### Question:
LinkResolver { public boolean isHeardBy(Node n1, Node n2) { return (n1.isWirelessEnabled() && n2.isWirelessEnabled() && n1.distance(n2) <= n1.getCommunicationRange()); }  boolean isHeardBy(Node n1, Node n2);  }### Answer:
@Test void isHeardBy_n1Null_throwsNullPointerException() { assertThrows(NullPointerException.class, () -> { resolver.isHeardBy(null, n2); }); }
@Test void isHeardBy_n2Null_throwsNullPointerException() { assertThrows(NullPointerException.class, () -> { resolver.isHeardBy(n1, null); }); }
@Test void isHeardBy_n1n2Null_throwsNullPointerException() { assertThrows(NullPointerException.class, () -> { resolver.isHeardBy(null, null); }); }