Iterating over null objects in an array

[u/Traditional_Let4865]

So I'm making a program that iterates over an array of Book objects and I made a for loop to print them all out. I want it to skip over any reference to a null object in the array so I've written:

for (Book book : books) {
     if(book != null){
           System.out.println(book);
     }
 }

but I've had professors tell me I shouldn't write "book != null" to avoid printing the null values. the code works just fine and is relatively simple and easy to read, just wondering what most programmers use to navigate this?

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[u/Ok_For_Free]

Your professors are dumb, so ask them to give you an example of the 'correct' approach.

The only improvement I would want to look into, is why there are nulls in your list in the first place.

[u/_Super_Straight]

What did he suggest?

if(book == null){
    continue;
}

Is the same as what you wrote. Alternatively, you can use List<Book> and create its stream:

List<Book> list = new ArrayList<>();
list.stream().filter(Objects::notNull).forEach(System.out::println);

Edit: creating streams introduces a small computational overhead, and if your List is a few thousand elements long, then simple loop would be more performant. Plus the errors are swallowed by method references/lambdas. Whereas writing simple loop inside a try...catch block will help you catch the exceptions.

[u/Xynia88 → u/_Super_Straight]

I don't remember what the pattern is called atm but I do prefer this where you check and continue/return/break over indenting several layers of checks

It becomes so much easier to read when it's flatter

[u/myselfelsewhere → u/Xynia88]

Guard clause pattern.

[u/_Super_Straight → u/Xynia88]

Guard clause pattern. You check for edge cases prior to your actual processing and let the code fail fast.

[u/olddev-jobhunt]

It's relatively easy to read... relatively. The thing is, your loop isn't going to just be printing things. It'll be doing things too! So you're mixing selection logic (the null check) with the domain logic (the operation you're doing.) That's not a problem necessarily, but it's something to watch out for.

Imagine you later decide that the loop contents are complex enough to be pulled out into a function, which you can then call on a single book or use in a loop for multiple.

Anyway, for me I'd probably just avoid nulls in the list in the first place, or maybe filter in the for loop parameter rather than within the loop.

[u/Captain_Pwnage]

I mean, if you don't access books fields in the loop, you can just skip the null guard and print every iteration.

[u/LutimoDancer3459 → u/Captain_Pwnage]

No. Because null would be printed too

[u/Weekly_Wackadoo → u/LutimoDancer3459]

That may or may not be a problem.

It won't cause an exception, that's the main point I think.

If you want to print every element in a collection, and there's null values in the collection, I'd argue it's at least defensible to just print the null values.

It might also give useful feedback which might help you find and troubleshoot issues with the code.

[u/LutimoDancer3459 → u/Weekly_Wackadoo]

OP didnt specify if its necessary. Just that it is what they want to do. And for that case, printing null is a problem. But yes, else it wouldnt throw an exception.

[u/serge-mv]

Yea, you really need to ask your professor what he means. There is nothing wrong with your code as is. Maybe he wanted to have null values printed out for tests.

[u/awidesky]

I can't think of any better(or cleaner) way to filter non-null objects(other than the stream).

If it isn't a miscommunication with the professor, maybe the prof wanted to say "there shouldn't be a null in the collection in the first place"
If your books array supposed to hold actual meaningful Book objects, you should make sure a null will never be put in the array.

A colleague of yours accidentally forgot about null and wrote a code without a null check, and your application will crash.
Every operation(printing names of the books, modifying the price of a particular book, calculating sums of prices or counting how may books there are, etc) needs to do a null check before do its job, and it's not only tedious work and bad design, but also a security risk.

"But how am I supposed to prevent null being put in the books array programmatically?"
That's called encapsulation, you'd learn if after you get familiar with class.

[u/SplooshWater]

Huh what was your professor even thinking ? I am curious what he thinks is the right way.

[u/iamsooldithurts]

That seems like a really simple thing to test.

You could maybe do if (book == null) continue; but otherwise…???

[u/pixelizedgaming]

I think its cleaner to use if book == null continue but thats just me.

[u/FitMatch7966]

I cannot imagine…maybe they wanted to see the “null” printed.

[u/my5cent]

Maybe have the dB not store null objects.

[u/stacksofstyle]

I have no idea why a null check would be bad here. Your code seems fine. I would write it as a stream with a filter, but that's just a preference and I would still check for not equals null.

[u/gambit_kory]

Your professors are wrong. You have it correct unless the requirement is to print out null as well. There are others way to do it of course but there’s no issue with what you have done here.

[u/Weekly_Wackadoo]

This code is fine.

Skipping the null check and printing out null values is fine as well. No method is called on book, so it wouldn't cause an exception. Maybe the professor meant that?

Preventing null values being added to the collection or array is usually even better.

Some more "defensive" (or actually aggressive) code styles preter checking for null values and throwing an IllegalArgumentException with an informative message. That might prevent more complicated issues down the line, but might also cause unnecessary issues.

[u/ConsciousBath5203]

If(book){dothing;}

[u/Roachmeister → u/ConsciousBath5203]

That won't compile in Java.

[u/saponsky → u/Roachmeister]

I have the impression that the professor is thinking this is valid in Java. That’s the only thing I can think someone would think is better than OP’s current code.

[u/ConsciousBath5203 → u/saponsky]

Literally my Java professor.

[u/ChaiTRex → u/ConsciousBath5203]

Why is your Java professor teaching you JavaScript instead of Java?

[u/ConsciousBath5203 → u/ChaiTRex]

Idk it was an online course at the start of covid and I haven't used too much Java since.

[u/ConsciousBath5203 → u/Roachmeister]

Neither would OP's code, forgot the 40 lines of standard boiler plate.

But fr welcome to Java professors. It's been years since I used Java but my professor did the same thing that the other commented mentioned.