Why does the code snippet produce the output \'132Exception in thread main MyExc1\' despite the fact that a MyExc2 exception is thrown in the catch block?

Exception Handling in Catch and Finally Blocks

In a recent university Java assignment, a student encountered the following code snippet and was asked to predict its output:

<code class="java">class MyExc1 extends Exception {}
class MyExc2 extends Exception {}
class MyExc3 extends MyExc2 {}

public class C1 {
    public static void main(String[] args) throws Exception {
        try {
            System.out.print(1);
            q();
        }
        catch (Exception i) {
            throw new MyExc2();
        }
        finally {
            System.out.print(2);
            throw new MyExc1();
        }
    }

    static void q() throws Exception {
        try {
            throw new MyExc1();
        }
        catch (Exception y) {
        }
        finally {
            System.out.print(3);
            throw new Exception();
        }
    }
}</code>

When the student answered with "13Exception in thread main MyExc2," they were surprised to find out that the correct answer was "132Exception in thread main MyExc1."

Understanding Exception Propagation in Catch and Finally Blocks

To comprehend the correct output, it's crucial to understand the behavior of exceptions within catch and finally blocks. According to Java's exception handling mechanism:

1. When an exception is thrown within a catch or finally block that will be propagated out of that block, the current exception is automatically aborted and forgotten.
2. The new exception begins unwinding up the stack, adhering to any catch or finally blocks along the way.
3. Notably, a new exception thrown in a catch block is still subject to that catch's finally block, if any.

Tracing Exception Flow

With this in mind, let's trace the execution of the code snippet step by step:

• The program starts in the main method and prints "1."
• An exception is thrown in the q method and caught by the catch block in main.
• Within the catch block, a new exception, MyExc2, is thrown. This aborts the original exception, MyExc1.
• The MyExc2 exception is propagated out of the catch block and into the finally block.
• The finally block prints "2." However, before exiting the q method, the finally block throws a new exception, which starts unwinding up the stack.
• The new exception is an Exception that is not caught in the q method's catch block.
• The Exception exception propagates out of the q method and into the finally block in the main method.
• The finally block in main prints "3" and then throws a MyExc1 exception.
• The MyExc1 exception propagates out of the main method and is caught by the virtual machine.

Understanding the Output

From this tracing, we can see that:

• The final exception that propagates out of the main method is MyExc1.
• The stack trace includes "132," which corresponds to the output printed by the program.

Therefore, the correct output is "132Exception in thread main MyExc1."

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