@Test
public void testAdd() {

  final Vector2 vec1 = new Vector2();
  final Vector2 vec2 = new Vector2(Vector2.ONE);
  vec1.addLocal(1, 2);
  assertEquals(new Vector2(1, 2), vec1);
  vec1.addLocal(-1, -2);
  assertEquals(Vector2.ZERO, vec1);
  vec1.zero();
  vec1.addLocal(vec2);
  assertEquals(Vector2.ONE, vec1);
  vec1.zero();
  final Vector2 vec3 = vec1.add(vec2, new Vector2());
  assertEquals(Vector2.ZERO, vec1);
  assertEquals(Vector2.ONE, vec3);
  final Vector2 vec4 = vec1.add(1, 0, null);
  assertEquals(Vector2.ZERO, vec1);
  assertEquals(Vector2.UNIT_X, vec4);
}