捕获多个异常 - 在捕获的异常上调用Exception中不存在的方法

Question

public class try
{
    public static void main(String[] args)
    {
        try
        {
            if(true)
                throw new A();
            if(true)
                throw new B();
        }
        catch( A | B e)
        {
            e.doit();
        }
    }
}

class A extends Exception
{
    public void doit() {}
}

class B extends Exception
{
    public void doit() {}
}

This doesn't compile

18: error: cannot find symbol
        e.doit();
         ^
symbol:   method doit()
location: variable e of type Exception

The variable e seems to end up as type Exception rather than the actual type - this seems logical because at compile type the compiler doesn't know what kind is going to be thrown. However, is there a way to make this work without making A & B both derive from some common base class or implement a common interface?

Answer 1

Well no, because Java doesn't support duck typing .

Doing an instanceof and casting e to either A or B is obviously going to work, but what you probably want to do in this case is doing it in the traditional way of writing two catch blocks .

I mean, it makes sense, right? Multi-catch is appropriate in the case where you want to treat different kinds of exceptions equally . In this case, the behaviour can be drastically different (even if the methods are named the same).

Answer 2

You should just create a super class that both A and B extend, give that class an doIt() method, and then implement that method for both A and B, like this:

  class A extends C {
    public void doit() {
    }
  }

  class B extends C {
    public void doit() {
    }
  }

  abstract class C extends Exception {
    public abstract void doit();
  }

You can then catch C, like this:

try
{
  if(true)
    throw new A();
  if(true)
    throw new B();
}
catch( C e)
{
  e.doit();
}

Answer 3

If not using interface, can I use instanceof instead. A little tedious. BTW, why you don't write two catch block to catch the two exceptions separately.

public class Try
{
    public static void main(String[] args)
    {
        try
        {
            if(true)
                throw new A();
            if(true)
                throw new B();
        }
        catch(A | B e)
        {
            if(e instanceof A){
                ((A) e).doit();             
            }else if (e instanceof B){
                ((B) e).doit();             
            }
        }
    }
}

class A extends Exception 
{
    public void doit() {}
}

class B extends Exception 
{
    public void doit() {}
}

---

Another way is using reflect.

package com.stackoverflow;

import java.lang.reflect.Method;

public class Try
{
    public static void main(String[] args) throws Exception
    {
        try
        {
            if(true)
                throw new A();
            if(true)
                throw new B();
        }
        catch(A | B e)
        {
            Class cls = e.getClass();  
            Method doit = cls.getMethod("doit");
            doit.invoke(e, null);
        }
    }
}

class A extends Exception
{
    public void doit() {}
}

class B extends Exception
{
    public void doit() {}
}

---

The interface may help.