Map.Entry entry = new DefaultMapEntry(key, value);
assertSame(key, entry.getKey());
assertSame(value, entry.getValue());
// 2. test pair constructor
KeyValue pair = new DefaultKeyValue(key, value);
assertSame(key, pair.getKey());
assertSame(value, pair.getValue());
// 3. test copy constructor
Map.Entry entry2 = new DefaultMapEntry(entry);
assertSame(key, entry2.getKey());
assertSame(value, entry2.getValue());