Java Generics in arguments -

i have following interface:

public interface iradiobuttongroup<t> {  list<iradiobutton<t>> getbuttons(); } 

now create method

protected object getdefaultradiobuttonvalue(iradiobuttongroup<?> field) { list<iradiobutton<?>> buttons = field.getbuttons(); 

now java complaining:

type mismatch: cannot convert list<iradiobutton<capture#1-of ?>> list<iradiobutton<?>> 

any suggestions?

the unbounded wildcard parameter type of method means that, accept iradiobuttongroup of unknown type. compiler doesn't know type come, @ compilation time, compiler generate , assign each wildcards placeholder, because although doesn't know type coming, sure there has single type replace ?. , placeholder capture#1-of ? see in error message.

basically, trying assign list<iradiobutton<cap#1-of-?>> list<iradiobutton<?>>, , not valid, in similar way how list<list<string>> cannot assigned list<list<?>>.

one known way solve these issues use capture helpers. create generic method, , delegate call method. generic method infer type parameter , able type safe operation. see this brian goetz's article more details.

so solve issue, provide method in below code:

protected object getdefaultradiobuttonvalue(iradiobuttongroup<?> field) {            return getdefaultradiobuttonvaluehelper(field); }  private <t> object getdefaultradiobuttonvaluehelper(iradiobuttongroup<t> field) {     list<iradiobutton<t>> buttons = field.getbuttons();     // write logic of original method here }