### Question:
LineSegment extends ALine implements ILineSegment { @Override public double getLength() { return directionVector().getLength(); } @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 testGetLength() { IVector2 start = Vector2f.ZERO_VECTOR; IVector2 end = Vector2.fromXY(21, 42); ILineSegment segment = LineSegment.fromPoints(start, end); double length = end.subtractNew(start).getLength(); assertThat(segment.getLength(), closeTo(length, ACCURACY)); ILineSegment invalidSegment = LineSegment.fromPoints(start, start); assertThat(invalidSegment.getLength(), closeTo(0.0d, ACCURACY)); }