Inheriting exceptions: a compiler intelligence question

[jay vas]

Can somebody explain to me why my compiler is allowing me to NOT catch the clearly
relevant exception in the Toyota.drive() method ?

I have a different exception in that method.... but there is no reason why the compiler couldnt force me
to catch both exceptions, or at least catch the Exception superclass.

    


public class MethodOverloading {

	
	public static class Car
	{
		public Car(){
			
		}
		public void drive() throws IOException {
			System.out.println("Driving superclass... watch out i might throw an io exception.");
		}
	}
	
	public static class Toyota extends Car
	{
		public void drive() throws EmptyStackException
		{
			System.out.println("Driving subclass .... I might throw an empty stack exception");
			throw new EmptyStackException();
		}
	}
	public static void testInheritance()
	{
		
		Toyota t = new Toyota();

		//uncaught exception below...  and the compiler doesnt care ???  This is the question that is confusing me.
		t.drive();
		
		
	}
	
	public static void main(String[] args)
	{
		testInheritance();
	}

[Ankit Garg]

EmptyStackException is an unchecked exception. And sice you are using a reference of type Toyota, so the method call will resolute to drive() in Toyota class. And since the drive method in Toyota class doesn't declare any checked exception in the throws clause, so you are not forced to catch any exception...

[George Coolidge]

When you override a method in the subclass, you can loose the specification by omitting an exception. In this case, Toyota class can throw either no exception at all or EmptyStackException, which is an unchecked exception. Remember that Toyota.drive() method doesn't "inherit" the IOException thrown in the superclass's drive method.

Always remember that a Java method's signature ONLY include the method name and its parameter type list, contrary to many people's intuition, access modifier, return type and thrown exception do NOT belong to the signature and therefore are not guaranteed to be the same in the overriden methods.

jay vas wrote:Can somebody explain to me why my compiler is allowing me to NOT catch the clearly
relevant exception in the Toyota.drive() method ?

I have a different exception in that method.... but there is no reason why the compiler couldnt force me
to catch both exceptions, or at least catch the Exception superclass.

    


public class MethodOverloading {

	
	public static class Car
	{
		public Car(){
			
		}
		public void drive() throws IOException {
			System.out.println("Driving superclass... watch out i might throw an io exception.");
		}
	}
	
	public static class Toyota extends Car
	{
		public void drive() throws EmptyStackException
		{
			System.out.println("Driving subclass .... I might throw an empty stack exception");
			throw new EmptyStackException();
		}
	}
	public static void testInheritance()
	{
		
		Toyota t = new Toyota();

		//uncaught exception below...  and the compiler doesnt care ???  This is the question that is confusing me.
		t.drive();
		
		
	}
	
	public static void main(String[] args)
	{
		testInheritance();
	}

[Salil Vverma]

Hey Jay,
As mentioned by Ankit, it is just that drive function in Toyota class does not throw a checked exception so the compiler is not forcing you to put that in try catch block. If you use the same function with Car type of reference, the compiler would give you the compilation error (ie - Unhandled exception type IOException)

 
import java.io.IOException;
import java.util.EmptyStackException;

public class MethodOverloading {

	public static class Car {
		public Car() {

		}

		public void drive() throws IOException {
			System.out
					.println("Driving superclass... watch out i might throw an io exception.");
		}
	}

	public static class Toyota extends Car {
		@Override
		public void drive() throws EmptyStackException {
			System.out
					.println("Driving subclass .... I might throw an empty stack exception");
			throw new EmptyStackException();
		}
	}

	public static void testInheritance() {

		Car t = new Toyota();
		// use the reference of super class
		t.drive();

	}

	public static void main(String[] args) {
		testInheritance();
	}
}

   

I hope this would clearify your confusion.