Map throw error on duplicate insertion

Question

I am creating a Map with stores some data and i want the map to throw an exception if a duplicate value is tried to insert.

Map <Integer, String> temp;
temp.put(1, "hi");
temp.put(1, "hello");

Here this map should throw an error since a key '1' is already present. It can throw an error or does not compile. Is there any map which has said functionality?

Answer 1

Using a contains-check prior to the put-operation is not an atomic-operation and therefore not thread-safe. Additionally, the map has to be accessed twice each time you add an entry.

To avoid this, use one of the functional methods like merge or compute:

map.merge(key, value, (v1, v2) -> {
    throw new IllegalArgumentException("Duplicate key '" + key + "'.");
});

Answer 2

Check containsKey method.

Map<Integer, String> map = new LinkedHashMap<>();

    map.put(1, "Value");
    if (map.containsKey(1)) {
        throw new Exception("Map already contains key 1");
    }

For your question, you can create your own implementation like:

public class MyCustomMap extends HashMap {

        @Override
        public Object put(Object key, Object value) {
            if (this.containsKey(key)) {
                System.out.println("Do whatevery you want when key exists.");
                return null;
            } else {
                return super.put(key, value);
            }
        }
    }

And then use it:

public static void main(String[] args) throws Exception {

        Map<Integer, String> map = new MyCustomMap();

        map.put(1, "Value");

        map.put(1, "Another value");

    }

Note: This is example, without null checking etc.

But please, always try to check JavaDoc or basic data structure use cases before asking here, I believe that on Google is a ton of examples ;)