pitest weak tests http://pitest.org/weak_tests/

example1.java

public static String foo(boolean b) {
  if ( b ) {
    performVitallyImportantBusinessFunction();
    return "OK";
  }

  return "FAIL";
}

@Test
public void shouldFailWhenGivenFalse() {
  assertEquals("FAIL", foo(false));
}

@Test
public void shouldBeOkWhenGivenTrue() {
  assertEquals("OK", foo(true));
}

example2.java

public static String foo(int i) {
  if ( i >= 0 ) {
      return "foo";
  } else {
      return "bar";
  }
}

@Test
public void shouldReturnBarWhenGiven1() {
  assertEquals("bar", foo(1));
}

@Test
public void shouldReturnFooWhenGivenMinus1() {
  assertEquals("foo", foo(-1));
}

example3.java

public static String foo(Collaborator c, boolean b) {
  if ( b ) {
      return c.performAction();
  }

  return "FOO";
}

@Test
public void shouldPerformActionWhenGivenTrue() {
  foo(mockCollaborator,true);
  verify(mockCollaborator).performAction();
}

@Test
public void shouldNotPerformActionWhenGivenFalse() {
  foo(mockCollaborator,false);
  verify(never(),mockCollaborator).performAction();
}