How do I add values to a Set inside a Map? [closed]

Answer 1

Something to keep in mind is that the value will initially be null, so the first time you use a key, you'll have to initialize it:

Map> map;  

To add a number to the key, you must do this:

String key = "apple"; // for example 

Set set = map.get(key); 
if (set == null) {
    set = new HashSet();
    map.put(key, set);
}
set.add(5);

Unfortunately, everywhere you interact with the set, you must null check. For example, if you want to check if a certain key had a certain number mapped to it, you couldn't safely do this:

if (map.get(key).contains(number))  // not safe

because the call to get() may return null (if there's no entry for the key) and you'd get a NullPointerException.

There is a way to make your map very convenient to use by internalising the null check inside the map, so that a call to get() will always return a non-null set; create an anonymous class that overrides get() accordingly:

Map> map = new HashMap> () {
    @Override
    public Set get(Object key) {
        Set set = super.get(key); 
        if (set == null) {
            set = new HashSet();
            put(key, set);
        }
        return set;
    }
}

With this in place, your main code becomes a lot simpler and clearer. To add:

map.get(key).add(number);

To check:

if (map.get(key).contains(number)) // now safe

The null checking code is no longer necessary.

——

Java 8 update:

To deal with the null entry problem when adding to the set:

map.computeIfAbsent( key, k -> new HashSet<>() ).add(number);

and for null safe checking:

if ( map.getOrDefault(key, Collections.emptySet() ).contains(number))