Why is this class mutable? [duplicate]

An arbitrary instance of Test isn’t guaranteed to be immutable, although direct instances of Test are. But consider this subclass:

public class MutableTest extends Test {
        private int mutable;
        public MutableTest(String url) {
                super(url);
        }

        @Override
        public String getUrl() {
                return super.getUrl() + mutable++;
        }
}

Then you can write something like this:

Test instance = new MutableTest("http://example.com/");
String firstGet = instance.getUrl();
String secondGet = instance.getUrl();
assertEquals(firstGet, secondGet); // Boom!