How link multiple keys to the same value in the HashMap

I am trying to link the value of a key to another key’s value but cannot seem to get it working.

For example, if I’m creating a HashMap and adding a key-value pair ("x", 0) to it. I then want to be able to add other keys mapped to the same value as the first one.

So if I have ("x", map.get("y")) and ("y", 0) I want to be able somehow to link it. So that if I now update a value for "y" key like that ("y", 10), then I expect that map.get("x") should also return 10.

HashMap<String, Integer> map = new HashMap<>();
map.put("x", 0);
map.put("y", 0);

//I now somehow want to link the value of x so its dependent on y

System.out.println(map.get("x"));
//Should return 0

map.put("y", 10);

System.out.println(map.get("x"));
//Should return 10 now

I can’t figure out how to get this working as x always gets the value of what y is now, and not what y is at the time of printing the value.

Answer

If you want to associate a group of keys with the same object, it can be achieved by using a mutable object as a value.

For instance, you can make use of StringBuilder or implement a custom class. It’ll be more performant and easier than an approach with implementing your own map which extends HashMap and is able to track these groups of keys and triggers a series of updates for each call of put(), replace() or remove().

Solution with a custom mutable Container can look like this:

HashMap<String, Container<Integer>> map = new HashMap<>();
Container<Integer> commonValue = new Container<>(0);
map.put("x", commonValue);
map.put("y", commonValue);

System.out.println("Value for 'x': " + map.get("x"));
System.out.println("Value for 'y': " + map.get("y"));

commonValue.setValue(10);

System.out.println("Value for 'x': " + map.get("x"));
System.out.println("Value for 'y': " + map.get("y"));

The Container class itself.

public class Container<T> {
    private T value;

    public Container(T value) {
        this.value = value;
    }

    public T getValue() {
        return value;
    }

    public void setValue(T value) {
        this.value = value;
    }

    @Override
    public String toString() {
        return String.valueOf(value);
    }
}

As I have already said, the alternative is to use a mutable class that is already provided by the JDK. The code is then almost the same:

HashMap<String, StringBuilder> map = new HashMap<>();
StringBuilder commonValue = new StringBuilder("0");
map.put("x", commonValue);
map.put("y", commonValue);

System.out.println("Value for 'x': " + map.get("x"));
System.out.println("Value for 'y': " + map.get("y"));

commonValue.replace(0, commonValue.length(), "10");

System.out.println("Value for 'x': " + map.get("x"));
System.out.println("Value for 'y': " + map.get("y"));

Output (by both versions)

Value for 'x': 0
Value for 'y': 0
Value for 'x': 10
Value for 'y': 10