Java 使用多图作为参数

Question

I wanted to create a multimap for each of the keys that I put, but I get this error Multimap is abstract; cannot be instantiated. Multimap is abstract; cannot be instantiated. Is there another way to do this?

package Graph;


import com.google.common.collect.Multimap;
import java.util.HashMap;

public class builder {

    public static void main(String[] args) {
        
        HashMap<String,Multimap<String,String>> map = new HashMap<>();
        map.put("Cars",new Multimap<String,String>());
        
    }
}

Answer 1

Interface, not a class

MultiMap in Google Guava is an interface. You cannot instantiate an interface.

Choose a concrete implementation of that interface to instantiate.

Guava supplies at least a dozen. And third-parties may also supply implementations (I don't know of any).

By the way, Eclipse Collections also provides multi-map functionality.

Answer 2

Here are the few things about Google's Multimap interface.

Note 0

Multimap is a collection-type data structure which can store more than 1 values against a key.

    multimap.get(0).add("Soumyajit");        
    multimap.get(0).add("Chatterjee");        
    System.out.println(multimap);

will returns: {0=[Soumyajit, Chatterjee]} as output.

Note 1

It has two child interfaces ListMultimap and SetMultimap . we can use Multimap's static method to create classes that will implement Multimap interface ie, ArrayListMultimap<K, V>, LinkedListMultimap<K, V> etc.

example: ListMultimap<String, Integer> m = ArrayListMultimap.create();

Note 2

We can get values from Multimap collection against a key in the form of collection using: Multimap.get(key) ; which returns value if present with its reference so if we modify the returned result set it will reflect back to original Map. If no value found for specific key, it will return an empty collection (Not NULL).

Therefore, the following code is absolutely fine:

        ListMultimap<Integer, String> multimap =  ArrayListMultimap.create();        
      multimap.get(0).add("Soumyajit");        
      multimap.get(4).add("Chatterjee");        

Output: {0=[Soumyajit], 4=[Chatterjee]}

Note 3

To add data in Multimap we can follow two ways:

map.get(index).add(value);
   or
map.put(index, value);

For deleting, we will follow same structure:

   map.get(index).remove(value);
     or
   map.remove(index, value);

Reference taken from: Official Google's Official Guava documentation

Answer 3

As mentioned by others, Multimap is an interface. You will need to use concrete classes such as HashMultimap or ImmutableMultimap . See below example:

HashMap<String, Multimap<String, String>> maps = new HashMap<>();

Multimap<String, String> multimap = HashMultimap.create();
multimap.put("KEY1", "V1");
    
maps.put("KEY", multimap);