close, destructor and finalize: Java contradictions

Question

I have the following code, which works nicely in my class reading a file line by line.

 try {  FileInputStream in = new FileInputStream(filename);
        BufferedReader reader=new BufferedReader(new InputStreamReader(in));
        String line;
        while((line=reader.readLine())!=null){
             // read the file
        }
    }
    catch (Exception e) {
        System.out.println(e);
    }

But if I try to add the command close, for after the file was finished reading, then I got the error:

    in.close();

Error:(131, 9) java: cannot find symbol
symbol:   variable in
location: class ReadFile

I searched about cleaning objects after use and the need to close files before your program ends. And found several posts on this for Java, but many are very contradictory. The point is that in the end I just get very confused.

Am I wrong, or Java programming is a little bit fuzzy and messy? I mean, there is apparently no real use of destructor, the use of finalize is very questionable, and the use of close is also suggested as unnecessary. Some of the posts on these issues are contradictory and non-conclusive.

So, how to proceed here? In the case I really need to close the file, how to get rid of this error message? Is it really dispensable and unnecessary to close files? What about cleaning up class instances for the the program finishes?

At which place you're calling `in.close()`? Remember in Java, scope matters. — Pradeep Simha, May 08 '14 at 06:19
@Vash - it is not a duplicate, as the post you mention doesn't address the issues I am quoting here, neither the command "close" — Emma Yazzie, May 08 '14 at 06:24
There is no destructor in Java. The finalize() method is indeed not something you should use or rely upon. And close() is absolutely necessary to call. Use the [try-with-resources](http://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html) statement, and learn about variable scopes. This is a notion that is important in every language. — JB Nizet, May 08 '14 at 06:24
@PradeepSimha - it just give another error if I follow your suggestion. — Emma Yazzie, May 08 '14 at 06:27
Also if you have JDK 7 you can use `try-with-resources` statement instead of writing the ugly try-catch-finally. — Siyuan Ren, May 08 '14 at 06:28
@JBNizet, not *every* language ... as I recall, PostScript doesn't have scope, per se, though you can fake it, and I don't *think* TeX does either. — dfeuer, May 08 '14 at 06:40
@JBNizet - Thanks! very insightful your comments. I really appreciate that. — Emma Yazzie, May 08 '14 at 06:43

SpringLearner · Answer 1 · 2014-05-08T08:17:34.760

5

you may be closing in.close() out side try block so obviously you will get error because in is defined in try block which is local.

Better to use try with resource so you need not to manually close file stream

edited May 08 '14 at 08:17

answered May 08 '14 at 06:21

SpringLearner

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thank you. Using "try with resource" is a good approach. – Emma Yazzie May 10 '14 at 23:41

score 5 · Accepted Answer · answered May 08 '14 at 06:22

5

You are getting the error because you have defined variable in inside the try block, so it is not visible in catch/finally/or anywhere outside that try. Move the declaration outside try:

Change this

try {  FileInputStream in = new FileInputStream(filename);

to

FileInputStream in = null;
try {  in = new FileInputStream(filename);

answered May 08 '14 at 06:22

Juned Ahsan

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Do not use comments to argue about mechanics of the site. – Flexo May 08 '14 at 07:03
@JunedAhsan - that solved the error message. – Emma Yazzie May 10 '14 at 23:42

Kuldeep Jain · Answer 3 · 2014-05-08T07:21:00.537

1

Ideally you must call the close() method inside the finally block, after the null check. And for that you need to declare the variable in outside the try block.

Calling the close() method in finally block ensures that it is called irrespective of whether an Exception is thrown or not by the code in try block.

EDIT : This is when you are using Java 6 or earlier version. For Java 7 and higher you could use try with resource as suggested by @JqueryLearner

edited May 08 '14 at 07:21

answered May 08 '14 at 06:23

Kuldeep Jain

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try-with-resources is better still. It's easier to get it right and harder to get it wrong, and I think there may be other advantages too. – dfeuer May 08 '14 at 06:45
I meant this for Java 6 or earlier version. Edited my answer. Thanks – Kuldeep Jain May 08 '14 at 07:21

close, destructor and finalize: Java contradictions

3 Answers3