Re: can this be done with generics?
On 11/24/13 4:55 PM, Andreas Leitgeb wrote:
I'd like to use the "call methods in a chain" pattern
across a class-subhierarchy, with most of the methods
defined in the base-class.
Suppose, I had two classes:
class Foo {
public Foo foo1() {
/* do something very interesting ... */
return this;
}
// assume there were like 100 such foo#() methods here
// (in practice they of course have more diverse names)
}
class Bar extends Foo {
public Bar bar() {
/* do something very interesting */
return this;
}
}
And then, somewhere else:
// the single-class chain works just fine:
new Foo().foo1().foo42().foo84();
// but if I start with a Bar, and eventually after some foo#() calls
// want to call method bar() which is not in Foo, then I'm out of luck:
new Bar().foo1().foo42().foo84().bar(); // doesn't work, of course!
Is there a way to use *generics* for the methods of class Foo such that
each foo#() returns the *static* type on which the compiler saw it
applied?
Alternatives, that do NOT satisfy me:
a) use an explicit cast to (Bar)
((Bar)(new Bar().foo1().foo42().foo84())).bar();
Would turn into a nightmare, when more of Bar's
methods got mixed into the chain.
b) override each of the 100 foo#()s in Bar with a Bar return-type.
Would turn into a nightmare, when more sub-classes like Bar
appeared, or new methods added to Foo.
(Note, that "bad performance" due to extra calls is NOT my concern here)
c) add "Foo bar() { return this; }" in Foo.
Would turn into a nightmare, when more sub-classes like Bar
appeared (and Foo would have to reflect all subclasses' methods),
or some chain accidentally called a subclass method on a chain
started from some other class's instance, and compiler wouldn't
detect the mistake.
I'm pretty sure, that this would be principially possible (as in: the
compiler has all the information that would be needed), but I can't seem
to find an approach for how to capture the static type of the expression
on which the (non-static) method is called.
Thanks in advance!
Yes, this *can* be done with generics, but its relatively ugly and hard
to maintain. I've been doing a lot of work in PHP lately, and they have
a return type hint "@return $this", which makes my IDE of choice
recognize the intent. Java, though superior in many ways, is lacking
that kind of ability.
To answer your question though:
abstract class Foo<T extends Foo<T>> {
public T foo1() { return getThis(); }
protected abstract T getThis();
}
class Bar extends Foo<Bar> {
public Bar bar1() { /* something interesting */ return this; }
protected T getThis() { return this; }
}
It gets uglier if you need multiple levels of inheritance, or if you
have two classes which refer to each other.