exception handling.
1 try catch block and
2 specifying the candidate exceptions in the throws clause ?
In the first approach as a programmer of the method, you urself are dealing with the exception. This is fine if you are in a best position to decide should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with it’s own exceptions, then do not use this approach. In this case use the second approach. In the second approach we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use.

13.Is it compulsory to use the finally block  ?
It is always a good practice to use the finally block. The reason for using the finally block is, any unreleased resources can be released and the memory can be freed. For example while closing a connection object an exception has occurred. In finally block we can close that object. Coming to the question, you can omit the finally block when there is a catch block associated with that try block. A try block should have at least a catch or a finally block.

14.How are try, catch and finally block organized  ?
A try block should associate with at least a catch or a finally block. The sequence of try, catch and finally matters a lot. If you modify the order of these then the code won’t compile. Adding to this there can be multiple catch blocks associated with a try block. The final concept is there should be a single try, multiple catch blocks and a single finally block in a try-catch-finally block.

15.What is a throw in an Exception block ?
“throw” is used to manually throw an exception (object) of type Throwable class or a subclass of Throwable. Simple types, such as int or char, as well as non-Throwable classes, such as String and Object, cannot be used as exceptions. The flow of execution stops immediately after the throw statement; any subsequent statements are not executed.

throw ThrowableInstance; ThrowableInstance must be an object of type Throwable or a subclass of Throwable.
throw new NullPointerException(“thrownException”);

16.What is the use of throws keyword ?
If a method is capable of causing an exception that it does not handle, it must specify this behavior so that callers of the method can guard themselves against that exception. You do this by including a throws clause in the method’s declaration. A throws clause lists the types of exceptions that a method might throw.

type method-name(parameter-list) throws exception-list {
// body of method

Here, exception-list is a comma-separated list of the exceptions that a method can throw.

static void throwOne() throws IllegalAccessException {
System.out.println(“Inside throwOne.”);

17.Is it necessary that each try block must be followed by a catch block ?
It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.

18.If I write return at the end of the try block, will the finally block still execute ?
Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The finally block will execute and then the control return.

19.If I write System.exit (0); at the end of the try block, will the finally block still execute ?
No in this case the finally block will not execute because when you say System.exit (0); the control immediately goes out of the program, and thus finally never executes.

20.What are Checked and UnChecked Exception ?
A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses.
Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream’s read() method·
Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn’t force client programmers either to catch the
exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String’s charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.

21.Give me some examples of Checked Exceptions and Unchecked Exceptions ?
Unchecked Exceptions. 

Checked Exception

22. What are Chained Exceptions ?
The chained exception feature allows you to associate another exception with an exception. This second exception describes the cause of the first exception. Lets take a simple example. You are trying to read a number from the disk and using it to divide a number. Think the method throws an ArithmeticException because of an attempt to divide by zero (number we got). However, the problem was that an I/O error occurred, which caused the divisor to be set improperly (set to zero). Although the method must certainly throw an ArithmeticException, since that is the error that occurred, you might also want to let the calling code know that the underlying cause was an I/O error. This is the place where chained exceptions come in to picture.

Throwable getCause( )
Throwable initCause(Throwable causeExc)