java - In a Transformer, how to specify that the types fo the classes being transformed are of the same type? -

java - In a Transformer, how to specify that the types fo the classes being transformed are of the same type? -

i'm having simple yet confuse question @ same time.

in transformer, there way specify types of classes beingness transformed same type?

to create clear, i'll share code:

transformer<set<?>, list<?>> transformer = new settolisttransformer();

is there way me specify set , list of same type?

also when writing transformer did this, , assume serves no purpose:

private static class settolisttransformer implements transformer<set<?>, list<?>> { @override public list<?> transform(final set<?> input) { homecoming this.dotransform(input); } public <t> list<t> dotransform(final set<t> input) { ... } }

the thing is, cannot type settolisttransformer since not care types inside, care same type.

any help appreciated!

ps: i'm not transforming set list, i'm using other types, used them clarify code :)

there's no way enforce desired constraint on transform implementation, since there no way impose relationship between generic wildcards. each of 2 ? in settolisttransformer declaration doomed each mean some unknown type no way bound them each other.

as others pointed out, easiest solution create settolisttransformer generic. example:

class settolisttransformer<t> implements transformer<set<? extends t>, list<t>> { @override public list<t> transform(final set<? extends t> input) { final list<t> output = new arraylist<t>(input.size()); output.addall(input); homecoming output; } }

of course of study requires instantiate transformers specific type arguments. should fine long settolisttransformer cheap. but, indicated want utilize 1 instance. here's how that:

class mytransformers { // there no reason expose settolisttransformer now. // maintain here anonymous class. private static final transformer<set<?>, list<?>> from_set_to_list = new transformer<set<?>, list<?>>() { @override public list<?> transform(final set<?> input) { homecoming dotransform(input); } private <t> list<t> dotransform(final set<t> input) { final list<t> output = new arraylist<t>(input.size()); output.addall(input); homecoming output; } }; private mytransformers() { } public static <t> transformer<set<? extends t>, list<t>> fromsettolist() { @suppresswarnings("unchecked")//this okay t because impl stateless final transformer<set<? extends t>, list<t>> withnarrowedtypes = (transformer<set<? extends t>, list<t>>)(transformer<?, ?>)from_set_to_list; homecoming withnarrowedtypes; } }

here's usage example:

set<integer> intset = new hashset<integer>(); intset.add(42); intset.add(1337); list<number> numlist = mytransformers.<number>fromsettolist().transform(intset);

java generics