计算给定字符串在ArrayList中的出现次数

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"

Answer 1

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"));

Answer 2

You should compare strings using equals instead of == . Ie change

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

to

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

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

---

Related questions:

• How do I compare strings in Java?
• Java Compare Strings
• Comparing strings in Java
• https://stackoverflow.com/questions/5116595/compare-two-strings-java
• What's the quickest way to compare strings in Java?
• String Comparison in Java
• Java String.equals versus ==
• How do I make my string comparison case insensitive?
• comparing strings in java
• Strings don't seem to be equal in Java on Android, even though they print the same
• Comparing java Strings with ==
• How can I compare String value with ArrayList of String type in Java?
• How can I compare two strings in java and define which of them is smaller than the other alphabetically?

Answer 3

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 );

Answer 4

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);

Answer 5

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 ==