Java Reference
In-Depth Information
Remember that in Java, class methods are declared with the
static keyword, and the terms static method and class
method are used interchangeably.
For example, when working with the Circle class you might find that you often
want to compute the area of a circle with a given radius but don't want to bother
creating a Circle object to represent that circle. In this case, a class method is more
public static double area ( double r ) { return PI * r * r ; }
It is perfectly legal for a class to define more than one method with the same name,
as long as the methods have different parameters. This version of the area()
method is a class method, so it does not have an implicit this parameter and must
have a parameter that specifies the radius of the circle. This parameter keeps it dis‐
tinct from the instance method of the same name.
As another example of the choice between instance methods and class methods,
consider defining a method named bigger() that examines two Circle objects and
returns whichever has the larger radius. We can write bigger() as an instance
method as follows:
// Compare the implicit "this" circle to the "that" circle passed
// explicitly as an argument and return the bigger one.
public Circle bigger ( Circle that ) {
if ( this . r > that . r ) return this ;
else return that ;
We can also implement bigger() as a class method as follows:
// Compare circles a and b and return the one with the larger radius
public static Circle bigger ( Circle a , Circle b ) {
if ( a . r > b . r ) return a ;
else return b ;
Given two Circle objects, x and y , we can use either the instance method or the
class method to determine which is bigger. The invocation syntax differs signifi‐
cantly for the two methods, however:
// Instance method: also y.bigger(x)
Circle biggest = x . bigger ( y );
Circle biggest = Circle . bigger ( x , y ); // Static method
Both methods work well, and, from an object-oriented design standpoint, neither of
these methods is “more correct” than the other. The instance method is more for‐
mally object oriented, but its invocation syntax suffers from a kind of asymmetry. In
a case like this, the choice between an instance method and a class method is simply
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