Declaration And Initialization On Separate Statement In A Class

I know that I can declare and initialize an member variable inside a class in a single statement but I can't do them on separate sentence. Why?

For example, I am allowed to do the following,

class A{
int a = 5;
// rest of the class
}

But I am not allowed to do this,
class A{
int a;
a = 5; // it doesn't compile even when the variable 'a' is static
// rest of the class
}

Why java don't allow me to do that?