What is Exception?

Exceptions in Java are any abnormal, unexpected events or extraordinary conditions that may occur at runtime. They could be file not found exception, unable to get connection exception and so on.
An exception can occur for many different reasons, including the following:
  • A user has entered invalid data.
  • A file that needs to be opened cannot be found.
  • A network connection has been lost in the middle of communications, or the JVM has run out of memory.
Exception Example
 class DivByZero {
 public static void main(String args[]) {
 System.out.println(3/0);
 System.out.println(“Pls. print me.”);
 }
 }
Example: Default Exception
Handling

● Displays this error message
Exception in thread "main"
java.lang.ArithmeticException: / by zero
at DivByZero.main(DivByZero.java:3)

● Default exception handler
– Provided by Java runtime
– Prints out exception description
– Prints the stack trace
 
● Hierarchy of methods where the exception occurred
– Causes the program to terminate
Some of these exceptions are caused by user error, others by programmer error, and others by physical resources that have failed in some manner.
To understand how exception handling works in Java, you need to understand the three categories of exceptions:
  • Checked exceptions: A checked exception is an exception that is typically a user error or a problem that cannot be foreseen by the programmer. For example, if a file is to be opened, but the file cannot be found, an exception occurs. These exceptions cannot simply be ignored at the time of compilation. 
  • Runtime exceptions: A runtime exception is an exception that occurs that probably could have been avoided by the programmer. As opposed to checked exceptions, runtime exceptions are ignored at the time of compilation.
  • Errors: These are not exceptions at all, but problems that arise beyond the control of the user or the programmer. Errors are typically ignored in your code because you can rarely do anything about an error. For example, if a stack overflow occurs, an error will arise. They are also ignored at the time of compilation.

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