java - Generically-typed interface for which lambdas can be used? -

i'm still new lambda expressions, , i've been trying practice them in order better understand how work. came across pdf online few exercises practice: http://www.java-programming.info/tutorial/pdf/java/exercises/exercises-lambdas-1.pdf . on last problem, , having difficulty understanding needed accomplish it.

here code have:

    interface twoelementpredicate <e> {     boolean bettercheck(e e1, e e2); }  public class lambdapractice4 <e> {      public static void main(string[] args)      {         car car1 = new car(50);         car car2 = new car(100);         employee worker1 = new employee(500, "john");         employee worker2 = new employee(1000, "tom");         string str1 = "string";         string str2 = "secondstring";          //these 3 lines not work, due duplicate variables. thought lambdas functions though?         twoelementpredicate<string> betterelement = (s1, s2) -> { return s1.length() > s2.length(); };         twoelementpredicate<car> betterelement = (c1, c2) -> { return c1.getprice() > c2.getprice(); };         twoelementpredicate<employee> betterelement = (w1, w2) -> w1.getsalary() > w2.getsalary(); //trying different syntaxes          /* these work, instead of different ending arguments (bettercar, betterstring, etc.) want 1 betterelement         system.out.println(betterentry(car1, car2, bettercar).getprice());         system.out.println(betterentry(str1, str2, betterstring));         system.out.println(betterentry(worker1, worker2, betterworker).getname());         */          //this have replace 3 statements above         system.out.println(betterentry(car1, car2, betterelement));         system.out.println(betterentry(str1, str2, betterelement));         system.out.println(betterentry(worker1, worker2, betterelement));     }      public e betterentry(e e1, e e2, twoelementpredicate genericface)     {         return genericface.bettercheck(e1, e2) ? e1 : e2;     }      /*     public static car betterentry(car c1, car c2, twoelementpredicate genericface)     {         return genericface.bettercheck(c1, c2) ? c1 : c2;     }      public static employee betterentry(employee e1, employee e2, twoelementpredicate genericface)     {         return genericface.bettercheck(e1, e2) ? e1 : e2;     }      public static string betterentry(string s1, string s2, twoelementpredicate genericface)     {         return genericface.bettercheck(s1, s2) ? s1 : s2;     }     */ } 

the employee , car classes defined underneath code; they're not relevant issues i'm having, exist wondering.