Map implementation with duplicate keys

Question

I want to have a map with duplicate keys.

I know there are many map implementations (Eclipse shows me about 50), so I bet there must be one that allows this. I know

Answer 1

You could simply pass an array of values for the value in a regular HashMap, thus simulating duplicate keys, and it would be up to you to decide what data to use.

You may also just use a MultiMap, although I do not like the idea of duplicate keys myself.

Answer 2

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

multimap.put(1, "A");
multimap.put(1, "B");
multimap.put(1, "C");
multimap.put(1, "A");

multimap.put(2, "A");
multimap.put(2, "B");
multimap.put(2, "C");

multimap.put(3, "A");

System.out.println(multimap.get(1));
System.out.println(multimap.get(2));       
System.out.println(multimap.get(3));

Output is:

[A,B,C,A]
[A,B,C]
[A]

Note: we need to import library files.

http://www.java2s.com/Code/Jar/g/Downloadgooglecollectionsjar.htm

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

or https://commons.apache.org/proper/commons-collections/download_collections.cgi

import org.apache.commons.collections.MultiMap;
import org.apache.commons.collections.map.MultiValueMap;

Answer 3

 1, Map<String, List<String>> map = new HashMap<>();

this verbose solution has multiple drawbacks and is prone to errors. It implies that we need to instantiate a Collection for every value, check for its presence before adding or removing a value, delete it manually when no values are left, etcetera.

2, org.apache.commons.collections4.MultiMap interface
3, com.google.common.collect.Multimap interface 

java-map-duplicate-keys

Answer 4

You are searching for a multimap, and indeed both commons-collections and Guava have several implementations for that. Multimaps allow for multiple keys by maintaining a collection of values per key, i.e. you can put a single object into the map, but you retrieve a collection.

If you can use Java 5, I would prefer Guava's Multimap as it is generics-aware.

Answer 5

No fancy libs required. Maps are defined by a unique key, so dont bend them, use a list. Streams are mighty.

import java.util.AbstractMap.SimpleImmutableEntry;

List<SimpleImmutableEntry<String, String>> nameToLocationMap = Arrays.asList(
    new SimpleImmutableEntry<>("A", "A1"),
    new SimpleImmutableEntry<>("A", "A2"),
    new SimpleImmutableEntry<>("B", "B1"),
    new SimpleImmutableEntry<>("B", "B1"),
);

And thats it. Usage examples:

List<String> allBsLocations = nameToLocationMap.stream()
        .filter(x -> x.getKey().equals("B"))
        .map(x -> x.getValue())
        .collect(Collectors.toList());

nameToLocationMap.stream().forEach(x -> 
do stuff with: x.getKey()...x.getValue()...

Answer 6

Learn from my mistakes...please don't implement this on your own. Guava multimap is the way to go.

A common enhancement required in multimaps is to disallow duplicate keys-value pairs.

Implementing/changing this in a your implementation can be annoying.

In Guava its as simple as:

HashMultimap<String, Integer> no_dupe_key_plus_val = HashMultimap.create();

ArrayListMultimap<String, Integer> allow_dupe_key_plus_val = ArrayListMultimap.create();