CODEWITHSUNDEEP
javacollectionsmaps
maryJane
maryJane

Reputation: 127

Working with Collections-Maps

Am trying to gives more than one value to a key in a map. I wanted something of this kind to work.. mapp.put("key1","value1") mapp.put("key1","value2"). The first value "value1" is getting over-ridden with "value2". How to retain both the values? Should I go for any other thing than maps for this to happen?

Upvotes: 1

Views: 202

Answers (5)

Paul Vargas
Paul Vargas

Reputation: 42060

You can use un map with list. Something like that:

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

For put a key1 with the first value:

if (!map.containsKey("key1")) {
    map.put("key1", new LinkedList<String>());
}
map.get("key1").add("value1");

For add the second value to key1

map.get("key1").add("value2");

You can have all this in one method

public void add(String key, String value) {
    if (!map.containsKey(key)) {
        map.put(key, new LinkedList<String>());
    }
    map.put(key, value);
}

If you want the values for the key1, you obtain a List

List<String> values = map.get("key1");

Upvotes: 1

Zhou Rui
Zhou Rui

Reputation: 47

HashMap<K, HashSet<V>> map = new HashMap<K, HashSet<V>>();
if(!map.containsKey(k)) {
  map.put(k, new HashSet<V>());
}
map.get(k).add(v);

if you have duplicate values, use list/vector instead of hashset.

Upvotes: 1

sgowd
sgowd

Reputation: 2262

From the Javadoc of Map,

A map cannot contain duplicate keys; each key can map to at most one value.

So, if you want to put multiple values for single key, you need to add the values to the list and put key1 and list to the map.

Upvotes: 1

dfb
dfb

Reputation: 13289

Also check out a Multimap if you want a map with that behavior

Upvotes: 0

DVK
DVK

Reputation: 129569

Put both valyes into another collection (Vector, Set, Stack, whatever fits your usage; or anything else), and store hat collection as a value in a map.

As a random example using Vector:

Vector<Object> myVector = new Vector<Object>();
myVector.add("value1");
myVector.add("value2");
mapp.put("key1", myVector);

Upvotes: 2

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