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POIn java, can one create a fluent extensible class hierarchy with methods that can be invoked in any order?
 

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  1. POIn java, can one create a fluent extensible class hierarchy with methods that can be invoked in any order?
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    <p>Can one create an extensible class hierarchy in java whose methods are fluent and can be invoked in any order? <strong><em>(YES! see answer below), even for existing classes when you don't have access to the source, provided the methods are fluant!</em></strong></p> <p>I'm retrofitting an existing hierarchy and hope to use a factory or at least a generic constructor and (eventually) immutable builder patterns <em>(JB P.14).</em> The methods that set fields return <code>void</code> - it would be better for them to return a generic <code>T</code> instead - that way we will gain the ability to do method chaining (they all call <code>super</code> now). </p> <p>Goals:<br> 1. <strong>Avoid having to create a static getFactory() method in every class.</strong><br> 2. <strong>Simple method signatures.</strong><br> 3. <strong>Create a factory method that is generic, yet will catch problems at compile time.</strong><br> 4. <strong>Get compile time errors instead of run time errors when mistakes are made.</strong> </p> <p>As requested, the non-generic code is very simple, but doesn't work.</p> <pre><code>public class A { private String a = null; protected A setA(String a){ this.a = a; return this;//&lt;== DESIRE THIS TO BE CHAINABLE } protected static A factory(){ return new A(); } } </code></pre> <p>.</p> <pre><code>public class B extends A { private String b = null; protected Foo setB(String b){ this.b = b; return this;//&lt;== DESIRE THIS TO BE CHAINABLE } protected static B factory(){ return new B(); } } </code></pre> <p>Now a caller could TRY to call <code>B.factory().setA("a").setB("b")//won't compile</code></p> <p>But that can't compile because <code>setA()</code> returns an <code>A</code>, not a <code>B</code>. You COULD make it work by overriding the <code>setA()</code> in <code>B</code>, calling <code>setB()</code> and returning <code>B</code> instead of the <code>A</code>. To avoid delegating for each of those methods is the point. I simply want an extensible group of chainable class methods that can be invoked in any order. <code>B.getFactory().B("b").A("a")</code> works obviously.</p>
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    1. COrelated: [Is there a way to refer to the current type with a type variable?](http://stackoverflow.com/questions/7354740/is-there-a-way-to-refer-to-the-current-type-with-a-type-variable)
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    2. COThere is no reason to write `interface Chainable<T extends Chainable<T>>` instead of just `interface Chainable<T>`; same with `Foo`, `Bar`, `Fubar`, and `FooBarBar`
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    3. CO"Correct because the this is the same as the type of Foo" No, it is not correct. `this` may not be a `T`.
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