ADDITIONAL SYSTEM INFORMATION :
windows 10 version 20H2,


A DESCRIPTION OF THE PROBLEM :
there is no error in program but it throws null pointer exception.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
to run smoothly.

---------- BEGIN SOURCE ----------
//TO DO: Complete this class, add JavaDocs

//Do not add any more imports!
import java.util.Iterator;
import java.util.Set;

public class MemMan implements Iterable<MemBlock> {

//******************************************************
//**** IMPORTANT: DO NOT CHANGE/ALTER/REMOVE ****
//**** ANYTHING PROVIDED LIKE THESE INTS HERE... ****
//******************************************************

public static final int FIRST_FIT = 0;
public static final int BEST_FIT = 1;
public static final int WORST_FIT = 2;
public static final int NEXT_FIT = 3;

public static BareNode head;

//******************************************************
//**** IMPORTANT: DO NOT CHANGE/ALTER/REMOVE ****
//**** ANYTHING PROVIDED LIKE THE INSTANCE ****
//**** VARIABLES IN THIS NESTED CLASS. ALSO ****
//**** DON'T REMOVE THE CLASS ITSELF OR ANYTHING ****
//**** STRANGE LIKE THAT. ****
//******************************************************

public static class BareNode {

public MemBlock block;
public BareNode next;
public BareNode prev;
public boolean marked;

public BareNode(MemBlock block) {
this.block = block;
}
}

public BareNode getHead(){
return head;

}

public BareNode malloc(int size){



BareNode node = new BareNode(new MemBlock(0,size,false));

node.next=head;
node.prev=null;

if(head!= null){
head.prev=node;
}

head=node;
return node;

}

public BareNode realloc(BareNode oldBn, int size){


     BareNode bNode = oldBn;
    
     bNode.block.size=size;

    
     return bNode;

}


public int garbageCollect(Set<Integer> addrs){

return 2;
}
public boolean free(BareNode node){
return true;
}
public static MemMan factory(int fit, BareNode head){

this.head= head;
}


public MemMan iterator() {
return new LinkedListIterator();
}

private class LinkedListIterator implements Iterator<MemBlock> {
       
        private BareNode current = head;

        public MemBlock next() {
            if (!hasNext()) {
             throw new NoSuchElementException();
            }
            MemBlock item = current.block;
            current = current.next;
            return item;
        }

        public boolean hasNext() {
         return current != null;
        }

        public void remove() {
         throw new UnsupportedOperationException();
        }
    }

}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
i have not found anyone.

FREQUENCY : always