Some threading problems were solved without extending java.lang.Object.
The methods wait(), notify() and notifyAll() are implemented on Object, which seems to me not to be the best decision since it pollutes the class's interface. It affects all classes in Java, which I think should have been avoided.
Next, the synchronized block takes an instance of type Object, allowing us to accidently pass shared instances (e.g. String), which can cause problems. They could have crafted a class Mutex/Lock in order to avoid this.
I wonder whether this comes with any technical advantages - e.g. performance - or whether it is just bad design? Is there somewhere an official documentation, e.g. a JEP or something similar - on why the Java language designers decided to work directly with java.lang.Object?