Prueba.java

import java.util.List;
import java.util.stream.Stream;
import static java.util.stream.Collectors.*;
public class Prueba {

	public static void main(String[] args) {
		Integer i=getInt(new Integer(2));
		int j=getInteger(3);
		
		System.out.println(idsToStringList(new Person(i),new Person(j)));
		
		idsToString(new Person(i),new Person(j)).forEach(System.out::println);
	}

	public static int getInt(int i) {
		return i;
	}
	
	public static Integer getInteger(Integer i) {
		return i;
	}
	
	public static Stream<String> idsToString(Person ...persons) {
	    return Stream.of(persons).map(x -> Integer.toString(x.id()));
	}
	
	public static List<String> idsToStringList(Person ...persons) {
	    return Stream.of(persons)
	              .collect(mapping(x -> Integer.toString(x.id()),toList()));
	}
	
}

class Person {
	
	private int id=0;
	
	public Person(Integer i) {
		this.id=i;
	}
	
	public int id() {return id;}
}