match array against string in java

Question

I'm reading a file using bufferedreader, so lets say i have

line = br.readLine();

I want to check if this line contains one of many possible strings (which i have in an array). I would like to be able to write something like:

while (!line.matches(stringArray) { // not sure how to write this conditional
  do something here;
  br.readLine();
}

I'm fairly new to programming and Java, am I going about this the right way?

Answer 1

Copy all values into a Set<String> and then use contains():

Set<String> set = new HashSet<String> (Arrays.asList (stringArray));
while (!set.contains(line)) { ... }

[EDIT] If you want to find out if a part of the line contains a string from the set, you have to loop over the set. Replace set.contains(line) with a call to:

public boolean matches(Set<String> set, String line) {
    for (String check: set) {
        if (line.contains(check)) return true;
    }
    return false;
}

Adjust the check accordingly when you use regexp or a more complex method for matching.

[EDIT2] A third option is to concatenate the elements in the array in a huge regexp with |:

Pattern p = Pattern.compile("str1|str2|str3");

while (!p.matcher(line).find()) { // or matches for a whole-string match
    ...
}

This can be more cheap if you have many elements in the array since the regexp code will optimize the matching process.

Answer 2

It depends on what stringArray is. If it's a Collection then fine. If it's a true array, you should make it a Collection. The Collection interface has a method called contains() that will determine if a given Object is in the Collection.

Simple way to turn an array into a Collection:

String tokens[] = { ... }
List<String> list = Arrays.asList(tokens);

The problem with a List is that lookup is expensive (technically linear or O(n)). A better bet is to use a Set, which is unordered but has near-constant (O(1)) lookup. You can construct one like this:

From a Collection:

Set<String> set = new HashSet<String>(stringList);

From an array:

Set<String> set = new HashSet<String>(Arrays.asList(stringArray));

and then set.contains(line) will be a cheap operation.

Edit: Ok, I think your question wasn't clear. You want to see if the line contains any of the words in the array. What you want then is something like this:

BufferedReader in = null;
Set<String> words = ... // construct this as per above
try {
  in = ...
  while ((String line = in.readLine()) != null) {
    for (String word : words) {
      if (line.contains(word)) [
        // do whatever
      }
    }
  }
} catch (Exception e) {
  e.printStackTrace();
} finally {
  if (in != null) { try { in.close(); } catch (Exception e) { } }
}

This is quite a crude check, which is used surprisingly open and tends to give annoying false positives on words like "scrap". For a more sophisticated solution you probably have to use regular expression and look for word boundaries:

Pattern p = Pattern.compile("(?<=\\b)" + word + "(?=\b)");
Matcher m = p.matcher(line);
if (m.find() {
  // word found
}

You will probably want to do this more efficiently (like not compiling the pattern with every line) but that's the basic tool to use.

Answer 3

Using the String.matches(regex) function, what about creating a regular expression that matches any one of the strings in the string array? Something like

String regex = "*(";
for(int i; i < array.length-1; ++i)
  regex += array[i] + "|";
regex += array[array.length] + ")*";
while( line.matches(regex) )
{
  //. . . 
}