java - operators in predicate as argument in lambda expression -

i need use predicate argument in lambda expression. tried example code seeing compiler error. see compiler treating same predicate differently different arguments. predicate arguments n -> true , n -> false works n -> n%4 == 0 doesn't work.

the compiler error is:

the operator % undefined argument type(s) object, int 

i fixed (see replacement code below) asking should have fix , why? not sure if missing basic.

here complete code:

import java.util.arraylist; import java.util.list; import java.util.function.predicate;  public class predicateasargumentinlambdaexpression {          public static int add(list<integer> numlist, predicate predicate) {             int sum = 0;             (int number : numlist) {                 if (predicate.test(number)) {                     sum += number;                 }             }             return sum;         }          public static void main(string args[]){             list<integer> numlist = new arraylist<integer>();             numlist.add(new integer(10));             numlist.add(new integer(20));             numlist.add(new integer(30));                    numlist.add(new integer(40));                    numlist.add(new integer(50));              system.out.println("add everything: "+add(numlist, n -> true));             system.out.println("add nothing: "+add(numlist, n -> false)); //          system.out.println("add less 25: "+add(numlist, n -> n < 25)); compiler says: operator < undefined argument type(s) object, int             system.out.println("add less 25: "+add(numlist, n -> integer.valueof((int)n) < integer.valueof("25"))); //          system.out.println("add 4 multiples: "+add(numlist, n -> n % 4 == 0)); //compiler says: operator % undefined argument type(s) object, int             system.out.println("add 4 multiples: "+add(numlist, n -> integer.valueof((int)n) % integer.valueof("4")==0));         } } 

commented out code what's not working , line below each replacement code. code works , expected expecting commented out code should have worked! what's isn't ok predicate in java.util.function.predicate here?. please provide link specification page if find answer in.

what happening you're using raw java.util.function.predicate, on test() method like:

public void test(object o) { ... } 

this why compile-time error: argument type object , numeric operators (<, >) not applicable type object.

however, if use generic java.util.function.predicate type-parameter of integer, test() method like:

public void test(integer i) { ... } 

in case, numeric operators (>, <) valid provided argument type (integer) , there's no need of casts.

also, i've taken advantage of stream api in java8 shorten method implementation:

public static int add(list<integer> numlist, predicate<integer> predicate) {      return numlist.stream().filter(predicate).maptoint(i -> i).sum(); } 

having method implemented this, these statements valid:

system.out.println("add everything: "+add(numlist, n -> true)); system.out.println("add nothing: "+add(numlist, n -> false)); system.out.println("add less 25: "+add(numlist, n -> n < 25)); system.out.println("add 4 multiples: "+add(numlist, n -> n % 4 == 0));