多图番石榴值

Question

I have this scenario:

ID={1,2,3}
Title ={T1,T2,T3} 
Place ={P1,P2,P3}

I want to have this result:

ID: 1 , Title: T1, Place: P1
ID: 2 , Title: T2, Place: P2
ID: 3 , Title: T3, Place: P3

I used Multimap (Guava) to store the ID and title with their multiple values:

Multimap<String, String> MultiMap = ArrayListMultimap.create();

However, I couldn't so far get the desired result as described above. I used this code as a try:

   for (Entry<String, String> key : MultiMap.entries()) 
    {
      System.out.println("name : " + key.getKey() + " value : " + key.getValue());
    }

However, using this code the result I got is like this:

name : ID value: 1
name : ID value: 2
name : ID value: 3 
name : Title value: T1
name : Title value: T2
name : Title value: T3
name : Place value: P1
name : Place value: P2
name : Place value: P3

How to get the result as I described above using Multimap?

Answer 1

I don't think you understand correctly what a Multimap is. A Multimap maps from one or more keys to multiple values, it's an n-to-m relation. What you seem to have is a relation from an ID to a Title, which is an n-to-1 relation. The right data structure for this is a Map, not a Multimap.

Map<Integer, String> titlesById = new TreeMap<>(); // keep map ordered by ID
titlesById.put(1, "T1");
titlesById.put(2, "T2");
titlesById.put(3, "T3");
for(Map.Entry<Integer, String> entry : titlesById.entrySet()){
  System.out.println("ID : " + entry.getKey() + " Title : " + entry.getValue());
}

This should produce the desired output.

Note: if you are likely to look up titles by ID from the Map, then you probably want to switch from TreeMap to HashMap, as HashMap has constant time lookup.

---

Update: apparently you do need a Multimap. Then change the above code to something like this:

Multimap<Integer, String> titlesById = TreeMultimap.create<>(); // keep map ordered by ID
titlesById.putAll(1, Arrays.asList("T1a", "T1b", "T1c"));
titlesById.putAll(2, Arrays.asList("T2a", "T2b"));
titlesById.put(3, "T3");
for(Map.Entry<Integer, String> entry : titlesById.entries()){
  System.out.println("ID : " + entry.getKey() + " Title : " + entry.getValue());
}

Again: TreeMultimap sorts the Map by the key, HashMultimap has efficient lookup.

---

It gets more and more interesting, now you want to have 3 values connected to each other. In that case, you should use a Table (3-dimensional Map). Eg

Table<Integer, String, String> titlesAndPlacesById = TreeBasedTable.create();
titlesAndPlacesById.put(1, "T1", "P1");
titlesAndPlacesById.put(2, "T2", "P2");
titlesAndPlacesById.put(3, "T3", "P3");

for(Table.Cell<Integer, String, String> cell : titlesAndPlacesById.cellSet()){
  System.out.println("ID : " + cell.getColumnKey() + " Title : " + cell.getRowKey() + ", Place: " + cell.getValue());
}

---

But this is not a perfect match. If you want to map an ID to a title and place, then you probably want to have a custom object encapsulating title and place, and use that as the value in a plain old Map from integer to custom type.

public class TitleAndPlace{
    private final String title;
    private final String place;

    TitleAndPlace(String title, String place) {
        this.title = title;
        this.place = place;
    }

    public String getTitle() { return title; }

    public String getPlace() { return place; }

    @Override public boolean equals(Object o) {
        if (this == o) return true;
        else if (o instanceof TitleAndPlace) {
            TitleAndPlace that = (TitleAndPlace) o;
            return Objects.equals(title, that.title)
                && Objects.equals(place, that.place);
        } else return false;
    }

    @Override public int hashCode() {
        return Objects.hash(title, place);
    }
}

Map<Integer, TitleAndPlace> map = new TreeMap<>();
map.put(1, new TitleAndPlace("T1", "P1"));
map.put(2, new TitleAndPlace("T2", "P2"));

for(Map.Entry<Integer, TitleAndPlace> entry : map.entrySet()){
  System.out.println("ID : " + entry.getKey() + " Title : " + entry.getValue().getTitle() + ", Place: " + entry.getValue().getPlace());
}