确保在构造函数抛出异常时初始化最终变量

Question

I have a final member data:

public final Foo foo;

in the constructor, foo is initialized as follow:

foo = new Foo();

Now, unfortunately, Foo's constructor might throw an exception:

try {
    foo = new Foo();
} catch (Exception e) {
    e.printStackTrace();
}

But now compiler complains that foo might not be initialized, which is true if Foo's constructor throws an exception. But if I put foo = null inside the catch braces, it complains that foo might have been initialized.

Foo is a third-party library that I cannot modify.

So, what's the most graceful way of handling this?

Answer 1

foo = createFoo();

...


private static Foo createFoo() {
    try {
        return new Foo();
    } catch (Exception e) {
        e.printStackTrace();
        return null;
    }
}

I make no comment about whether it makes sense to continue with construction of your outer object if the inner object's constructor has failed...

Answer 2

final Foo foo ;
Foo foo_temp = null ;
try {
    foo_temp = new Foo();
} catch (Exception e) {
    e.printStackTrace();
} finally {
    foo = foo_temp ;
}

Answer 3

Maybe I don't understand, but I don't get the error. Note that ConTest's constructor throws the exception that Foo raises. This to me is most elegant (to answer your question.) If Foo fails to initialize, isn't something terribly wrong?

ConTest.java:

   public class ConTest {
       public final Foo foo;
       public ConTest() throws Exception {
           foo = new Foo(3);
       }
   }

Foo.java:

   public class Foo {
       public Foo(int i) throws Exception {
           if (i < 0) throw new Exception("yah");
       }
   }

and it all compiles fine.