Exception Handling

What is an Exception?

An exception is an unwanted or unexpected event, which occurs during the execution of a program i.e at run time, that disrupts the normal flow of the program’s instructions.

Error vs Exception

Error: An Error indicates a serious problem that a reasonable application should not try to catch.
Exception: Exception indicates conditions that a reasonable application might try to catch.

Exception Hierarchy

All exception and error types are sub classes of class Throwable, which is the base class of the hierarchy. One branch is headed by Exception. This class is used for exceptional conditions that user programs should catch. NullPointerException is an example of such an exception. Another branch, Error are used by the Java run-time system(JVM) to indicate errors having to do with the run-time environment itself(JRE). StackOverflowError is an example of such an error.

How Programmer handles an exception?

Customized Exception Handling: Java exception handling is managed via five keywords: try, catch, throw, throws, and finally. Briefly, here is how they work. Program statements that you think can raise exceptions are contained within a try block. If an exception occurs within the try block, it is thrown. Your code can catch this exception (using catch block) and handle it in some rational manner. System-generated exceptions are automatically thrown by the Java run-time system. To manually throw an exception, use the keyword throw. Any exception that is thrown out of a method must be specified as such by a throws clause. Any code that absolutely must be executed after a try block completes is put in a finally block.

Consider the following java program.

 class Scaler{
      public static void main (String[] args) {
                    // array of size 4
                    int[] arr = new int[4];

                 // this statement causes an exception
                  int i = arr[10];

                // the following statement will never execute
                System.out.println("Hello World");
           } 
 }

Output :

 Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 10 at Scaler.main(Scaler.java:9)

Explanation: In the above example an array is defined with size i.e. you can access elements only from index 0 to 3. But you trying to access the elements at index 10(by mistake) that’s why it is throwing an exception. In this case, JVM terminates the program abnormally. The statement System.out.println(“Hello World”); will never execute. To execute it, we must handle the exception using try-catch. Hence to continue the normal flow of the program, we need a try-catch clause.

Task:

Which of these keywords is not a part of exception handling?