Path: csiph.com!eternal-september.org!feeder.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: Eric Sosman <esosman@comcast-dot-net.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: Generics
Date: Wed, 17 Aug 2016 15:22:10 -0400
Organization: A noiseless patient Spider
Lines: 63
Message-ID: <np2dhf$tvo$1@dont-email.me>
References: <681a67de-8a78-46ab-a028-5176a48df402@googlegroups.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
Injection-Date: Wed, 17 Aug 2016 19:22:24 -0000 (UTC)
Injection-Info: mx02.eternal-september.org; posting-host="22e5ab429c5a4bb738666b9c52e40d69"; logging-data="30712"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX18uX24SOq9vo63n3I7XrfDW"
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0
In-Reply-To: <681a67de-8a78-46ab-a028-5176a48df402@googlegroups.com>
Cancel-Lock: sha1:GszqIse4nNJ+xxzpu70TtDQU8uU=
Xref: csiph.com comp.lang.java.programmer:35380

On 8/17/2016 2:59 PM, Eric Douglas wrote:
> Say we have 3 classes.
>
> public class TestA<E extends TestB> {
> }
>
> public abstract class TestB {
> }
>
> public class TestC extends TestB {
> }
>
> Now we create an instance.
>
> TestA<TestC> testMe = new TestA();

     TestA<TestC> testMe = new TestA<TestC>();

or, more briefly,

     TestA<TestC> testMe = new TestA<>();

> Now within class TestA how do we tell it instance testMe is referencing TestC?
> The only thing I could find is to add a method or constructor which passes in a reference to TestC.class or the class name, so there's that for last resort, but I do want to make sure if I have to pass it in separately that it matches the value of E.

     I'm not sure what you're asking, so I'll try answers to two
different questions -- if neither is helpful, please post again.

     1) The methods and fields and whatnot within TestA should
traffic in things of type "E", like

	public class TestA<E extends TestB> {
	    private final E theTest;

	    public TestA(E theTest) {
	        this.theTest = theTest;
	    }

	    public E getTheTest() {
	        return theTest;
	    }

	    // ...
	}

     2) If some code inside TestA needs to be aware of the TestC-ness
of some reference or other, use "instanceof":

	public class TestA<E extends TestB> {
	    private boolean isItTestC(E thing) {
	        return thing instanceof TestC;
	    }
	}

However, I'd view this need with suspicion.  The "instanceof" operator
(and its reflective counterparts) are often signs that the design
should be reconsidered -- and perhaps retained, but only after due
thought has lowered the raised eyebrows.  More commonly, it's a sign
that polymorphism is not being used to best advantage.

-- 
esosman@comcast-dot-net.invalid
The Tooth Fairy has gone high-tech, now pays in Bitecoin.