Odd case in Java generics - should class level generic affect getters? -

this question has answer here:

• what raw type , why shouldn't use it? 12 answers
• why calling method generic return on generic class considered unsafe javac? 4 answers

i came across weird case while writing unit test today (simplified below).

import java.util.collection; import java.util.collections; public class test {      public static class testclass<t> {         private final collection<string> field = collections.emptylist();          public collection<string> getfield(){             return this.field;         }     }      public static void main(string[] args){         // unchecked assignment, why?         testclass case1 = new testclass();         collection<string> field1 = case1.getfield();          // ok         testclass case2 = new testclass();         collection field2 = case2.getfield();          // adding type parameter class makes unchecked assignment dissapear         testclass<string> case3 = new testclass<>();         collection<string> field3 = case3.getfield();     } } 

running javac test.java -xlint:unchecked (or pasting ide) yields following message:

test.java:17: warning: [unchecked] unchecked conversion         collection<string> field1 = case1.getfield();                                                   ^   required: collection<string>   found:    collection 1 warning 

it looks removing generic type parameter class variable declaration leaves compiler unable infer type of getfield(), though declared returning collection<string>. bug? if not, can explain why happening? totally thrown loop while debugging hamcrest matcher. last thing expected causing problem.