One of the less safe features of C is function pointers — it is usually not possible to write generic and
typesafe code with function pointers. (In C++, templates can be used to for typesafe generic code.) For
example, consider qsort in the standard C library, or a callback in Windows or X Windows. These
callbacks use unchecked void* parameters. For that reason, Java did not have function pointers. But they
slipped into Java 1.1 with the reflection feature.
Tip
Of course, normally you would use a dynamically bound method with a known name and signature, and then use inheritance to call different functions with similar properties, But sometimes, that is too constraining. Consider this example — we want to print tables of mathematical functions.
{ double dx = (to - from) / 20;
for (double x = from; x <= to; x += dx)
{
double y = f(x); // wrong syntax
Format.print(System.out, "%12.4f |", x);
Fozmat.print(System.out, "%12.4f\n", y);
}
static void printTable(double from, double to, Method f)
}
printTable(0, 10, Math.sin); // wrong syntax
In principle, this is now possible in Java, but the syntax is more complex.
Here is how you can get a method pointer:
printTable(0, 10, java.lang.Math.class.getMethod("sqrt", new Class[] { double.class }));
And here is how you make the call:
static void printTable(double from, double to, Method f)
{ . . .
// compute y = f (x);
Object [] args = ( new Double (x) );
Double d = (Double) f. invoke (null, args);
double y = d.doubleValue ();
}
However, keep in mind that invoking a method is very slow. Only use this as a last resort, if there is no better design involving an inter face and a dynamically bound method.
