java - Generic return type - unable to compile -

i'm reading oracle generics tutorial, , i'm having problem getting following code work:

import java.util.*; public class genericreturn {     public static void main(string[] args){             integer index = 1;             arraylist<integer> al = new arraylist<>();             al.add(10);             al.add(20);             al.add(30);             number s = choose(index, al);             system.out.println(s);     }     static <t, u, s> s choose(t a1, u a2) { return a2.get(a1); } } 

it won't compile errors are:

javac genericreturn.java  genericreturn.java:12: error: cannot find symbol     static <t, u> t choose(t a1, u a2) { return a2.get(a1); }                                                   ^ symbol:   method get(t) location: variable a2 of type u t,u type-variables: t extends object declared in method <t,u>choose(t,u) u extends object declared in method <t,u>choose(t,u) 1 error 

can please me out here?

someone's want mark duplicate, please don't - generics hard enough without ploughing through similar question doesn't quite cover need know!

change :

static <t, u, s> s choose(t a1, u a2) { return a2.get(a1); } 

to :

static <u extends list<s>, s> s choose(integer a1, u a2) { return a2.get(a1); } 

or more simplified version :

static <s> s choose(integer a1, list<s> a2) { return a2.get(a1); } 

that being said, collection classes in java generic there no need write such wrapper method in first place.