CODEWITHSUNDEEP
javainstanceofruntimeexceptionthrowable
user1768830
user1768830

Reputation:

Why is RuntimeException not assignable from Throwable?

I have a method that (unfortunately) accepts an Object instance as its sole parameter. I need to add logic that handles it differently if the Object is any type of Throwable (that means all Exceptions - checked or unchecked, as well as all Errors).

public void handle(Object toHandle) {
    if(toHandle.getClass().isAssignableFrom(Throwable.class))
        handleThrowable(toHandle);
}

When I invoke this method like so:

RuntimeException rte = new RuntimeExceptio("Panic!");
handle(rte);

The isAssignableFrom check returns false and handleThrowable never gets invoked. Why?

Instead, I have to use:

public void handle(Object toHandle) {
    if(toHanlde instanceof Throwable)
        handleThrowable(toHandle);
}

This works, as I expected it to. But still stumped as to why the isAssignableFrom isn't working...thanks in advance!

Upvotes: 1

Views: 2185

Answers (3)

M Sach
M Sach

Reputation: 34424

Its other way around. here is the java doc of isAssignableFrom(...) method

Determines if the class or interface represented by this Class object is either the same as, or is a superclass or superinterface of, the class or interface represented by the specified Class parameter. It returns true if so; otherwise it returns false.

So when you do

if (toHandle.getClass().isAssignableFrom(Throwable.class))

Its actually checking whether RuntimeException class object is either the same as, or is a superclass or superinterface of, the class or interface represented by Throwable.

Upvotes: 1

Ray Stojonic
Ray Stojonic

Reputation: 1270

isAssignableFrom checks if this class is the same as, or a parent of, the argument class.

In your code, you're asking if toHandle is the same as or a parent of Throwable, which is probably not what you want.

Upvotes: 0

Cyrille Ka
Cyrille Ka

Reputation: 15533

Look at the documentation of isAssignableFrom:

Determines if the class or interface represented by this Class object is either the same as, or is a superclass or superinterface of, the class or interface represented by the specified Class parameter.

So you are actually checking if the class of toHandle is a superclass of Throwable. This is not what you want.

You should do it the other way around:

if (Throwable.class.isAssignableFrom(toHandle.getClass()))

or simply use instanceof.

Upvotes: 3

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