CLASSES
Method Modifiers
8.4.3
An instance method that is not
abstract
 can be overridden by an
abstract
method. 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
 ( 20.1.2). (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
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
 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
.
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