Strange Set Behaviour in Apex

set_behaviour.apex

Integer nCallsToEquals = 0;

public class HashingTest {
    private String s;

    public HashingTest(HashingTest ht) {
        this.s = ht.s;
    }

    public HashingTest(String s) {
        this.s = s;
    }

    public Integer hashCode() {
        return s.hashCode();
    }

    public Boolean equals(Object o) {
        ++nCallsToEquals;
        if(o instanceof HashingTest) {
            return s == ((HashingTest)o).s;
        } else {
            return false;
        }
    }

}

HashingTest k1 = new HashingTest('a');
HashingTest k2 = new HashingTest('b');

System.debug('k1.hashCode(): ' + k1.hashCode());
System.debug('k2.hashCode(): ' + k2.hashCode());

Set<HashingTest> aSet = new Set<HashingTest>();

aSet.add(k1);
aSet.add(k2);

nCallsToEquals = 0;
aSet.contains(k1);
System.debug('nCallsToEquals: ' + nCallsToEquals);
System.assertEquals(1, nCallsToEquals);
nCallsToEquals = 0;
aSet.contains(k2);
System.debug('nCallsToEquals: ' + nCallsToEquals);
System.assertEquals(1, nCallsToEquals);

// Comment out this debug line to get expected number of calls to equals()
System.debug('aSet: ' + aSet);

nCallsToEquals = 0;
aSet.contains(k1);
System.debug('nCallsToEquals: ' + nCallsToEquals);
System.assertEquals(1, nCallsToEquals);
nCallsToEquals = 0;
aSet.contains(k2);
System.debug('nCallsToEquals: ' + nCallsToEquals);
System.assertEquals(1, nCallsToEquals);