Question

我正在为餐馆创建一个Android应用程序，让服务员接受客户的订单。我使用扩展列表视图来显示菜单，并使用hashmap填充扩展列表视图。当我运行应用程序时，没有错误，但尝试使用扩展列表视图打开活动时应用程序崩溃。调试之后，我发现有空指针异常导致它崩溃，但我很难解决它。以下是我的代码：

菜单活动：

public class Menu extends AppCompatActivity {

    HashMap<String, List<String >> Food_menu;
    List<String> Food_list;
    ExpandableListView Exp_list;
    FoodMenuAdapter adapter;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        Exp_list = (ExpandableListView) findViewById(R.id.exp_list);
        Food_menu = DataProvider.getInfo();
        Food_list = new ArrayList<String>(Food_menu.keySet());
        Exp_list.setAdapter(adapter);
        adapter = new FoodMenuAdapter(this, Food_menu, Food_list);


        Exp_list.setOnGroupExpandListener(new ExpandableListView.OnGroupExpandListener() {
            @Override
            public void onGroupExpand(int groupPosition)
            {
                Toast pass2 = Toast.makeText(getBaseContext(),Food_list.get(groupPosition) + "is Expanded", Toast.LENGTH_SHORT);
                pass2.show();;
            }
        });


    }
}

FoodMenuAdapter类：

public class FoodMenuAdapter  extends BaseExpandableListAdapter{

    private Context ctx;
    private HashMap<String, List<String>> Food_Category;
    private List<String> Food_List;

    public FoodMenuAdapter(Context ctx, HashMap<String, List<String>> Food_Category, List<String> Food_List)
    {
        this.ctx = ctx;
        this.Food_Category = Food_Category;
        this.Food_List = Food_List;
    }


    @Override
    public int getGroupCount()
    {
        return Food_List.size();
    }

    @Override
    public int getChildrenCount(int arg0)
    {
        return Food_Category.get(Food_List.get(arg0)).size();
    }

    @Override
    public Object getGroup(int arg0)
    {
        return Food_List.get(arg0);
    }

    @Override
    public Object getChild(int parent, int child)
    {
        return Food_Category.get(Food_List.get(parent)).get(child);
    }

    @Override
    public long getGroupId(int arg0)
    {
        return arg0;
    }

    @Override
    public long getChildId(int parent, int child)
    {
        return child;
    }

    @Override
    public boolean hasStableIds() {
        return false;
    }

    @Override
    public View getGroupView(int parent, boolean isExpanded, View convertView, ViewGroup parentView)
    {
        String group_title = (String) getGroup(parent);
        if (convertView == null)
        {
            LayoutInflater infalter = (LayoutInflater) ctx.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = infalter.inflate(R.layout.parent_layout, parentView, false);
        }
        TextView parent_textview = (TextView) convertView.findViewById(R.id.parent_txt);
        parent_textview.setTypeface(null, Typeface.BOLD);
        parent_textview.setText(group_title);
        return convertView;
    }

    @Override
    public View getChildView(int parent, int child, boolean lastChild, View convertView, ViewGroup parentView)
    {
        String child_title = (String) getChild(parent, child);
        if(convertView == null)
        {
            LayoutInflater inflater = (LayoutInflater) ctx.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = inflater.inflate(R.layout.child_layout, parentView, false);

        }

        TextView child_textview = (TextView) convertView.findViewById(R.id.child_txt);
        child_textview.setText(child_title);

        return convertView;
    }

    @Override
    public boolean isChildSelectable(int groupPosition, int childPosition)
    {
        return true;
    }
}

DataProvider类：

public class DataProvider {

    public static HashMap<String, List<String>> getInfo() {
        HashMap<String, List<String>> FoodDetails = new HashMap<String, List<String>>();

        List<String> Starter_Menu = new ArrayList<String>();
        Starter_Menu.add("Cheesy Garlic Bread");
        Starter_Menu.add("Liver Pate");
        Starter_Menu.add("Soup of the Day");
        Starter_Menu.add("Caesar Salad");
        Starter_Menu.add("Prawn Cocktail");

        List<String> Main_Menu = new ArrayList<String>();
        Main_Menu.add("Steak Burger");
        Main_Menu.add("Rib of Beef");
        Main_Menu.add("10oz Steak");
        Main_Menu.add("Pappardelle Pasta");
        Main_Menu.add("Fish and Chips");

        List<String> Desstert_Menu = new ArrayList<String>();
        Desstert_Menu.add("Sticky Toffee Pudding");
        Desstert_Menu.add("Chocolate Brownee with Ice Cream");
        Desstert_Menu.add("Cheesecake");
        Desstert_Menu.add("Ice Cream");
        Desstert_Menu.add("Strawberry Meringue");

        List<String> Sides_Menu = new ArrayList<String>();
        Sides_Menu.add("Onion Rings");
        Sides_Menu.add("Chips");
        Sides_Menu.add("Garlic Bread");
        Sides_Menu.add("Garlic Potatoes");
        Sides_Menu.add("Tobacco Onions");

        List<String> Drinks_Menu = new ArrayList<String>();
        Drinks_Menu.add("Tennants");
        Drinks_Menu.add("Coke");
        Drinks_Menu.add("Red Wine");
        Drinks_Menu.add("White Wine");
        Drinks_Menu.add("Guiness");
        Drinks_Menu.add("Magners");

        FoodDetails.put("Starter Menu", Starter_Menu);
        FoodDetails.put("Main Menu", Main_Menu);
        FoodDetails.put("Dessert Menu", Desstert_Menu);
        FoodDetails.put("Sides Menu", Sides_Menu);
        FoodDetails.put("Drinks Menu", Drinks_Menu);

        return FoodDetails;
    }
}

它在菜单活动中说Exp_list，Food_menu和Food_list都为空。有没有人看到问题或知道如何解决它？

修改

添加了Logcat

04-12 10:56:45.177 1984-1984/com.example.ben.restaurantapp E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.ben.restaurantapp, PID: 1984
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.ben.restaurantapp/com.example.ben.restaurantapp.Menu}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ExpandableListView.setAdapter(android.widget.ExpandableListAdapter)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
    at android.app.ActivityThread.access$800(ActivityThread.java:144)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5221)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
    Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ExpandableListView.setAdapter(android.widget.ExpandableListAdapter)' on a null object reference
    at com.example.ben.restaurantapp.Menu.onCreate(Menu.java:39)
    at android.app.Activity.performCreate(Activity.java:5937)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
    at android.app.ActivityThread.access$800(ActivityThread.java:144)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5221)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

Answer 1

一个问题可能是您在设置适配器后初始化适配器并且其中的所有项都为空

$(function() {
    $("#accordion").accordion({
        collapsible: true,
        active: false heightStyle: "content"
    });
});

$(function() {
    var icons = {
        header: "ui-icon-circle-arrow-e",
        activeHeader: "ui-icon-circle-arrow-s"
    };

    $("#accordion").accordion({
        icons: icons
    });

    $("#toggle").button().click(function() {
        if ($("#accordion").accordion("option", "icons")) {
            $("#accordion").accordion("option", "icons", null);
        } else {
            $("#accordion").accordion("option", "icons", icons);
        }
    });
});

这可能会解决问题：

Exp_list.setAdapter(adapter);
adapter = new FoodMenuAdapter(this, Food_menu, Food_list);

Answer 2

我认为您应首先初始化适配器，然后将其传递给setadapter。

SELECT * FROM dev.products
WHERE prodid IN (SELECT prodid from dev.kit_products where kitid = 2);

使用HashMap填充ListView

修改

2 个答案: