guava之Multimap

package com.test;

import java.util.Collection;

import com.google.common.collect.ArrayListMultimap;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.Multimap;

public class MutliMapTest {
    public static void main(String... args) {
        Multimap myMultimap = ArrayListMultimap.create();

        // Adding some key/value
        myMultimap.put("Fruits", "Bannana");
        myMultimap.put("Fruits", "Apple");
        myMultimap.put("Fruits", "Pear");
        myMultimap.put("Fruits", "Pear");
        myMultimap.put("Vegetables", "Carrot");

        // Getting the size
        int size = myMultimap.size();
        System.out.println(size); // 5

        // Getting values
        Collection fruits = myMultimap.get("Fruits");
        System.out.println(fruits); //  [Bannana, Apple, Pear, Pear]
        System.out.println(ImmutableSet.copyOf(fruits));// [Bannana, Apple, Pear]
        // Set set = Sets.newHashSet(list);
        // Set foo = new HashSet(myList);

        Collection vegetables = myMultimap.get("Vegetables");
        System.out.println(vegetables); // [Carrot]

        // Iterating over entire Mutlimap
        for (String value : myMultimap.values()) {
            System.out.println(value);
        }

        // Removing a single value
        myMultimap.remove("Fruits", "Pear");
        System.out.println(myMultimap.get("Fruits")); // [Bannana, Apple, Pear]

        // Remove all values for a key
        myMultimap.removeAll("Fruits");
        System.out.println(myMultimap.get("Fruits")); // [] (Empty Collection!)
    }

}

这里有一点你可能会疑惑，就是为何get方法返回的是一个collection而不是list，这是因为前者会更加有用。如果你需要基于multimap直接操作list或者set，那么可以使用在定义类型的时候使用子类名称：ListMultimap，SetMultimap和SortedSetMultimap。例如：
ListMutlimap myMutlimap = ArrayListMultimap.create();

List myValues = myMutlimap.get("myKey");  // Returns a List, not a Collection.

另外，静态工具类 Multimaps的asMap可以返回具体类型的map。
static Map> asMap(ListMultimap multimap)
Returns multimap.asMap(), with its type corrected from Map> to Map>.