What changes need to be made to the data structure to support a composite key?

To support a composite key in a data structure, it is necessary to modify the key representation so that it can contain multiple fields. In Java, this is usually achieved by creating a separate class that combines all the key fields and overriding the equals() and hashCode() methods for correct comparison and hashing.

Example:

public class CompositeKey {
    private String part1;
    private int part2;

    public CompositeKey(String part1, int part2) {
        this.part1 = part1;
        this.part2 = part2;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        CompositeKey that = (CompositeKey) o;
        return part2 == that.part2 && Objects.equals(part1, that.part1);
    }

    @Override
    public int hashCode() {
        return Objects.hash(part1, part2);
    }
}

Such a key can be used, for example, in HashMap<CompositeKey, Value> for correct storage and lookup by a composite key.