The Java Language Specification, Third Edition

8.4.3 Method Modifiers CLASSES
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For example, we can declare an abstract class Point that requires its subclasses
to implement toString if they are to be complete, instantiable classes:
abstract class Point {
int x, y;
public abstract String toString();
}
This abstract declaration of toString overrides the non-abstract toString
method of class Object. (Class Object is the implicit direct superclass of class
Point.) Adding the code:
class ColoredPoint extends Point {
int color;
public String toString() {
return super.toString() + ": color " + color; // error
}
}
results in a compile-time error because the invocation super.toString() refers
to method toString in class Point, which is abstract and therefore cannot be
invoked. Method toString of class Object can be made available to class
ColoredPoint only if class Point explicitly makes it available through some
other method, as in:
abstract class Point {
int x, y;
public abstract String toString();
protected String objString() { return super.toString(); }
}
class ColoredPoint extends Point {
int color;
public String toString() {
return objString() + ": color " + color; // correct
}
}
8.4.3.2 static Methods
DRAFT
A method that is declared static is called a class method. A class method is
always invoked without reference to a particular object. An attempt to reference
the current object using the keyword this or the keyword super or to reference
the type parameters of any surrounding declaration in the body of a class method
results in a compile-time error. It is a compile-time error for a static method to
be declared abstract.
A method that is not declared static is called an instance method, and sometimes
called a non-static method. An instance method is always invoked with