FULL PRODUCT VERSION :
9

A DESCRIPTION OF THE PROBLEM :
http://book2s.com/java/src/package/jdk/nashorn/internal/runtime/jstype.html#1f2ca1218bfe8fd34d4b40a8fba83acc

    /**
     * Convert a long to the narrowest JavaScript Number type. This returns either a
     * {@link Integer} or {@link Double} depending on the magnitude of {@code l}.
     * @param l a long value
     * @return the value converted to Integer or Double
     */
    public static Number toNarrowestNumber(final long l) {
        return isRepresentableAsInt(l) ? Integer.valueOf((int) l) : Double
                .valueOf(l);
    }


Because of the unboxing rules for numeric conditional expressions (https://docs.oracle.com/javase/specs/jls/se9/html/jls-15.html#jls-15.25.2), the operands here will be unboxed and widened to doubles, and thus the result will be boxed back to Double. As such, Integer isn't returned.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
https://ideone.com/Opw18m

    public static Number toNarrowestNumber(boolean condition, final long l) {
        return condition ? Integer.valueOf((int) l) : Double
                .valueOf(l);
    }
 
public static void main (String[] args) throws java.lang.Exception
{
System.out.println(toNarrowestNumber(true, 0).getClass());
System.out.println(toNarrowestNumber(false, 0).getClass());
}

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
class java.lang.Integer
class java.lang.Double
ACTUAL -
class java.lang.Double
class java.lang.Double

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------

class Ideone
{
    public static Number toNarrowestNumber(boolean condition, final long l) {
        return condition ? Integer.valueOf((int) l) : Double
                .valueOf(l);
    }
 
public static void main (String[] args) throws java.lang.Exception
{
System.out.println(toNarrowestNumber(true, 0).getClass());
System.out.println(toNarrowestNumber(false, 0).getClass());
}
}
---------- END SOURCE ----------