15.11.2

Compile Time Step 2: Determine Method Signature

EXPRESSIONS

/**

Copy all relevant fields of the argument into

this

ColoredPoint

 object.

 */

public void adopt(Point p) { x = p.x; y = p.y; }

public String toString() {

String s = "," + COLORS[color];

return super.toString(s);

}

}

Now consider a third compilation unit that uses

ColoredPoint

:

import points.*;

class Test {

public static void main(String[] args) {

ColoredPoint cp =

new ColoredPoint(6, 6, ColoredPoint.RED);

ColoredPoint cp2 =

new ColoredPoint(3, 3, ColoredPoint.GREEN);

cp.adopt(cp2);

System.out.println("cp: " + cp);

}

}

The output is:

cp: (3,3,red)

The application programmer who coded class

Test

 has expected to see the

word

green

, because the actual argument, a

ColoredPoint

, has a

color

 field,

and

color

 would seem to be a  relevant field  (of course, the documentation for

the package

Points

 ought to have been much more precise!).

Notice, by the way, that the most specific method (indeed, the only applicable

method) for the method invocation of

adopt

 has a signature that indicates a

method of one parameter, and the parameter is of type

Point

. This signature

becomes part of the binary representation of class

Test

 produced by the compiler

and is used by the method invocation at run time.

Suppose the programmer reported this software error and the maintainer of

the

points

 package decided, after due deliberation, to correct it by adding a

method to class

ColoredPoint

:

public void adopt(ColoredPoint p) {

adopt((Point)p); color = p.color;

}

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