### Question:
LineSegment extends ALine implements ILineSegment { @Override public boolean isValid() { return !directionVector().isZeroVector(); } @SuppressWarnings("unused") private  LineSegment(); private  LineSegment(final IVector2 start, final IVector2 end); static ILineSegment fromOffset(final IVector2 supportVector, final IVector2 offset); static ILineSegment fromPoints(final IVector2 start, final IVector2 end); @Override boolean isValid(); @Override final boolean equals(final Object other); @Override final int hashCode(); @Override IVector2 getStart(); @Override IVector2 getEnd(); @Override IVector2 getCenter(); @Override double getLength(); @Override IVector2 directionVector(); @Override IVector2 closestPointOnLine(final IVector2 point); @Override ILine toLine(); @Override IHalfLine toHalfLine(); @Override ILineSegment copy(); @Override Optional<IVector2> intersectLine(final ILine line); @Override Optional<IVector2> intersectHalfLine(final IHalfLine halfLine); @Override Optional<IVector2> intersectSegment(final ILineSegment other); @Override IVector2 stepAlongLine(final double stepSize); @Override ILineSegment withMargin(final double margin); @Override String toString(); @Override List<IVector2> getSteps(final double stepSize);  }### Answer:
@Test public void testIsValid() { IVector2 start = Vector2.fromXY(10, -20); IVector2 end = Vector2f.ZERO_VECTOR; ILineSegment properLine = LineSegment.fromPoints(start, end); ILineSegment invalidLine = LineSegment.fromPoints(start, start); assertThat(properLine.isValid(), is(true)); assertThat(invalidLine.isValid(), is(false)); properLine = LineSegment.fromOffset(start, end.subtractNew(start)); invalidLine = LineSegment.fromOffset(start, Vector2f.ZERO_VECTOR); assertThat(properLine.isValid(), is(true)); assertThat(invalidLine.isValid(), is(false)); }