Re: Question about Effective Java
On Thursday, February 9, 2012 6:08:51 PM UTC-8, Eric Sosman wrote:
On 2/9/2012 7:40 PM, Novice wrote:
At first glance it seems simple enough; in any other class, "this" refers
to an instance of the class that contains the "this", just like we see
here. But I'm not used to seeing several methods in a class all return
"this".
Get used to it.
Have you considered Googling for "builder pattern"? You might be surprised by
what just a few minutes of reading can reveal.
Analogies are never perfect, but often helpful. So, imagine a
group of friends ordering pizza: They've got a slip of paper listing
possible toppings, and they're going to put check marks next to the
items they want. "Pepperoni," says George, "gotta have pepperoni,"
making his mark on the paper and passing it to Gina. "Mushrooms
make my breath fresher," she says, adding her mark and passing the
paper along to Gail. She opts for extra cheese and hands the paper
to Guy, who's worried about his weight and says "Sounds good, let's
just go with these three." He passes the paper over the counter to
Guido, who makes them a pluperfectly personalized pizza.
Now, what do the friends pass among themselves during this
colloquy? Do they pass the pizza? No, it hasn't even been made
yet. Do they pass their chosen ingredients? No, George doesn't
have a handful of pepperoni, nor does Gina have mushrooms nor Gail
extra cheese, and Guy doesn't have plenty of nothing. What they
pass is the slip of paper, the specification of the pizza they would
like Guido to build for them.
... and that's very much like what's going on with Builder and
NutritionFacts. The friends "make marks on" a Builder, then hand
the Builder to Guido, who makes them a NutritionFacts built to its
specification.
That's the idea; now to the returning of "this". It's just a
convenience, really. Imagine that all those Builder methods were
"void", returning nothing at all. Then you'd write
NutritionFacts.Builder b = new NutritionFacts.Builder(300, 4);
b.calories(950);
b.sodium(800);
b.carbohydrate(1025);
NutritionFacts facts = b.build();
That is, you'd use the variable "b" to hold a reference to the
Builder while you "make your marks" on it. Eventually, you call
its build() method to "hand it to Guido" and get a pizza -- er,
that is, a NutritionFacts -- made to order. This would work just
fine, and there's nothing fundamentally wrong with it.
But if each of the Builder method returns a reference to the
Builder it has just "marked" things can be written a little more
compactly:
NutritionFacts facts =
new NutritionFacts.Builder(300, 4)
.calories(950)
.sodium(800)
.carbohydrate(1025)
.build();
Observe that you no longer need the "b" variable: The "new" creates
a Builder and produces a reference to it, the calories() method
modifies that Builder and returns a reference that the sodium()
method can use, ..., until eventually the build() method takes all
the information stored in the Builder and uses it to generate a
Pizza -- er, that is, a NutritionFacts. Since there's no "b" to
refer to the Builder, it becomes eligible for garbage collection
as soon as its fulfilled its purpose, namely, specifying what's
wanted in the new NutritionFacts.
In short, the returning of "this" enables a convenience, but
is not truly fundamental to the pattern of using a builder of some
kind to hold the parameters for a hidden constructor.
I'm also not used to named classes nested within classes. (I'm
Read the Java tutorials.
Then study the Java Language Specification (JLS) about "member types".
The keyword is "scope". The scope of a nested class is defined like any other
class or instance member: 'public', 'protected', package-private or 'private'.
There are also some special rules about access to the enclosing type's members.
familiar with anonymous inner classes like window listeners and with
It's the same, but anonymous classes don't have a name (hence "anonymous").
Do note that not all nested classes are inner classes; some are static.
putting additional named classes within the same source file in a
sequential fashion but putting a named class within another is still
feels a little odd.)
"Not familiar", "feels a little odd". Jeez, get over it. You sound like a
whiney little baby.
If you haven't got the ability to learn new stuff, you won't last 10 minutes in
software development. Really, get over it.
You'll get used to it. The pattern works even if the builder
class is completely separate from the built class -- consider, for
example, StringBuilder and String. (And note how StringBuilder's
append() method, for instance, returns a reference to the same
StringBuilder that the append() modified.)
I'll post back if I can't make my peace with this slightly exotic
technique ;-)
Why don't you first study up on it? The newsgroup is not a help desk, and it
certainly is no substitute for study.
And you call it "exotic" only because you're incompletely trained. Don't make
such judgments given your lack of experience. You are in no position yet to
decide what is "exotic" and what isn't. You need to check your ego at the door.
You'll get used to it. (Is there an echo in here?)
Bottom line: Study, and get used to learning new things.
--
Lew