— Scott McNealy, Sun Microsystems Chairman, President and ceo

constructors and 
gc
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The role of superclass constructors 
in an object’s life.
All the constructors in an object’s inheritance 
tree must run when you make a new object.
Let that sink in.
That means every superclass has a constructor 
(because every class has a constructor), and each 
constructor up the hierarchy runs at the time an 
object of a subclass is created. 
Saying 
new 
is a Big Deal. It starts the 
whole constructor chain reaction. And yes, 
even abstract classes have constructors. 
Although you can never say new on an 
abstract class, an abstract class is still 
a superclass, so its constructor runs 
when someone makes an instance of a 
concrete subclass.
The super constructors run to build 
out the superclass parts of the object. 
Remember, a subclass might inherit 
methods that depend on superclass state 
(in other words, the value of instance variables 
in the superclass). For an object to be fully-
formed, all the superclass parts of itself must be 
fully-formed, and that’s why the super constructor 
must run. All instance variables from every class 
in the inheritance tree have to be declared and 
initialized. Even if Animal has instance variables 
that Hippo doesn’t inherit (if the variables are 
private, for example), the Hippo still depends on 
the Animal methods that use those variables.
When a constructor runs, it immediately calls its 
superclass constructor, all the way up the chain 
until you get to the class Object constructor. 
On the next few pages, you’ll learn how superclass 
constructors are called, and how you can call 
them yourself. You’ll also learn what to do if your 
superclass constructor has arguments!