Java：如何重写方法并引发异常？

Question

I'm trying to override a method, with throwing an exception:

class A {

    public doSomething(){
        // some of logic
    }

}


class B extends A {

    public doSomething() throws MyCustomizedException {
        try {
             // some of logic
        } catch(ExceptionX ex ) {
             throw new MyCustomizedException(" Some info ", ex);
        }
    }      
}

But I get this compile time error :

Exception MyCustomizedException is not compatible with throws clause in A

The two constraints are :

• Using the same name of the function and the same arguments if they exist: doSomething()
• Throwing my customized exception

How can I get rid of the exception?

Thank you a lot.

Answer 1

Cannot be done.

When you override a method, you can't break the original contract and decide to throw a checked exception.

You can make MyCustomizedException unchecked. You can throw it, but you can't require that users handle it the way you can with a checked exception. The best you can do is add it to the javadocs and explain.

Answer 2

This is not a overloading case. To overload you need methods with same signature but different argument list. This is Overriding, Use public doSomething() throws MyCustomizedException {} in class A if Class A can modify.

Now you can override with your current implementation for class B.

Answer 3

The only way you can do this is to make the exception that you are throwing extend RuntimeException instead of Exception.

This way you will be able to throw it as you don't change the compact of the method, unfortunately the compiler will not automatically detect that you need to catch that exception but depending on how you are using it that may not be a problem.

Answer 4

There is actually a way to do this - using composition rather than inheritance:

class B {
   A a = new A();
   void doSomething() throws MyException {
      a.doSomething();
      throw MyException();
   }
}

Of course by doing this your B no longer counts as an A so cannot be passed to anything expecting an A. You could use a B throughout your code though and just wrap As on demand.