Question

It puzzles me how the following segment can lead to a null value of the Boolean mandatory, although it is not null at the corresponding key in the actual hashtable:

for (List<List<A>> a : hashMap.keySet()) {  
    Boolean mandatory = hashMap.get(a);
}

Answer 1

A HashMap will return null if the key specified is not bound to a value.

Issue is almost certainly that comparison op on a — a List — against keys is failing.

Let me guess: are you modifying these lists (the key object) after you have called a put? Did you remove all entries in one of the keys? Remember an empty list is equal to all empty ArrayLists. Further remember that List.equals() compares list content (one by one) to test equality.

package sof_6462281;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * Demonstrate the fact that the Map uses key.equals(k) to
 * test for key equality.  Further demonstrate that it is a 
 * very bad idea to use mutable collections are keys to maps.
 */
public class ListAsKey {
    public static void main(String[] args) {
        Map<List<A>, Boolean>  map = new HashMap<List<A>, Boolean>();

        List<A> alist = new ArrayList<A>();
        map.put(alist, true);
        for (List<A> a : map.keySet()) {
            Boolean b = map.get(a);
            System.out.format("\t%s(ArrayList@%d) => %s\n",a, a.hashCode(), map.get(a)); 
        }

        // you changed your list after the put, didn't you?
        alist.add(new A());
        for (List<A> a : map.keySet()) {
            Boolean b = map.get(a);
            System.out.format("\t%s(ArrayList@%d) => %s\n",a, a.hashCode(), map.get(a)); 
        }

        alist.clear();
        for (List<A> a : map.keySet()) {
            Boolean b = map.get(a);
            System.out.format("\t%s(ArrayList@%d) => %s\n",a, a.hashCode(), map.get(a)); 
        }
    }
    public static final class A { /* foo */ }
}

Results:

[](ArrayList@1) => true
[sof_6462281.ListAsKey$A@4b71bbc9](ArrayList@1265744872) => null
[](ArrayList@1) => true

edit: added more ops to above and added console out.