Question

Is there an implementation of java.util.Map that only allows a value to be read once? What I’d like to do is something like this:

Map map = new ReadOnceMap();
map.put("key", "value")
System.out.println(map.get("key")); // prints "value"
System.out.println(map.get("key")); // prints null

EDIT: requirements:

• existing implementation
• values are guaranteed to be read at most one time

Answer 1

Something like this?

public class GetOnceHashMap<K,V> extends HashMap<K,V> {
    @Override
    public V get(Object key) {
        return remove(key);
    }

    @Override
    public Collection<V> values() {
        Collection<V> v = new ArrayList<V>(super.values());
        clear();
        return v;
    }

    @Override
    public Set<Map.Entry<K, V>> entrySet() {
        Set<Map.Entry<K, V>> e = new HashSet<Map.Entry<K,V>>(super.entrySet());
        clear();
        return e;
    }
}