java单例枚举实例重置实例列表

Question

有人能帮我解决enum单例实例的问题吗？实例列表被还原或清除，我不知道如何或为什么：

public enum ObjectMap {

INSTANCE;

ObjectMap() {
    this.displayObjectList = new ArrayList<DisplayObject>();
}

private final List<DisplayObject> displayObjectList;

public boolean addObject(final DisplayObject paramDisplayObject) {
    boolean success = false;
    if (null != paramDisplayObject && !this.isTaken(paramDisplayObject.x, paramDisplayObject.y)) {
        this.displayObjectList.add(paramDisplayObject);
        success = true;
    }
    return success;
}

public boolean isTaken(final int paramX, final int paramY) {
    for (final DisplayObject displayObject : this.displayObjectList) {
        if (displayObject.x == paramX && displayObject.y == paramY) {
            return true;
        }
    }
    return false;
}

}

以下junit测试失败：

@Test
public void testBombEvent() {
    Player player = new Player(PLAYER_X, PLAYER_Y);

    // calls ObjectMap.INSTANCE.addObject(DisplayObject bomb), debugging this successfully adds the object to the list
    player.placeBomb(); 

    // AssertionError because ObjectMap.INSTANCE.displayObjectList is empty???
    Assert.assertTrue(ObjectMap.INSTANCE.isTaken(PLAYER_X, PLAYER_Y));
    final DisplayObject object = ObjectMap.INSTANCE.getObject(PLAYER_X, PLAYER_Y);
    Assert.assertEquals(object.objectType, GameObjectType.BOMB);

    try {
        Thread.sleep(2010);
        Assert.assertFalse(ObjectMap.INSTANCE.isTaken(PLAYER_X, PLAYER_Y));
    } catch (final InterruptedException e) {

    }
}