How to include duplicate keys in HashMap?

Question

I need linked and multiple keys in key set. I tried this:

LinkedHashMap<Integer, String> map = new LinkedHashMap< Integer,String>();

map.put( -1505711364,"4");
map.put(294357273, "15"); map.put(-1593134417, "28"); map.put(-1231165758, "45");
map.put(121046798, "58");
map.put(294357273, "71"); map.put(-1593134417, "82"); map.put(-1231165758, "95");
map.put(121046798, "108");

I need duplicate keys which is order preserved. What is the way to do this??

score 38 · Answer 1 · edited Jul 19 '20 at 13:03

38

You can't have duplicate keys in a Map. You can rather create a Map<Key, List<Value>>, or if you can, use Guava's Multimap.

Multimap<Integer, String> multimap = ArrayListMultimap.create();
multimap.put(1, "rohit");
multimap.put(1, "jain");

System.out.println(multimap.get(1));  // Prints - [rohit, jain]

And then you can get the java.util.Map using the Multimap#asMap() method.

edited Jul 19 '20 at 13:03

drac_o

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answered Sep 20 '13 at 17:06

Rohit Jain

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will insertion order is preserved in this one?? – LoneWolf Sep 20 '13 at 17:10
3

@vijji It doesn't hurt to try it out :) – Konstantin Yovkov Sep 20 '13 at 17:14
The order of key insertion won't be preserved with `ArrayListMultimap`; you might want `LinkedListMultimap`. – Louis Wasserman Sep 20 '13 at 17:34
@LouisWasserman. But what's the meaning of the very 2nd line of the ArrayListMultimap doc? Also, when I tried inserting different keys, it was giving out the keys in the insertion order. Am I missing something? – Rohit Jain Sep 20 '13 at 17:36
1

@RohitJain: 1) The meaning is that the values for a given key are kept in insertion order, but the keys themselves might not be kept in insertion order. `ArrayListMultimap` is like a `HashMap>`, not a `LinkedHashMap>`. You got lucky with your test data, that insertion order was preserved. – Louis Wasserman Sep 20 '13 at 17:38
@LouisWasserman. Oh. Thanks. It's clear now :) – Rohit Jain Sep 20 '13 at 17:42
How can we find the duplicate pair? from multimap – Ashok kumar Ganesan Aug 06 '20 at 10:55

Luiggi Mendoza · Answer 2 · 2017-09-26T14:07:45.030

21

Use Map<Integer, List<String>>:

Map<Integer, List<String>> map = new LinkedHashMap< Integer, List<String>>();

map.put(-1505711364, new ArrayList<>(Arrays.asList("4")));
map.put(294357273, new ArrayList<>(Arrays.asList("15", "71")));
//...

To add a new key/value pair in this map:

public void add(Integer key, String newValue) {
    List<String> currentValue = map.get(key);
    if (currentValue == null) {
        currentValue = new ArrayList<String>();
        map.put(key, currentValue);
    }
    currentValue.add(newValue);
}

edited Sep 26 '17 at 14:07

answered Sep 20 '13 at 17:05

Luiggi Mendoza

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Then to add a value for an **existing** key one will need `get`, `concatenate`, and `put`, right? – PM 77-1 Sep 20 '13 at 17:29
@PM77-1 just `get` the `List list` and call `list#add(theNewString)`. – Luiggi Mendoza Sep 20 '13 at 17:30
@PM77-1 I've updated the answer to show that. Thanks. – Luiggi Mendoza Sep 20 '13 at 17:32
Why do you write `Arrays.asList(new String[] { "15", "71" } )` instead of `Arrays.asList("15", "71")`? – Holger Sep 20 '13 at 17:38
@Holger I forgot the method accepts a varargs... – Luiggi Mendoza Sep 20 '13 at 17:41
1. Why am I getting `java.lang.UnsupportedOperationException` at this step `currentValue.add(newValue);` ? 2. How does it come we update `currentValue` after adding it to `map` ? will this actually change the `map` ? Aren't we supposed to put the `currentValue` in the `map` again – Radhwen Sep 26 '17 at 13:46
@Radhwen fixed. – Luiggi Mendoza Sep 26 '17 at 14:07
@LuiggiMendoza Great ! Now can you please answer my second question ? – Radhwen Sep 26 '17 at 14:22
1

@Radhwen since the value is a mutable list, then what you only need to do is to update the state of the list. If the list is immutable (as it was before), then in that case you would need to update the map by updating the value in that key. – Luiggi Mendoza Sep 26 '17 at 14:24

score 14 · Answer 3 · edited Jul 12 '20 at 15:37

14

Map does not supports duplicate keys. you can use collection as value against same key.

Associates the specified value with the specified key in this map (optional operation). If the map previously contained a mapping for the key, the old value is replaced by the specified value.

Documentation

you can use any kind of List or Set implementation according to your requirement.

If your values might be also duplicate you can go with ArrayList or LinkedList, in case values are unique you can use HashSet or TreeSet etc.

Also In google guava collection library Multimap is available, it is a collection that maps keys to values, similar to Map, but in which each key may be associated with multiple values. You can visualize the contents of a multimap either as a map from keys to nonempty collections of values:

a → 1, 2
b → 3

Example -

ListMultimap<String, String> multimap = ArrayListMultimap.create();
multimap.put("a", "1");
multimap.put("a", "2");
multimap.put("c", "3");

edited Jul 12 '20 at 15:37

drac_o

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answered Sep 20 '13 at 17:06

Subhrajyoti Majumder

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Does HashSet support this? – LoneWolf Sep 20 '13 at 17:07
No. HashSet does not. – BlackHatSamurai Sep 20 '13 at 17:08
how will you remove from this multimap with key? – Sathish Aug 03 '15 at 06:25
`Multimap#removeAll(Object key)` - Removes all values associated with the key key. `Multimap#remove(Object key,Object value)` - Removes a single key-value pair with the key key and the value value from this multimap, if such exists. If multiple key-value pairs in the multimap fit this description, which one is removed is unspecified. – Subhrajyoti Majumder Aug 03 '15 at 10:07

score 1 · Answer 4 · answered Sep 20 '13 at 17:08

1

hashMaps can't have duplicate keys. That said, you can create a map with list values:

Map<Integer, List<String>>

However, using this approach will have performance implications.

answered Sep 20 '13 at 17:08

BlackHatSamurai

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How to include duplicate keys in HashMap?

4 Answers4

Linked

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