Exceptions in Java

Introduction

Customers have high expectations for the code we produce.

Users will use our programs in unexpected ways.

Due to design errors or coding errors, our programs may fail in unexpected ways during execution.

It is our responsibility to produce quality code that does not fail unexpectedly.

Consequently, we must design error handling into our programs. 

Errors and Error handling

An Error is any unexpected result obtained from a program during execution.

Unhandled errors may manifest themselves as incorrect results or behavior, or as abnormal program termination.

Errors should be handled by the programmer, to prevent them from reaching the user.


Memory errors (i.e memory incorrectly allocated, memory leaks…)

File system errors (i.e. disk is full…)

Network errors (i.e. network is down…)

Calculation errors (i.e. divide by 0)

Array errors (i.e. accessing element –1)

Types of Exceptions

Unchecked Exceptions

Checked Exceptions It is not required that these types Must either be caught by a of exceptions be caught or method or declared in
its declared on a method. signature.


Runtime exceptions can be

Placing exceptions in the generated by methods or by method signature harkens the JVM itself.

Errors are generated from deep within the JVM.

Keywords

• throws Describes the exceptions which can be raised by a method.
  

• throw Raises an exception to the first available handler in the call stack, unwinding the stack along the way.
  

• try Marks the start of a block associated with a set of exception handlers.
  

• catch If the block enclosed by the try generates an exception of this type, control moves here; watch out for implicit subsumption.
  

• finally Always called when the try block concludes, and after any necessary catch handler is complete.

There are two types of errors:

Compile time errors
Runtime error



Compile time errors

Instead of declaring int a; you mistakenly declared it as in a; for which compiler will throw an error.



Runtime Errors Example

A Runtime error is called an Exceptions error. It is any event that interrupts the normal flow of program execution.

Example for exceptions are, arithmetic exception, Nullpointer exception, Divide by zero exception, etc.

class JavaException {
   public static void main(String args[]){
      int d = 0;
      int n = 20;
      int fraction = n/d;
     System.out.println("End Of Main");
   }

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