When Constructors Are Called?

When Constructors Are Called?

When a class hierarchy is created, in what order are the constructors for the classes that make up the hierarchy called?

 

For example, given a subclass called B and a superclass called A, is A's constructor called before B's, or vice versa? The answer is that in a class hierarchy, constructors are called in order of derivation, from superclass to subclass.

 

Further, since super( ) must be the first statement executed in a subclass' constructor, this order is the same whether or not super( ) is used. If super( ) is not used, then the default or parameterless constructor of each superclass will be executed.

 

The following program illustrates when constructors are executed:

 

// Demonstrate when constructors are called.

// Create a super class.

class A {

A() {

System.out.println("Inside A's constructor.");

}

}

// Create a subclass by extending class A.

class B extends A {

B() {

System.out.println("Inside B's constructor.");

}

}

// Create another subclass by extending B.

class C extends B {

C() {

System.out.println("Inside C's constructor.");

}

}

class CallingCons {

public static void main(String args[]) {

C c = new C();

}

}

 

The output from this program is shown here:

 

Inside A's constructor

Inside B's constructor

Inside C's constructor

 

As you can see, the constructors are called in order of derivation. If you think about it, it makes sense that constructors are executed in order of derivation. Because a superclass has no knowledge of any subclass, any initialization it needs to perform is separate from and possibly prerequisite to any initialization performed by the subclass. Therefore, it must be executed first.