How to make a hash table throw an exception when inserting a null key (Java)?

Question

For an assessment I need to create a hash table, which I have done. However, I need to throw an exception when inserting a null key.

The test I ran, which I need to pass, was:

@Test(expected = IllegalArgumentException.class)
public void testNullKey() {
    Hashtable<Boolean> h = new Hashtable<>(10);
    h.put(null, true);
}

Which gives me the error:

java.lang.Exception: Unexpected exception, expected<java.lang.IllegalArgumentException> but was<java.lang.NullPointerException>

Caused by: java.lang.NullPointerException: Cannot invoke "String.hashCode()" because "key" is null
    at Hashtable.hash(Hashtable.java:270)
    at Hashtable.put(Hashtable.java:84)
    at test.TestHT.testNullKey(TestHT.java:52)

Hashtable.hash(Hashtable.java:270) is:

private int hash(String key)
{
    int hashVal = key.hashCode();
    hashVal %= max;

    if (hashVal < 0)

        hashVal += max;

    return hashVal;
}

and Hashtable.put(Hashtable.java:84) is:

public void put(String key, V value)
{
    int h = 0;

    if (getLoadFactor() > maxLoad)
        resize();

    h = probeType == PROBE_TYPE.LINEAR_PROBE ? hash(key) : doubleHash(key);

    if (get(key) != null)
    {
        arr[h] = new Pair(key, value);
    } else
    {
        arr[findEmpty(h, 0, key)] = new Pair(key, value);
        itemCount++;
    }
}

I am at a total loss for throwing an exception when inserting a null key. I've tried adding throw new IllegalArgumentException();and other attemps like that within put, but I can't get it to work.

Does anyone have any ideas or pointers?

Answer 1

OK. So this Hashtable class is your code, and not java.util.Hashtable. (You had me tricked for a minute there ...)

The solution is simple. Explicitly test the key to see if it is null and throw the required exception:

if (key == null) {
    throw new IllegalArgumentException("key is null");
}

I'll leave you to figure out where the best place to put this code is.

---

The other approach would be to make NullPointerException the expected exception ... but my reading of your question is that that testcase is checking that a requirement is satisfied.