Java Reference
In-Depth Information
Triangle
. The
super( )
in
Triangle
calls the constructor in
TwoDShape
. In a class hier-
archy, if a superclass constructor requires parameters, then all subclasses must pass those
parameters “up the line.” This is true whether or not a subclass needs parameters of its own.
When Are Constructors Executed?
In the foregoing discussion of inheritance and class hierarchies, an important question may
have occurred to you: When a subclass object is created, whose constructor is executed
first, the one in the subclass or the one defined by the superclass? For example, given a
subclass called
B
and a superclass called
A
, is
A
's constructor executed before
B
's, or vice
versa? The answer is that in a class hierarchy, constructors complete their execution in or-
der of derivation, from superclass to subclass. Further, since
super( )
must be the first state-
ment executed in a subclass' constructor, this order is the same whether or not
super( )
is
used. If
super( )
is not used, then the default (parameterless) constructor of each superclass
will be executed. The following program illustrates when constructors are executed:
