Exceptions

Introduction

• Exceptions are a mechanism to detect and handle errors without writing special code to test return values.
• Error handling code for common errors can be handled in a single place.

Structure

• A try clause has to have either a catch clause or a finally clause or both.
• catch must follow try.
• finally must follow the last catch clause.

Exception Hierarchy

• All exception classes are subtypes of java.lang.Exception.
• RunTimeException is a subtype of Exception which represents unchecked exceptions. All unchecked exception will inherit from this class.
• Classes which derive from java.lang.Error indicate serious problems that an application should not try to catch. e.g OutOfMemory error.
• Both Error and Exception classes are derived from the java.lang.Throwable class.

Exception Handling

• The order of catch clauses is important.
• If the exceptions are in the same class hierarchy, The most-specific exception type should be first, followed by the more general exceptions. Otherwise, the code will not compile.
• If exceptions are in different class hierarchy, then the order does not matter.
• Each method must follow the handle or declare law for checked exceptions.

• The compiler will check if the code within the try block can throw any checked exceptions in the catch block, if not it will flag an error. See below:
  • This is true only for checked exceptions. The compiler wont complain for catch clauses for Exception, Error and Throwable.

public static void main(String[] args) {
 try {
	f();
 } catch(FileNotFoundException f) {
			
 } catch(InterruptedException e) { //This will be a compiler error, since f() can never throw InterruptedException

 }
}
	
private static void f() throws FileNotFoundException {	
}

• Any exceptions thrown from within a catch block will not be handled by the other catch clauses of the main try block.
• These exceptions have to be handled or declared separately.

	public static void main(String[] args) {
		try {
			f();
		} catch(FileNotFoundException f) {
			throw new IOException(); //The below catch block will not handle this IOException. It has to be caught here or the method has to declare that it throws IOException.
		} catch(IOException e) {
			
		}
	}
	
	private static void f() throws FileNotFoundException {
		
	}

• Although a finally block is always executed, it cannot "handle" exceptions, a catch block must be present.

        public static void main(String[] args) {
		
		try {
			foo(); //ILLEGAL, CATCH BLOCK MUST BE PRESENT
		} finally {
			
		}
	}
	
	private static void foo() throws IOException {
		
	}

finally block

• The finally block is always called, no matter what. (Other than a System.exit() ! )

import java.io.IOException;

public class Finale {

	public static void main(String[] args) {

		try {
			foo(); 
		} catch (Exception e) {
			System.out.println("EEE");
			return;
		} finally {
			System.out.println("Finale");
		}
	}

	private static void foo() throws IOException {
		throw new IOException();
	}
	
	/* Output:
	 *  
	 * EEE
	 * FINALE
	 */

}