Count occurrences of a given string in an ArrayList

Question

I have a list of strings , I browse it and count number of "x" strings as below but the count doesn't print me the expected value:

ArrayList<Integer> list = new ArrayList<Integer>();

List<String> strings = table.getValue(); //this gives  ["y","z","d","x","x","d"]

int count = 0;
for (int i = 0; i < strings.size(); i++) {
    if ((strings.get(i) == "x")) {
        count++;
        list.add(count);
    }
}

System.out.println(list);

this gives [] it should be 2 as I have 2 occurrences of "x"

I wonder how much less traffic SO would get if the Java compiler were to warn on `==` comparison of strings... — NPE, Oct 07 '11 at 12:17
ok, I 've changed to equals, the problem now is that print me [1,2] instead of[2] ? — lola, Oct 07 '11 at 12:32

JRL · Accepted Answer · 2011-10-07T13:35:08.900

13

There already is an existing method for this:

Collections.frequency(collection, object);

In your case, use like this (replace all of your posted code with this):

System.out.println(java.util.Collections.frequency(table.getValue(), "x"));

edited Oct 07 '11 at 13:35

answered Oct 07 '11 at 12:24

JRL

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ok, thanks! so we can use collections directly as you've mention ? – lola Oct 07 '11 at 13:32
@Iola: yes, it's a static method, that's how you use it. – JRL Oct 07 '11 at 13:33

score 4 · Answer 2 · edited May 23 '17 at 11:59

You should compare strings using equals instead of ==. I.e. change

if ((list.get(i) == "x"))
                 ^^

to

if ((list.get(i).equals("x")))
                 ^^^^^^

== compares references, while .equals compares actual content of strings.

Related questions:

ok, I 've changed to equals, the problem now is that print me [1,2] instead of[2] ? — lola, Oct 07 '11 at 12:32
That's because you *add* `count` to the list each time you increment it. Either just print `count` instead of `list`, or remove the previous `count` from `list` before you add it again. — aioobe, Oct 07 '11 at 12:34

score 1 · Answer 3 · answered Oct 07 '11 at 12:16

1

You need to use:

list.get(i).equals("x");

!= / == only checks the reference.

I don't knwo why you're using a ArrayList to count. You would probably something like that:

int count = 0;
for (String s : table.getValue()) {
    if (s.equals("x")) {
        count++;
    }
}
System.out.println( count );

answered Oct 07 '11 at 12:16

MasterCassim

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i need array list, as getValue returns me an Arraylist – lola Oct 07 '11 at 12:36

score 0 · Answer 4 · answered Oct 07 '11 at 12:24

0

For String you should use equals method.

int ct = 0;
for (String str : table.getValue()) {
    if ("x".equals(str)) { // "x".equals to avoid NullPoniterException
        count++;
    }
}
System.out.println(ct);

answered Oct 07 '11 at 12:24

Vaandu

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score 0 · Answer 5 · answered Oct 07 '11 at 12:32

Since you are looking for both the elements as well as the size, I would recommend Guava's Iterables.filter method

List<String> filtered = Lists.newArrayList(
                     Iterables.filter(myList, 
                                      Predicates.equalTo("x")));
int count = filtered.size();

But as everyone else has pointed out, the reason your code is not working is the ==

Count occurrences of a given string in an ArrayList

5 Answers5

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