Storing and Retrieving ArrayList values from hashmap

Question

I have a hashmap of the following type

HashMap> map=new HashMap>();    
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Answer 1

Our variable:

Map> map = new HashMap>();

To store:

map.put("mango", new ArrayList(Arrays.asList(0, 4, 8, 9, 12)));

To add numbers one and one, you can do something like this:

String key = "mango";
int number = 42;
if (map.get(key) == null) {
    map.put(key, new ArrayList());
}
map.get(key).add(number);

In Java 8 you can use putIfAbsent to add the list if it did not exist already:

map.putIfAbsent(key, new ArrayList());
map.get(key).add(number);

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Use the map.entrySet() method to iterate on:

for (Entry> ee : map.entrySet()) {
    String key = ee.getKey();
    List values = ee.getValue();
    // TODO: Do something.
}