java为什么我的几何体不会出现？（jMonkeyEngine）

Question

我刚刚拿起jMonkeyEngine，遇到了一个似乎无法解决的问题

在main类中的simpleInitApp方法中，我可以使用以下代码成功地呈现一个长方体：

    Box playerBase = new Box(Vector3f.ZERO,1f,1f,1f);
    Geometry playerBaseGeom = new Geometry("playerBase", playerBase);
    Transform fixBaseHeight = new Transform(
            new Vector3f(0f,(0.5f * 2f),0f));
    playerBaseGeom.setLocalTransform(fixBaseHeight);
    Material playerBaseMaterial = new Material(assetManager,
            "Common/MatDefs/Misc/Unshaded.j3md");
    playerBaseMaterial.setColor("Color", ColorRGBA.Yellow);
    playerBaseGeom.setMaterial(playerBaseMaterial);
    rootNode.attachChild(playerBaseGeom);

我试图使用一个名为Tower的类来生成几个代表塔的盒子（对于一个简单的塔防御游戏）。tower类如下所示：

public class Tower {

    private static final float HEIGHT = 0.5f;
    private static final float WIDTH = 0.2f;
    private static final float DEPTH = 0.2f;

    private Geometry towerGeom;
    private Material towerMaterial;
    private Box tower;

    public Tower(AssetManager assetManager, float x_coord, float z_coord) {

        tower = new Box();
        towerGeom = new Geometry("tower", tower);
        towerMaterial = new Material(assetManager,
                "Common/MatDefs/Misc/Unshaded.j3md");
        towerMaterial.setColor("Color", ColorRGBA.Green);
        towerGeom.setMaterial(towerMaterial);

        towerGeom.setLocalTranslation(x_coord, (0.5f * .5f),z_coord);
        towerGeom.setLocalScale(WIDTH, HEIGHT, DEPTH);
    }

    public Geometry getGeometry() {
        return towerGeom;
    }
}

在主类中，在simpleInitApp方法中，我尝试使用我的新Tower类，如下所示：

    List <Tower> towers = new ArrayList<Tower>();
    towers.add(new Tower(assetManager, 10f,8f));
    for(Tower t:towers) {
        rootNode.attachChild(t.getGeometry());
    }

但是，不会渲染任何立方体。为什么？我使用了与开头所示完全相同的程序，效果很好