Why does order matter when catching exceptions?

Question

I had to answer this question with some code:

Suppose I wrote the following method specification:
public void manipulateData ( ) throws java.sql.SQLException, java.sql.SQLDataException

You are writing code for a database program that will use this method and you want to handle each specifically. What should the try/catch clause look like?
You can use no-ops-- empty blocks {}--for the catch clause contents.
We are only interested in the syntax and structure of the statements here.

I answered with this:

try {  } catch(java.sql.SQLException e) {  } catch(java.sql.SQLDataException e) {  } 

He did not accept the answer for this reason:

"Your catch clauses are in the wrong order. Can you explain why the order matters?"

Is he correct in his answer?

Answer 1

Yes he is correct. As you can see in the Javadoc, SQLDataException is a subclass of SQLException. Therefore, your answer is wrong, since it would create a block of unreachable code in the second catch.

In Java, this code would not even compile. In other languages (e.g. python) such code would create a subtle bug, because SQLDataException would actually be caught in the first block, and not in the second one (as would be the case if it was not a subclass).

Had you answered catch(java.sql.SQLException | java.sql.SQLDataException e) { }, it would still be incorrect, since the question asks to handle each exception specifically.

The correct answer is in Grijesh's answer

Answer 2

In Java you have to put the least inclusive Exception first. The next exceptions must be more inclusive (when they are related).

For instance: if you put the most inclusive of all (Exception) first, the next ones will never be called. Code like this:

try {      System.out.println("Trying to prove a point");      throw new java.sql.SqlDataException("Where will I show up?"); }catch(Exception e){      System.out.println("First catch"); } catch(java.sql.SQLException e) {      System.out.println("Second catch"); } catch(java.sql.SQLDataException e) {      System.out.println("Third catch"); } 

will never print the message you'd expect it to print.