How to compare multiple strings?

Question

I was wondering how I can compare multiple strings in one line. I tried using the || but it doesn't work for booleans or strings. this is what my code is like:

}else if(question != "a" || "b") {
    System.out.println("Sorry that isn't an A or a B");

For those who marked it duplicate, I checked over 200 questions here on stack overflow, and none worked. The one @Chrylis posted actually didn't help. they were just asking about the difference in == and .equals()

score 9 · Accepted Answer · answered Nov 20 '13 at 00:50

9

First of all, don't use == for strings. You'll learn why later. You want to compare strings by their contents, not where they are in memory. In rare cases a string of "a" could compare false to another string called "a".

Second, split it up so you are performing boolean logic on the comparison results:

else if(!(question.equals("a") || question.equals("b")) {

answered Nov 20 '13 at 00:50

nanofarad

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Presumably the OP is asking about an approach that would scale well as the number of strings is increased. – arshajii Nov 20 '13 at 00:52
2

@arshajii If that's an assumption you wish to make, go ahead. I'm going to leave this with my own assumptions and interpretation however. – nanofarad Nov 20 '13 at 00:54
hexafraction, I was hoping this would work, and while it exempted me form errors, but when I put an input of c, it doesnt run the code v.v – SaraMaeBee Nov 20 '13 at 01:02
also, I usually don't use ==. I just was not sure how else to do it except for != I am fairly new to Java (while ive been coding for awhile, but i never really went in depth(im still only 13)) i wasnt sure if you could do an if(!(ques..."b")) { – SaraMaeBee Nov 20 '13 at 01:09
@hexafraction if i was to upload a picture, could you figure it out possibly? I have not the slightest clue why it wont run correctly – SaraMaeBee Nov 20 '13 at 01:15
The cases where two strings `"a"` aren't `==` aren't even rare. It's probably more rare that they _are_ `==`. – yshavit Nov 20 '13 at 01:26
@yshavit if i have my code running, but it doesnt continue after accepting the unknown variable (it wont run the code) how can i fix it? – SaraMaeBee Nov 20 '13 at 01:33
`}else if(!(question.equals("a") || question.equals("b"))) { System.out.println("Sorry that isn't an A or a B"); try { Thread.sleep(2000); } catch(InterruptedException ex) { Thread.currentThread().interrupt(); }` – SaraMaeBee Nov 20 '13 at 01:37
What code won't it run? What's the value of `question`? – yshavit Nov 20 '13 at 01:52
@ yshavitthe value of question was c, and it wouldnt run the System.out.println("Sorry that isn't an A or a B"); – SaraMaeBee Nov 20 '13 at 11:46

arshajii · Answer 2 · 2013-11-20T00:58:38.967

5

You can try using Arrays.asList():

else if (!Arrays.asList("a", "b").contains(question)) {
    ...
}

edited Nov 20 '13 at 00:58

answered Nov 20 '13 at 00:50

arshajii

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3

Isn't that a bit overkill? – nanofarad Nov 20 '13 at 00:50
@hexafraction This seems to be the whole point of the question; an approach that works for an arbitrary number of strings. – arshajii Nov 20 '13 at 00:51
1

Correct logic, but not a good way to code it. (`ImmutableSet` would be great, though.) – chrylis -cautiouslyoptimistic- Nov 20 '13 at 00:51
1

@chrylis No I disagree: this has very little overhead. Much less than an `ImmutableSet` or something of the kind. – arshajii Nov 20 '13 at 00:51
Is `Arrays.asList()` considered a compile-time constant? (`ImmutableSet` is just a wrapper around a fixed-size array anyway.) – chrylis -cautiouslyoptimistic- Nov 20 '13 at 00:53
@chrylis No, the JLS does not consider it such. – nanofarad Nov 20 '13 at 00:54
@chrylis What class are you talking about anyway? Guava's `ImmutableSet`? – arshajii Nov 20 '13 at 00:57
@arshajii Yes, Guava's. These days Guava is on the list of near-reflexive dependencies, like slf4j and commons-lang. – chrylis -cautiouslyoptimistic- Nov 20 '13 at 01:04

score 1 · Answer 3 · answered Nov 20 '13 at 00:52

Two things wrong: You can't just specify multiple values with || (or &&) like that. You need to specify both the left side and the right side explicitly each time.

Second, use equals to compare String values, not the == (or in this case !=) operators. == compares two object references to see if they are the same object.

} else if (!("a".equals(question) || "b".equals(question)))

Or an alternative is to make a temporary List and use contains, which might be clearer for longer lists of things to test:

} else if (!Arrays.asList("a", "b").contains(question))

Or make your own helper function: `boolean isOneOf(String s, String ... a) { for (String as : a) if (as.equals(s)) return true; return false; }` — Per Lindberg, May 20 '14 at 07:56

score 1 · Answer 4 · answered Nov 20 '13 at 00:56

String[] options = {"a", "b"}; // Must be sorted.
if (java.util.Arrays.binarySearch(options, question) < 0) {
  System.out.println("Sorry that isn't an A or a B");
}

Alternatively (assuming your strings don't contain |:

if ("a|b".indexOf(question) == -1) {
  System.out.println("Sorry that isn't an A or a B");
}

dkatzel · Answer 5 · 2013-11-20T00:58:59.970

0

As an aside, you should use equals for objects not ==

To answer your question, you have to repeat the equals call on both sides of the ||

}else if( ! (question.equals("a") || question.equals("b"))  ) {

edited Nov 20 '13 at 00:58

answered Nov 20 '13 at 00:50

dkatzel

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Michael Yaworski · Answer 6 · 2013-11-20T21:06:34.460

}else if( !(question.equals("a") || question.equals("b")) {
    System.out.println("Sorry that isn't an A or a B");

You can't do NOT equals a OR b
You have to do NOT(equals a OR equals b)

Secondly, you are comparing strings with !=, but you should be comparing strings using the .equals(String) method. This has been said millions of times, but: == and != are comparing object references, whereas .equals(String) is comparing String values.

How to compare multiple strings?

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