15.10.2

Accessing Superclass Members using

super

EXPRESSIONS

t.z()=1 when t holds a class T at run time.

s.z()=0 when s holds a class S at run time.

s.z()=1 when s holds a class T at run time.

The last line shows that, indeed, the method that is accessed

does

 depend on the

run time class of referenced object; when

s

 holds a reference to an object of class

T

, the expression

s.z()

 refers to the

z

 method of class

T

, despite the fact that the

type of the expression

s

 is

S

. Method

z

 of class

T

 overrides method

z

 of class

S

.

The following example demonstrates that a null reference may be used to

access a class (

static

) variable without causing an exception:

class Test {

static String mountain = "Chocorua";

static Test favorite(){

System.out.print("Mount ");

return null;

}

public static void main(String[] args) {

System.out.println(favorite().mountain);

}

}

It compiles, executes, and prints:

Mount Chocorua

Even though the result of

favorite()

 is

null

, a

NullPointerException

 is

not

thrown. That  

Mount

  is printed demonstrates that the

Primary

 expression is

indeed fully evaluated at run time, despite the fact that only its type, not its value,

is used to determine which field to access (because the field

mountain

 is

static

).

15.10.2   Accessing Superclass Members using

super

The special form using the keyword

super

 is valid only in an instance method or

constructor, or in the initializer of an instance variable of a class; these are exactly

the same situations in which the keyword

this

 may be used ( 15.7.2). The form

involving

super

 may not be used anywhere in the class

Object

, since

Object

 has

no superclass; if

super

 appears in class

Object

, then a compile time error results.

Suppose that a field access expression

super.name

 appears within class

C

,

and the immediate superclass of

C

 is class

S

. Then

super.name

 is treated exactly

as if it had been the expression

((S)this).name

; thus, it refers to the field

named

name

 of the current object, but with the current object viewed as an

instance of the superclass. Thus it can access the field named

name

 that is visible

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