Only a inner class can be defined as static, the top level class cannot be static.(Please correct me if I am wrong)
Also the constructor cannot be static, the compiler will complain in both these cases.
What's the use of adding it? It's already implicitly static since you cannot call it on an existing instance. Instead of allowing it, adding to more confusion, Sun decided that it's simply not possible. They could also have made it required but Sun know that programmer's are inherently lazy, and leaving out the static keyword for each constructor will save a lot of typing.