Java NullPointerException in class using HashMap

1.The problem statement, all variables and given/known data + 2. Relevant equations
Hello,
I really need your help, because I'm a Java newbie and I've got stuck in this Java homework.
I should implement class ObjectCounter, which has to include two methods:

Code (Text):

// increase the counter for given object
public void add(Object o)
// returns the current number of objects, i.e. current value of counter
public int getCount(Object o)
 

Implemented class ObjectCounter has to use the HashMap and all of it has to be called this way:

Code (Text):

ObjectCounter objectCounter = new ObjectCounter();
objectCounter.add("hi");
// has to print 1
System.out.println(objectCounter.getCount("hi"));
// has to print 0
System.out.println(objectCounter.getCount("hello"));
objectCounter.add("hi");
// has to print 2
System.out.println(objectCounter.getCount("hi"));
objectCounter.add("hello");
// has to print 1
System.out.println(objectCounter.getCount("hello"));
 

3. The attempt at a solution

Code (Text):

public class ObjectCounter {

    private HashMap<Integer, Integer> map = new HashMap();

    public void add(Object o) {
        int counter = getCount(o);
        if (counter == 0) {
            map.put(o.hashCode(), 1);
        } else {
            map.remove(o.hashCode());
            map.put(o.hashCode(), counter++);
        }
    }

    public int getCount(Object o) {
        System.out.println(map.get(o.hashCode()));
        return map.get(o.hashCode());
    }
}
 

But it always ends with java.lang.NullPointerException, but I don't know how to repair it.

Thank you very much!

 

Your advice was really helpful!
Thank you so much!