URL decoding: UnsupportedEncodingException in Java

What I understand from the documentation is that UnsupportedEncodingException can only be thrown if I specify a wrong encoding as the second parameter to URLDecoder.decode(String, String) method. Is it so? I need to know cases where this exception can be thrown.

Basically, I have this code segment in one of my functions:

if (keyVal.length == 2) {
    try {
        value = URLDecoder.decode(
            keyVal[1],
            "UTF-8");
    } catch (UnsupportedEncodingException e) {
          // Will it ever be thrown?
    }
}

Since I am explicitly mentioning "UTF-8", is there any way this exception can be thrown? Do I need to do anything in the catch block? Or, if my understanding is completely wrong, please let me know.

It cannot happen, unless there is something fundamentally broken in your JVM. But I think you should write this as:

try {
    value = URLDecoder.decode(keyVal[1], "UTF-8");
} catch (UnsupportedEncodingException e) {
    throw new AssertionError("UTF-8 is unknown");
    // or 'throw new AssertionError("Impossible things are happening today. " +
    //                              "Consider buying a lottery ticket!!");'
}

The cost of doing this is a few bytes of code that will "never" be executed, and one String literal that will never be used. That a small price for the protecting against the possibility that you may have misread / misunderstood the javadocs (you haven't in this case ...) or that the specs might change (they won't in this case ...)

That's because of the odd choice to make UnsupportedEncodingException checked. No, it won't be thrown.

I usually do as follows:

} catch (UnsupportedEncodingException e) {
  throw new AssertionError("UTF-8 not supported");
}

In your special case - no, it won't be thrown. Unless you execute your code in a Java runtime that does not support "UTF-8".

To answer an old question for newer readers:

Java 11 now has URLDecoder.decode(String, Charset); which does not throw. So you don't have to use a try-catch block at all. Just do:

URLDecoder.decode(keyVal[1], StandardCharsets.UTF_8);