java - What exactly is <T> in the following method declaration? -

i'll admit parameterized classes , methods combined extends , super confuse heck out of me. said, appreciate if explain why following doesn't work, , direct me definitive guide understanding these things.

here's barebones coding of problem. in main class:

public class app {      public <t extends object> boolean isnull(t obj)     {         return obj == null;     }      public static void main(string[] args)     {      }  } 

in junit test class have:

import static org.junit.assert.assertequals;  import org.junit.assert.*; import org.junit.test;  public class testclass1 {     @test     public void shouldbenull()     {         app theapp = new app();         org.junit.assert.assertfalse(theapp.isnull(new string())); //eclipse flags line     } } 

eclipse warns me method isnull(string) undefined app. under impression in definition of isnull, i'm telling instance of t should regarded object, , since string object, should ok. that's not case. please enlighten me. thanks. , it's worth, added "extends object" modification original method definition thinking help, didn't. thousand thank you's whoever can make should obvious (but isn't), obvious.