CODEWITHSUNDEEP
javaexceptiontry-catch
user16655
user16655

Reputation: 1941

Try catch in Java - what happens when an exception occurs?

I have a question regarding try/catch and exceptions in Java. I know this should be basic knowledge, but I think I miss a part of the understand of how this works. Given this code for example:

String res = "";

try {
    res = methodThatCanThrowException();
    res = res + ".txt";
} catch (Exception e) {
    res = "somethingwentwrong.txt";
}

return res;

Am I guaranteed that 'res' will never be set both in the try AND the catch block? If an exception is thrown inside the methodcall in the try block, the code control goes directly to the catch block, right? Is there any scenarios where 'res' will be given a value both in the try and the catch block?

Upvotes: 4

Views: 6581

Answers (5)

Japheth Ongeri - inkalimeva
Japheth Ongeri - inkalimeva

Reputation: 4979

When an exception occurs inside a try block, control goes directly to the catch block, so no other code will be executed inside the try block and the value of res will not change. Also when a method throws an exception it does not return anything. So in case of an exception, res will only be set once when it is initialised outside the try catch block, and then later when inside the catch block.

Never more than twice

Upvotes: 0

Maroun
Maroun

Reputation: 96016

The best answer you can get is from the JLS - 14.20.1. Execution of try-catch:

A try statement without a finally block is executed by first executing the try block. Then there is a choice:

...

If the run-time type of V is assignment compatible with (§5.2) a catchable exception class of any catch clause of the try statement, then the first (leftmost) such catch clause is selected. The value V is assigned to the parameter of the selected catch clause, and the Block of that catch clause is executed...

I don't want to paste the whole section because it's big, I highly advise you to go through it to better understand the mechanism of try-catch-finally.

Upvotes: 2

Eran
Eran

Reputation: 394146

If methodThatCanThrowException throws an exception, res won't get assigned in the try block, so only the catch block would assign it (assuming you fix your catch block).

However, even if the exception was thrown after res is already assigned, the catch block would overwrite it with a new value, so it doesn't matter if it is assigned by both.

Upvotes: 2

npinti
npinti

Reputation: 52205

If no exception takes place within the try block, the catch block will never be executed.

If, on the other hand, an exception does take place, control flow moves from the try block to your catch block, and the res variable would be overwritten with whatever it is you have in the catch block.

In your case, res will be have either whatever methodTatCanThrowException() returns with .txt appended to it or somethingwentwrong.txt.

As a side note, you might also want to look into the finally block and what it does.

Upvotes: 0

Si mo
Si mo

Reputation: 989

You are right. If methodThatCanThrowException throws an execption it will jump to res = "somethingwentwrong.txt";. There is never a scenario with both lines excecuted

Upvotes: 0

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