CODEWITHSUNDEEP
javahashmap
ta4le
ta4le

Reputation: 101

How do I handle duplicate keys in HashMaps in Java?

I have a HashMap. I wrote a method for reading files and printing them, but I just thought that HashMap does not tolerate duplicate keys in the file, so I need to do something about it, e.g. saving the same key but with some kind of a character in the end (like just _ or something like that so they differ from each other). I can't come up with the solution (maybe I could catch an exception of just write an if-block). Could you please help me? 

public static HashMap<String, String> hashmapReader(File test3) {
    HashMap<String, String> data = new HashMap<>(); 
    try (BufferedReader hmReader = new BufferedReader(new FileReader(test3))) {
        String line;
        while ((line = hmReader.readLine()) != null) {
            String[] columns = line.split("\t");
            String key = columns[0];
            String value = columns[1];
            data.put(key, value); 
        }
    } catch (Exception e) {
        System.out.println("Something went wrong"); 
    }
    return data;
}

Upvotes: 0

Views: 2135

Answers (2)

Andy Turner
Andy Turner

Reputation: 140318

To achieve what you actually ask for:

Before your put line:

while (data.containsKey(key)) key += "_";
data.put(key, value);

This will keep on checking the map to see if key already exists, and if it does, it adds the _ to the end, and tries again.

You can do these two lines in one:

while (data.putIfAbsent(key, value) != null) key += "_";

This does basically the same, but it just avoids having to look up twice in the case that the key isn't found (and thus the value should be inserted).

However, consider whether this is actually the best thing to do: how will you then look up things by "key", if you've essentially made up the keys while reading them.

You can keep multiple values per key by using a value type which stores multiple values, e.g. List<String>.

HashMap<String, List<String>> data = new HashMap<>(); 
// ...
data.computeIfAbsent(key, k -> new ArrayList<>()).add(value);

Upvotes: 0

lettomobile
lettomobile

Reputation: 316

You can add a control on the key if it already exist in the HashMap data. In order to do this you can use get(key) method of the HashMap Java Class which returns null if the key doesn't exist:

if(data.get(key) != null)
         key = key + "_";
data.put(key, value); //adding the split line array to the ArrayList

If it already exists (didn't return null) then you can change his name by adding a character at the end, e.g. "_" as you said.

EDIT: The answer above mine pointed out to me a fact: "What if there are more than 2 identical keys?". For this reason, I recommend following his solution instead of mine.

Upvotes: 1

Related Questions