i'm trying to make a exception handler that takes different types of exception argugment, i declared a function type that takes any types that extends Throwable. It can compile when it's declared:

Function<? extends Throwable, Integer> handler = (IllegalStateException e) -> {
    return 0;
};

but i could't pass throwable or any exception object to it and i don't understand why...

// cannot pass exception object
int result = handler.apply(new IllegalStateException("test"));

Anyone knows how to pass the argument correctly?

[jdk version]: 1.8

[error message]: i got 2 messages:

1. The method apply(capture#6-of ? extends Throwable) in the type Function<capture#6-of ? extends Throwable,Integer> is not applicable for the arguments (IllegalStateException)

2. Type mismatch: cannot convert from Function<capture#5-of ? extends Throwable,R> to Function<Throwable,R>

CodePudding user response：

i'm trying to make a exception handler that takes different types of exception argugment, i declared a function type that takes any types that extends Throwable

No, you haven't.

The function type you've declared:

Function<? extends Throwable, Integer> handler

says "this is a Function which accepts a specific subclass of Throwable - but I don't know which".

You can't pass anything to handler.accept, aside from literal null, because the compiler doesn't know if the subclass of Throwable you're passing is the right subclass of Throwable.

For example, these are legal assignments:

Function<? extends Throwable, Integer> handler1 = (NullPointerException e) -> 0;
Function<? extends Throwable, Integer> handler2 = (IllegalStateException e) -> 0;

But those functions can only work correctly if they are passed a NullPointerException and IllegalStateException respectively. Given just handler1 and handler2, the compiler has no information about which types are expected.

A generic function which accepts any kind of Throwable would be:

Function<Throwable, Integer> handler

No need for bounds.

Don't forget that instances of all subtypes of Throwable are also instances of Throwable.

CodePudding user response：

Afterwards,
i found a way that can pass argument without compilation error
and it can work.

// the original one
Function<? extends Throwable, Integer> handler = (IllegalStateException e) -> {
    System.out.println("handle IllegalStateException, msg: "   e.getMessage());
    return 0;
};

// cast it to one that accept specific type
// (instead of just a generic type upbound)
Function<Throwable, Integer> handler_ = (Function<Throwable, Integer>) handler;

// perform call on it
handler_.apply(new IllegalStateException("test"));

output:
handle IllegalStateException, msg: test

maybe it's solved now
while i'm not sure if there's other better solution...