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
– NPE, Commented Oct 7, 2011 at 12:17
ok, I 've changed to equals, the problem now is that print me [1,2] instead of[2] ? — lola
– lola, Commented Oct 7, 2011 at 12:32

JRL · Accepted Answer · 2011-10-07 13:35:08Z

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 7, 2011 at 13:35

answered Oct 7, 2011 at 12:24

JRL

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2 Comments

lola Over a year ago

ok, thanks! so we can use collections directly as you've mention ?

JRL Over a year ago

@Iola: yes, it's a static method, that's how you use it.

Community · Accepted Answer · 2017-05-23 11:59:41Z

4

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:

edited May 23, 2017 at 11:59

CommunityBot

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answered Oct 7, 2011 at 12:14

aioobe

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2 Comments

lola Over a year ago

ok, I 've changed to equals, the problem now is that print me [1,2] instead of[2] ?

aioobe Over a year ago

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.

Tobias · Accepted Answer · 2011-10-07 12:16:07Z

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 7, 2011 at 12:16

Tobias

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1 Comment

lola Over a year ago

i need array list, as getValue returns me an Arraylist

Vaandu · Accepted Answer · 2011-10-07 12:24:54Z

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 7, 2011 at 12:24

Vaandu

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Comments

John B · Accepted Answer · 2011-10-07 12:32:43Z

0

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 ==

answered Oct 7, 2011 at 12:32

John B

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