On page 200, the code can use boxed() instead of the (somewhat mysterious) maptoObject(e -> e)
One should also reveal to the reader that we are computing perfect numbers 
package chapter12;
import org.junit.jupiter.api.Test;
import java.util.List;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.LongStream;
public class Chapter12 {
// Confusing code on page 200
// but with mapToObj(e -> e) replaced with boxed()
private static List<Long> confusingCode(long number) {
return LongStream.rangeClosed(1, number)
.filter(i -> { //Bad Code, don't do this
long factor = 0;
for (int j = 1; j < i; j++) {
if (i % j == 0) {
factor += j;
}
}
return factor == i;
})
.boxed()
.toList();
}
// Confusing code on page 201
// but with mapToObj(e -> e) replaced with boxed()
private static List<Long> refactoredConfusingCode(long number) {
return LongStream.range(1, number)
.filter(i -> LongStream.range(1, i) //Not good
.filter(j -> i % j == 0)
.sum() == i)
.boxed()
.toList();
}
// Nonconfusing code on page 201, in two methods
private static Long sumOfDivisors(long number) {
return LongStream.range(1, number)
.filter(i -> number % i == 0)
.sum();
}
// Create list of those x <= limit such that the sum of the divisors of x equals x
// https://en.wikipedia.org/wiki/Perfect_number
// https://oeis.org/A000396
private static List<Long> nonConfusingCode(long limit) {
return LongStream.rangeClosed(1, limit)
.filter(i -> sumOfDivisors(i) == i)
.boxed()
.toList();
}
// A record to pass data along the stream
private record InAndOut(Long in, List<Long> out) {
public String toString() {
return "f(" + in + ")=[" + out.stream().map(it -> Long.toString(it)).collect(Collectors.joining(",")) + "]";
}
}
private static String exerciseCode(Function<Long, List<Long>> f, List<Long> input) {
return input.stream()
.map(it -> new InAndOut(it, f.apply(it)))
.map(InAndOut::toString)
.collect(Collectors.joining(", "));
}
@Test
public void exerciseAllCode() {
var input = List.of(0L, 1L, 2L, 10L, 13L, 100L, 1000L, 10000L);
{
String str = exerciseCode(Chapter12::confusingCode, input);
System.out.println(str);
}
{
String str = exerciseCode(Chapter12::refactoredConfusingCode, input);
System.out.println(str);
}
{
String str = exerciseCode(Chapter12::nonConfusingCode, input);
System.out.println(str);
}
}
}