PiTest "changed conditional boundary mutation survived" without reason?

I have a small Java 11 example with a JUnit 5 test that results in a pitest result of:

changed conditional boundary → SURVIVED

Main class:

public final class CheckerUtils
 {
  private CheckerUtils()
   {
    super();
   }


  public static int checkPort(final int port)
   {
    if (port < 0)
     {
      throw new IndexOutOfBoundsException("Port number out of range!");
     }
    return port;
   }

 }

Test class:

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertThrows;

import org.junit.jupiter.api.Test;

import de.powerstat.security.CheckerUtils;


public final class CheckerUtilsTests
 {
  @Test
  public void checkPortOk()
   {
    final int port = 1023;
    final int resultPort = CheckerUtils.checkPort(port);
    assertEquals(port, resultPort, "Port not as expected!");
   }


  @Test
  public void checkPortNegative1()
   {
    final int port = -1;
    assertThrows(IndexOutOfBoundsException.class, () ->
     {
      CheckerUtils.checkPort(port);
     }
    );
   }


  @Test
  public void checkPortNegative2()
   {
    final int port = -1;
    int resultPort = 0;
    try
     {
      resultPort = CheckerUtils.checkPort(port);
     }
    catch (final IndexOutOfBoundsException e)
     {
      // ignore
     }
    assertEquals(0, resultPort, "Port is not 0");
   }

 }

From my point of view the mutation should not survive, because:

1. checkPortOk() is the normal path for a legal value that is not negative
2. checkPortNegative1() is the path for the negative value when noting is mutated an the exception is thrown.
3. checkPortNegative2(): When nothing is mutated, the exception is thrown and resultPort is still 0 - so the assertion is ok here
4. checkPortNegative2(): When the < 0 is mutated to < -1 or something lower, then no exception will be thrown, so resultPort will become -1 and the assert will fail (mutation killed)
5. checkPortNegative2(): When the < 0 is mutated to < 1 or something higher, the same as under 3.

So my question is did I miss something here or is it a bug in pitest (1.4.9)?

Solution

As statet by @henry,adding the following test solves the issue:

@Test
public void checkPortOk2()
 {
  final int port = 0;
  final int resultPort = CheckerUtils.checkPort(port);
  assertEquals(port, resultPort, "Port not as expected!");
 }

The conditional boundary mutation will mutate

if (port < 0)

To

if (port <= 0)

As none of the tests provide an input of 0 they are not able to distinguish the mutant from the unmutated program and the mutant will survive.

Adding a test case that describes the expected behaviour when port is 0 should kill the mutant.