Following this example, we would do the following to create an
instance of the Car object:
//create a new instance of the Car object
Car car = new Car();
In this, the instance variable of the Car class named model and
also the instance variable registration of the super class
Vehicle both will get initialized to their default values. By
default, the Java Virtual Machine will allocate enough space for
all the instance variables of the Object’s own class and all the
instance variable in all its super classes. Do not forget that
in Java you can have multi-level inheritance, that is a Car IS A
Automobile IS A Vehicle. In such cases, the same initialization
mechanism will be followed. The initialization chain finally
leads to the java.lang.Object class as all the classes in Java
implicitly inherits from the Object class.
The important thing is however the order of initialization.
According to Java Language Specification, the initialization
starts with super class fields and end with the fields declared
in the Object’s own class. Thus, in our previous example, the
order of initialization will be:
- All variables in the Object class. (No fields declared)
- All variables in the Vehicle class. (field registration).
- All variables in the Car class (field model).
The logic behind this order of initialization is to ensure that
you can use a super class variable within a sub class
initialization properly.
The Initialization Mechanism…
In Java, the initialization of objects can be done in two ways:
- The static initialization blocks
- The constructor
The static initialization block is invoked only when the Class
Loader in Java loads the class. The constructor is not invoked
unless you explicitly invoke them in order to initialize an
instance of an Object.
In Java the first thing that a constructor does is to invoke
another constructor in the super class. In order to fully
understand this, I would try to iterate the mechanism in the
following points to note.
- In Java if no constructor is explicitly defined for a class,
the compiler provides a default no-argument constructor.
However, if you explicitly specify a public constructor,
compiler will not place any default constructor.
- When the constructor of the sub class is invoked, at first
the compiler will invoke the default constructor in the super
class recursively up in the tree.
- The super class’s constructor can also be invoked explicitly
by using the super() key word.
- From within a constructor, you are free to call a super
class constructor with any number of arguments. It does not have
to be the default constructor only.
- It is important however to note that the order of
constructor invocation starts from the Object’s class and
proceeds recursively up in the tree. However, if you remember
that the order of instance variable initialization proceeds from
the base class down to the Object’s class. (Figure 1).
- The previous point coincides with the fact that eventually
before a constructor of the Object’s class has finished its job,
a constructor of the super class is always called, which will
initialize the super class fields.
Figure 1: The Constructor invocation and Object initialisation order
Now that we understand how the inheritance in Java really
happens behind the scene, it is time to think more about the
implications. At first sight, inheritance is an attractive
mechanism. But on second thoughts, we will reveal some of the
problems that might arise with the user of inheritance.
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