我怎样才能 model 一个 class 具有另一个 class 的特定实例计数？

Question

If I have two different classes (say, person class and commodity class), how do I represent each person's individual demand (a number) for each good?

If I initialize say 10 instances of the person class, and then 5 different types of good: each person has their own personal demand for each type of good, but if I only initialize the good class as a separate class (so we'll have 5 instances of the good class in this example), how do I give each instance of the person their own specific demand for each of the 5 good instances?

I don't really know how to ask this in clearer language and hope I was specific enough that you know what I'm trying to ask. Please reply if you have questions!

Answer 1

I can think of a couple of simple ways to implement this; which one is more appropriate might depend on how you need to use a Person 's commodity demands.

The first idea is for each Person to hold a Map of Commodity to count, to indicate which commodities, and how much of of each, that person demands. Something like this:

import lombok.AllArgsConstructor;
import lombok.EqualsAndHashCode;
import lombok.Getter;

@EqualsAndHashCode
@AllArgsConstructor
public class Commodity {

    @Getter
    private UUID id;

    @Getter
    private String name;
}

import java.util.HashMap;
import java.util.Map;
import java.util.Optional;

public class Person {

    private Map<Commodity, Integer> demands = new HashMap<>();

    public void setDemand(Commodity commodity, Integer count) {
        demands.put(commodity, count);
    }

    public Optional<Integer> getDemand(Commodity commodity) {
        return Optional.ofNullable(demands.get(commodity));
    }
}

The other option, which might make more sense if you need to, for example, track a Demand as a distinct class in your model, would be for Person to have a set of Demand s, with each Demand referencing a Commodity and a count. So, given the same Commodity class above, something like this:

import lombok.AllArgsConstructor;
import lombok.Getter;

@AllArgsConstructor
@Getter
public class Demand {

    private Commodity commodity;
    private int count;
}

import java.util.LinkedList;
import java.util.List;

public class Person {

    private List<Demand> demands = new LinkedList<>();

    public void addDemand(Commodity commodity, Integer count) {
        addDemand(new Demand(commodity, count));
    }

    public void addDemand(Demand demand) {
        demands.add(demand);
    }
}