Problems using contains() method for arraylist

Question

I am trying to check if an arraylist B contains a string from arrayA with this, but it doesn't work:

String match = arrayA[i];
if (! B.contains(match)) {
    outputFile.print(match);
}

Is this because I used the contains() method incorrectly?
Can we put the name of the variable in the contains() method like contains(match) here?
Or do we have to put a string in the contains(), like contains("name")?

The condition in your if statement is negated (indicated by the "!"), meaning that you only print stuff to the file if they DONT match. Are you sure that that's what you want? — f1sh
– f1sh, Commented Apr 16, 2014 at 7:31
You say it "doesn't work". Could you be more specific? What happens? — David Knipe
– David Knipe, Commented Apr 16, 2014 at 7:33
Note that "name" and a String with the value "name" will work the same way here. If I'm not mistaken they'll even point to the same String literal in memory. The difference is that the second one has a variable associated with it. — keyser
– keyser, Commented Apr 16, 2014 at 7:34

Christian Kullmann · Accepted Answer · 2014-04-16 07:30:16Z

1

In

  if (! B.contains(match))

you check, if B does NOT contain match. Apart from that, the approach is valid.

answered Apr 16, 2014 at 7:30

Christian Kullmann

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4 Comments

user3474606 Over a year ago

Thanks Christian! Yes I am trying to print out the elements that are not in the arraylist B.

keyser Over a year ago

Then your contains is correct. Is your program crashing?

user3474606 Over a year ago

No it's not crashing, but it does not print out the String match properly. Maybe it's because of the other methods but not this contains(). I will check it again. Thanks, the confirmation really helpful!

Kevin Krumwiede Over a year ago

I'm guessing from the index i that you're doing this in a loop iterating over arrayA. Are you sure that arrayA is initialized and contains what you think it contains? Try adding an else clause that prints elements that are not in B. If nothing is printed, then you'll know that arrayA doesn't contain what you expected.

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