How can you guarantee the uniqueness of keys when using a HashMap data structure?

In Java, the HashMap data structure guarantees the uniqueness of keys by using the hashCode() and equals() methods of the key objects. When adding a key-value pair, HashMap computes the hash code of the key to determine the bucket where the element will be stored. If a bucket already contains an element with the same hash code, the equals() method is called to check for key equality.

To ensure key uniqueness:

• Properly override the hashCode() and equals() methods in the key class so that objects considered equal have the same hash code.
• Avoid mutability of keys after placing them in the HashMap, as changing the state of a key can break the correctness of lookups.

Example:

class Key {
    private final int id;

    public Key(int id) {
        this.id = id;
    }

    @Override
    public int hashCode() {
        return Integer.hashCode(id);
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
        Key other = (Key) obj;
        return this.id == other.id;
    }
}

HashMap<Key, String> map = new HashMap<>();
map.put(new Key(1), "value1");
map.put(new Key(1), "value2"); // will overwrite the value for the key with id=1