Returning a list of wildcard matches from a HashMap in java

Question

I have a Hashmap which may contain wildcards (*) in the String.

For instance,

HashMap students_;

can have

Answer 1

You can just iterate your Map without converting it into a list, and use the String matches function, wih uses a regexp.

If you want to avoid the loop, you can use guava like this

@Test
public void hashsetContainsWithWildcards() throws Exception {
Set<String> students = new HashSet<String>();
students.add("John*");
students.add("Jo*Smith");
students.add("Bill");

Set<String> filteredStudents = Sets.filter(students, new Predicate<String>() {
  public boolean apply(String string) {
    return "JohnSmith".matches(string.replace("*", ".*"));
  }
});

assertEquals(2, filteredStudents.size());
assertTrue(filteredStudents.contains("John*"));
assertTrue(filteredStudents.contains("Jo*Smith"));

}

Answer 2

You can use regex to match, but you must first turn "John*" into the regex equivalent "John.*", although you can do that on-the-fly.

Here's some code that will work:

String name = "John Smith"; // For example
Map<String, Student> students_ = new HashMap<String, Sandbox.Student>();

for (Map.Entry<String, Student> entry : students_.entrySet()) {
    // If the entry key is "John*", this code will match if name = "John Smith"
    if (name.matches("^.*" + entry.getKey().replace("*", ".*") + ".*$")) {
        // do something with the matching map entry
        System.out.println("Student " + entry.getValue() + " matched " + entry.getKey());
    }
}

Answer 3

It's not possible to achieve with a hasmap, because of the hashing function. It would have to assign the hash of "John*" and the hash of "John Smith" et al. the same value.

You could make it with a TreeMap, if you write your own custom class WildcardString wrapping String, and implement compareTo in such a way that "John*".compareTo("John Smith") returns 0. You could do this with regular expressions like other answers have already pointed out.

Seeing that you want the list of widlcard matchings you could always remove entries as you find them, and iterate TreeMap.get()'s. Remember to put the keys back once finished with a name.

This is just a possible way to achieve it. With less than 200 elements you'll be fine iterating.

UPDATE: To impose order correctly on the TreeSet, you could differentiate the case of comparing two WildcardStrings (meaning it's a comparation between keys) and comparing a WildcardString to a String (comparing a key with a search value).