Java - 如何尝试和捕获异常处理运行

Question

I'm almost totally new to Java programming and I'm trying to understand how try and catch exception handling runs.

My doubt is about the fact that when I create a customized exception class and I throw it in a try block of a main class I can instantiate it without reference, but then when I write the catch block I can use a member of the same kind of the instantiated exception class without instantiate it.

For example :

public void main.....{

.....

 try{

    if(division==0){

         throw new DivisionByZero() ;
         //Divisionbyzero is my customized exception class

     } 

 }

catch(DivisionByZero e) {

   e. methodDivisionByZero;

 }

}

The point is that I can use directly the e member without instantiate it by means of new. Is e somehow instantiate before when I throw the exception (even if I don't write any reference in the try block)?

Thanks in advance for any answer.

Answer 1

When an exception is thrown, an exception object is created and further lines are not executed in the try block. This exception object is matched with the catch block parameters the most suitable one receives the object in the parameter variable just like method parameters . In this case catch(DivisionByZero e) the correct catch clause should be catch(ArithmeticException e) to catch the divide by zero exception and the parameter e is the reference to the exception object.

Answer 2

You are creating a new object:

throw new DivisionByZero() ;

Throwing/Catching is just a "mean of transportation". In "essence", it is like writing:

DivisionByZero dbz = new DivisionByZero() ;
dbz.someMethod();

You create an exception, you throw "it", you catch "it" and then "it" is nothing else but a reference to some object.

Answer 3

For your first question I can instantiate it without reference : it's not necessary to assign an instance to any reference variable. We use reference variables so that we can use it's members in future.

And for the second question I can use a member of the same kind of the instantiated exception class without instantiate it : When you are writing

throw new DivisionByZero() ;` this is equal to 
DivisionByZero dBZException = new DivisionByZero() ;
throw dBZException;

which means you are passing an instance to the catch block where you have the reference variable of class DivisionByZero .

Thus in catch block your code works like this :

DivisionByZero e = dBZException;

Hope it's clear now.

Answer 4

The point is that I can use directly the e member without instantiate it by means of new.

Well you don't need any instance here, in the catch block like in methods call we pass a **[reference][1]** , here it's a reference to the Exception that should be handled.

You can see that if you take a look at The catch Blocks Oracle Docs :

Each catch block is an exception handler that handles the type of exception indicated by its argument. The argument type, ExceptionType, declares the type of exception that the handler can handle and must be the name of a class that inherits from the Throwable class. The handler can refer to the exception with name.