12.5

Creation of New Class Instances

EXECUTION

ColoredPoint() { super(); }

is provided for it automatically by the Java compiler.

This constructor then invokes the

Point

 constructor with no arguments. The

Point

 constructor does not begin with an invocation of a constructor, so the com 

piler provides an implicit invocation of its superclass constructor of no arguments,

as though it had been written:

Point() { super(); x = 1; y = 1; }

Therefore, the constructor for

Object

 which takes no arguments is invoked.

The class

Object

 has no superclass, so the recursion terminates here. Next,

any instance variable initializers and static initializers of

Object

 are invoked.

Next, the body of the constructor of

Object

 that takes no arguments is executed.

No such constructor is declared in

Object

, so the compiler supplies a default one,

which in this special case is:

Object() { }

This constructor executes without effect and returns.

Next, all initializers for the instance variables of class

Point

 are executed. As

it happens, the declarations of

x

 and

y

 do not provide any initialization expres 

sions, so no action is required for this step of the example. Then the body of the

Point

 constructor is executed, setting

x

 to

1

 and

y

 to

1

.

Next, the initializers for the instance variables of class

ColoredPoint

 are

executed. This step assigns the value

0xFF00FF

 to

color

. Finally, the rest of the

body of the

ColoredPoint

 constructor is executed (the part after the invocation

of

super

); there happen to be no statements in the rest of the body, so no further

action is required and initialization is complete.

Unlike C++, the Java language does not specify altered rules for method dis 

patch during the creation of a new class instance. If methods are invoked that are

overridden in subclasses in the object being initialized, then these overriding

methods are used, even before the new object is completely initialized. Thus,

compiling and running the example:

class Super {

Super() { printThree(); }

void printThree() { System.out.println("three"); }

}

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