为什么默认构造函数无法处理异常类型Exception？

Question

I want to know that why i have to define an explict constructor because i am getting error which says that default constructor cannot handle exception type Exception thrown by implicit super constructor.

class A {
    A() throws Exception {
        System.out.println("A Class");
    }
}

public class Example extends A {
    public static void main(String args[]) throws Exception {
        Example t = new Example();
    }
}

Answer 1

Yes - your Example class is effectively declaring:

public Example() {
    super();
}

That won't compile, because the super() call is calling the A() constructor which is declared to throw Exception , which is a checked exception. That's just as much a mistake in a constructor as it is to call a method which declares that it throws a checked exception from within a method which neither catches the exception nor declares that it throws it itself.

So you need to declare the exception in an explicitly declared constructor in Example .

public Example() throws Exception {
    super(); // This is implicit; you can remove it if you want.
}

instead. Note that this is only relevant if the constructor throws a checked exception... unchecked exceptions don't need to be declared, so the "compiler-provided" default exception is fine.

Also note that you can't catch an exception thrown by a super-constructor.

Answer 2

You have to add an explicit constructor with throws clause to the Example class.

public Example() throws Exception {
}