function - Generic method to perform a map-reduce operation. (Java-8) -

how overload function generic parameter in java 8?

public class test<t> {      list<t> list = new arraylist<>();      public int sum(function<t, integer> function) {         return list.stream().map(function).reduce(integer::sum).get();     }       public double sum(function<t, double> function) {         return list.stream().map(function).reduce(double::sum).get();     } } 

error: java: name clash: sum(java.util.function.function<t,java.lang.double>) , sum(java.util.function.function<t,java.lang.integer>) have same erasure

the example present in question has got nothing java 8 , how generics work in java. function<t, integer> function , function<t, double> function go through type-erasure when compiled , transformed function. rule of thumb method overloading have different number, type or sequence of parameters. since both methods transform take function argument, compiler complains it.

that being said, srborlongan has provided 1 way resolve issue. problem solution have keep modifying test class each , every type of operation (addition,subtraction,etc) on different types (integer,double, etc). alternate solution use method overriding instead of method overloading :

change test class bit follows :

public abstract class test<i,o extends number> {      list<i> list = new arraylist<>();      public o performoperation(function<i,o> function) {         return list.stream().map(function).reduce((a,b)->operation(a,b)).get();     }      public void add(i i) {         list.add(i);     }      public abstract o operation(o a,o b); } 

create subclass of test add 2 integers.

public class mapstringtointaddtionoperation extends test<string,integer> {      @override     public integer operation(integer a,integer b) {         return a+b;     }  } 

client code can use above code follows :

public static void main(string []args) {     test<string,integer> test = new mapstringtointaddtionoperation();     test.add("1");     test.add("2");     system.out.println(test.performoperation(integer::parseint)); } 

the advantage of using approach test class in line open-closed principle. add new operation such multiplication, have add new subclass of test , override operation method multiply 2 numbers. club decorator pattern , can minimize number of sub-classes have create.

note example in answer indicative. there lot of areas of improvement (such make test functional interface instead of abstract class) beyond scope of question.