How do I append to a Java exception?

Question

I am new to Java and exceptions in general.

In my prior C/Perl programming days, when I wrote a library function, errors were passed back by a boolean flag, plus some kind of string with a human-friendly (or programmer-friendly) error message. Java and C++ have exceptions, which is handy because they include stack traces.

I often find when I catch an exception, I want to add my two cents, then pass it along.

How can this be done? I don't want to throw away the whole stack trace... I don't know how deep the failure occurred and for what reason.

I have a little utility library to convert a stack track (from an Exception object) into a string. I guess I could append this to my new exception message, but it seems like a hack.

Below is an example method. Advices?

    public void foo(String[] input_array) {
        for (int i = 0; i < input_array.length; ++i) {
            String input = input_array[i];
            try {
                bar(input);
            }
            catch (Exception e) {
                throw new Exception("Failed to process input [" 
                        + ((null == input) ? "null" : input)
                        + "] at index " + i + ": " + Arrays.toString(input_array) 
                        + "\n" + e);
            }
        }
    }

Answer 1

Exceptions can be chained:

try {
    ...
} catch (Exception ex) {
    throw new Exception("Something bad happened", ex);
}

It makes original exception the cause of the new one. Cause of exception can be obtained using getCause(), and calling printStackTrace() on the new exception will print:

Something bad happened
... its stacktrace ...
Caused by:
... original exception, its stacktrace and causes ...

Answer 2

Typically you throw a new exception which includes the old exception as a "cause". Most exception classes have a constructor which accept a "cause" exception. (You can get at this via Throwable.getCause().)

Note that you should almost never be catching just Exception - generally you should be catching a more specific type of exception.

Answer 3

Just use a different constructor:

Exception(String message, Throwable cause)

The message is your "two cent" and you include the catched exception, which will be shown in a stacktrace printout