17.5.2 Static and instance methods

When a method declaration includes a static modifier, that method is said to
be a static method. When no
static modifier is present, the method is said to be an instance method.
A static method does not operate on a specific instance, and it is a
compile-time error to refer to this in a static
method.
An instance method operates on a given instance of a class, and that
instance can be accessed as this (§14.5.7).
When a method is referenced in a member-access (§14.5.4) of the form E.M,
if M is a static method, E must
denote a type that has a method M, and if M is an instance method, E must
denote an instance of a type that has a
method M.
The differences between static and instance members are discussed further
in §17.2.5.