Exception note

Runtime Exception

• An class is called Runtime Exception if it extends RuntimeException.
  Example (from Java Document)
  

  java.lang Class NullPointerException java.lang.Object | +--java.lang.Throwable | +--java.lang.Exception | +--java.lang.RuntimeException | +--java.lang.NullPointerException

• More examples : ArrayIndexOutOfBoundsException, NullPointerException, ArithmeticException,NumberFormatException
• Runtime exceptions are those exceptions that occur within the Java runtime system. This includes arithmetic exceptions (such as when dividing by zero), pointer exceptions (such as trying to access an object through a null reference), and indexing exceptions (such as attempting to access an array element through an index that is too large or too small).
• it is not necessary to "take care" of runtime exception example

  public class Ex1 { public static void main(String[] args) { String s = "hi"; int a = Integer.parseInt(s); } }

  • No compiling error.
  • Runtime error occurs when you run the program.
  
  
  
  

  public class Ex1 { public static void main(String[] args) { String s = "hi"; int a; try { a = Integer.parseInt(s); } catch (NumberFormatException ex) { System.out.println("error"); } } }

  • No compiling error.
  • no runtime error.

Checked Exception

• A class that entends Throwable but doesn't extend RuntimeException. Example:

  java.io Class IOException java.lang.Object | +--java.lang.Throwable | +--java.lang.Exception | +--java.io.IOException

• You must take care of the exception. Fail to do so will cause a compiling error.
  Example
  

  import java.io.*; public class Ex2 { public static void main(String[] args) { String filename = "file.txt"; // the following constructor throws FileNotFoundException FileInputStream in = new FileInputStream(filename); } }

  Compling error

  Ex2.java:7: unreported exception java.io.FileNotFoundException; must be caught o r declared to be thrown FileInputStream in = new FileInputStream(filename); ^ 1 error

  Explanation:
  • The construction FileInputStram(String filename) throws FileNotFoundException
  • FileNotFoundException is not runtime exception
  Correction:

  import java.io.*; public class Ex2 { public static void main(String[] args) { String filename = "file.txt"; try { // the following constructor throws FileNotFoundException FileInputStream in = new FileInputStream(filename); } catch (FileNotFoundException ex) { System.out.println("File not found"); } } }