This code should not compile:

class Main {
public boolean func(Object obj) {
return obj == 0;
}
}

But javac (since JDK 7 accepts it).

The following variant:

class Main {
public boolean func(Object obj) {
return 0 == obj;
}
}

should also not compile - but note that this was accepted since JDK 5.

Relevant JLS section is 15.21. Javac seems to treat this as a reference comparison, but a reference comparison is only allowed when BOTH operands are reference types.