EXPRESSIONS

Field Access Using a Primary

15.10.1

s = t;

System.out.println("s.x=" + s.x + when("s", s));

}

static String when(String name, Object t) {

return " when " + name + " holds a "

+ t.getClass() + " at run time.";

}

}

produces the output:

t.x=1 when t holds a class T at run time.

s.x=0 when s holds a class S at run time.

s.x=0 when s holds a class T at run time.

The last line shows that, indeed, the field that is accessed does not depend on the

run time class of the referenced object; even if

s

 holds a reference to an object of

class

T

, the expression

s.x

 refers to the

x

 field of class

S

, because the type of the

expression

s

 is

S

. Objects of class

T

 contain two fields named

x

, one for class

T

and one for its superclass

S

.

This lack of dynamic lookup for field accesses allows Java to run efficiently

with straightforward implementations. The power of late binding and overriding is

available in Java, but only when instance methods are used. Consider the same

example using instance methods to access the fields:

class S { int x = 0; int z() { return x; } }

class T extends S { int x = 1; int z() { return x; } }

class Test {

public static void main(String[] args) {

T t = new T();

System.out.println("t.z()=" + t.z() + when("t", t));

S s = new S();

System.out.println("s.z()=" + s.z() + when("s", s));

s = t;

System.out.println("s.z()=" + s.z() + when("s", s));

}

static String when(String name, Object t) {

return " when " + name + " holds a "

+ t.getClass() + " at run time.";

}

}

Now the output is:

321