package Shapes;
public class BasicShape{
...
}


package BasicShape;
public class Shape2D extends BasicShape{
...
}

你把BasicShape放到了Shapes package里面, 又把Shape2D放到了 BasicShape package里面.
这样Shape2D根本看不到BasicShape的存在...
你把package BasicShape换成 package Shapes 或者干脆2个package都不要了, 就没问题了