Java nested templates like A<B, C extends A> usage

Question

I want to define the following Java class:

public class DummyTemplate<S, D extends DummyTemplate<S,D>> {

S value;
D next;
public DummyTemplate(S value, D next) {
    super();
    this.value = value;
    this.next = next;
}


public static DummyTemplate<String, DummyTemplate>  factory(){

    return  new DummyTemplate<String, DummyTemplate>("wohoo", null);
}

}

created so i can subclass:

public class DummyTemplateSubclass<S,  D extends DummyTemplateSubclass<S,D>> extends DummyTemplate<S, D>

(and the factoty of subclass returns DummyTemplateSubclass).

But the definition creates compile error:

Bound mismatch: The type DummyTemplate is not a valid substitute for the bounded parameter <D extends DummyTemplate<S,D>> of the type DummyTemplate<S,D>

Probably because the DummyTemplate must have parameters, how to define it then? i get the error

it compiles howewer only :

public static<D extends DummyTemplate<String,D>> D  factory()

but then i got trouble on subclass:

public static<D extends DummyTemplateSubclass<String,D>> D  factory(){

Name clash: The method factory() of type DummyTemplateSubclass has the same erasure as factory() of type DummyTemplate but does not hide it

Andreas · Accepted Answer · 2018-01-29 04:25:08Z

2

When you make a self-referencing template parameter like that, you can't use the template directly, because it's ever-expanding:

DummyTemplate<String, DummyTemplate<String, DummyTemplate<String, ...>>>

To use it, you must create a subclass:

public class DummyTemplate<S, D extends DummyTemplate<S,D>> {
    S value;
    D next;
    public DummyTemplate(S value, D next) {
        this.value = value;
        this.next = next;
    }
}

public class StringTemplate extends DummyTemplate<String, StringTemplate> {
    public StringTemplate(String value, StringTemplate next) {
        super(value, next);
    }
    public static StringTemplate factory() {
        return new StringTemplate("wohoo", null);
    }
}

answered Jan 29, 2018 at 4:25

Andreas

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Comments

Jacob G. · Accepted Answer · 2018-01-29 04:35:32Z

0

Because the factory method is static and the second parameter of DummyTemplate is D extends DummyTemplate<S, D>, you need to declare a new generic type, as follows:

public static <T extends DummyTemplate<String, T>> DummyTemplate<String, T> factory() {
    return new DummyTemplate<String, T>("wohoo", null);
}

Then the subclass compiles just fine:

public class DummyTemplateSubclass<S, D extends DummyTemplateSubclass<S, D>> extends DummyTemplate<S, D> {
    public DummyTemplateSubclass(S value, D next) {
        super(value, next);
    }

    public static <T extends DummyTemplateSubclass<String, T>> DummyTemplateSubclass<String, T> factorySubclass() {
        return new DummyTemplateSubclass<String, T>("wohoo", null);
    }
}

If you want to remove the static keyword from factory, then the code simplifies:

public class DummyTemplate<S, D extends DummyTemplate<S, D>> {
    S value;
    D next;

    public DummyTemplate(S value, D next) {
        this.value = value;
        this.next = next;
    }

    public DummyTemplate<String, D> factory() {
        return new DummyTemplate<String, D>("wohoo", null);
    }
}

public class DummyTemplateSubclass<S, D extends DummyTemplateSubclass<S, D>> extends DummyTemplate<S, D> {
    public DummyTemplateSubclass(S value, D next) {
        super(value, next);
    }

    @Override
    public DummyTemplateSubclass<String, D> factory() {
        return new DummyTemplateSubclass<String, D>("wohoo", null);
    }
}

edited Jan 29, 2018 at 4:35

answered Jan 29, 2018 at 4:04

Jacob G.

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6 Comments

user184868 Over a year ago

public static DummyTemplate<String, D> factory() does not compile, also does not public static<D> DummyTemplate<String, D> factory()

Jacob G. Over a year ago

@user184868 My apologies, I accidentally removed static from it earlier without noticing. You should now be free to create a factory method for your subclass, but remember that static methods cannot be overridden in Java.

user184868 Over a year ago

nah i still got Name clash: The method factory() of type DummyTemplateSubclass<S,D> has the same erasure as factory() of type DummyTemplate<S,D> but does not hide it

Jacob G. Over a year ago

Yeah, you can't use the same name factory in the subclass, as static methods cannot be overridden. I've named the method factorySubclass in DummyTemplateSubclass.

user184868 Over a year ago

hmm, ok. now i tryed to use it not static like: public DummyTemplate<String, D> factory(){ return new DummyTemplate<String, D>("DummyTemplate", null); } then i subclass: public class DummyTemplateSubclass<S, D extends DummyTemplateSubclass<S,D>> extends DummyTemplate<S, D>{ and then DummyTemplateSubclass<String,?> dummy = new DummyTemplateSubclass<>("noob", null); DummyTemplate<String, ?> fa = dummy.factory(); System.out.println(fa.getClass()); returns class templates.DummyTemplate, but D must be DummyTemplateSubclass

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