Re: unnecessary code in Oracle example?

From:
Robert Klemme <shortcutter@googlemail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Wed, 09 Oct 2013 21:11:21 +0200
Message-ID:
<bblo2qFqgc8U1@mid.individual.net>
On 10/09/2013 08:19 PM, Daniel Pitts wrote:

On 10/9/13 9:38 AM, Jim Janney wrote:

I'm reading up on Java 7's try-with-resource statement and looking at
the tutorial at

http://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html

which includes the following code:

static String readFirstLineFromFileWithFinallyBlock(String path)
                                                    throws IOException {
    BufferedReader br = new BufferedReader(new FileReader(path));
    try {
        return br.readLine();
    } finally {
        if (br != null) br.close();
    }
}


As a matter of habit, I always write that pattern as

static String readFirstLineFromFileWithFinallyBlock(String path)
                                                     throws
IOException {
    BufferedReader br = new BufferedReader(new FileReader(path));
    try {
        return br.readLine();
    } finally {
        br.close();
    }
}


on the theory that if you reach that point br can never be null, so the
test is both redundant and confusing. On the other hand, I might be
wrong. Is there a reason to test for null in the finally block?


If you declare the variable "final" then there is no reason. The way
you showed it "br" can actually become null.

You are correct for this case, the other case comes from a block which
uses multiple resources.

MyResource r1 = null;
MyResource r2 = null;

try {
   r1 = openResource1();
   r2 = openResource2();
} finally {
   if (r1 != null) r1.close();
   if (r2 != null) r2.close();
}

Of course, some close() methods can throw, which could break this idiom
too.


The proper (i.e. robust) way to do it would be to spend one try finally
block per resource. That may not be the prettiest of idioms because of
the indentation but it's robust.

This is one reason why C++ forbids destructors from throwing.


Does it?

$ cat -n x.cxx
      1
      2
      3 #include <iostream>
      4
      5 class Ex {
      6 };
      7
      8 class Foo {
      9 public:
     10 ~Foo() throw (Ex) {
     11 std::cout << "destructor" << std::endl;
     12 throw Ex();
     13 }
     14 };
     15
     16 int main() {
     17 try {
     18 Foo f;
     19 std::cout << "foo" << std::endl;
     20 }
     21 catch (Ex e) {
     22 std::cout << "caught" << std::endl;
     23 }
     24
     25 return 0;
     26 }
     27
robert@fussel:~$ g++ x.cxx && ./a.out
foo
destructor
caught

Cheers

    robert

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