定义异常类列表-类型不匹配错误

Question

i am trying to define a list of exception classes like so:

private static final List<Class<? extends Exception>> SOME_ERRORS = Arrays.asList(NumberFormatException.class, NullPointerException.class);

the error i get from Eclipse is this:

Type mismatch: cannot convert from List<Class<? extends RuntimeException>> to List<Class<? extends Exception>>

Could you please advise? I don't see why it can't convert a list of Exceptions to a list of Exceptions...

Answer 1

You should use:

List<Class<? extends RuntimeException>> SOME_ERRORS = Arrays.asList(NumberFormatException.class, NullPointerException.class);

The generics type returned from Arrays.asList will always use the most specific type, ie RuntimeException . If you added a checked exception to the list, this statement would become:

List<Class<? extends Exception>> SOME_ERRORS = Arrays.asList(IOException.class, NumberFormatException.class, NullPointerException.class);

Answer 2

+1 for Reimeus's answer . However, if you'd prefer to keep the list declared using the parent Exception type, this is the best I can come up with (using Java 7 syntax):

private static final List<Class<? extends Exception>> SOME_ERRORS = new ArrayList<>();
static{
    SOME_ERRORS.add(NumberFormatException.class);
    SOME_ERRORS.add(NullPointerException.class);
}

Your issue is due to Arrays.asList automatically determining the proper list type for you. To compare, this also works without error:

private static final List<Class<? extends Exception>> SOME_ERRORS = Arrays.asList(NumberFormatException.class, NullPointerException.class, Exception.class);

Especially dealing with "constants" ( static final s), you're usually best advised to ensure that such arrays / collections can't be modified. At worst, this can save running into some difficult issues. For example:

private static final List<Class<? extends Exception>> SOME_ERRORS;
static{
    List<Class< ? extends Exception>> errors = new ArrayList<>();
    errors.add(NumberFormatException.class);
    errors.add(NullPointerException.class);

    SOME_ERRORS = Collections.unmodifiableList(errors);
}