问题
I am creating an android app for a restaurant that lets the waiters take the customers orders. I am using a extended list view to show the menu and have used a hashmap to populate the extended list view. When I run the app, there are no errors but the app crashes when trying to open the activity with the extended list view on it. After debugging, I found that there were null pointer exceptions causing it to crash but I'm struggling to fix it. Below is my code:
Menu activity:
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 Class:
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 Class:
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;
}
}
It says in the Menu Activity that Exp_list, Food_menu and Food_list are all null. Does anyone see the problem or know how to fix it?
Edit
Logcat has been added
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)
回答1:
One problem could be that you're initializing the adapter after you set it and all items in it are null
Exp_list.setAdapter(adapter);
adapter = new FoodMenuAdapter(this, Food_menu, Food_list);
This might solve the problem:
adapter = new FoodMenuAdapter(this, Food_menu, Food_list);
Exp_list.setAdapter(adapter);
回答2:
I think you should initialize the adapter first and then pass it to setadapter.
adapter = new FoodMenuAdapter(this, Food_menu, Food_list);
Exp_list.setAdapter(adapter);
来源：https://stackoverflow.com/questions/36571025/npe-while-using-hashmap-to-populate-listview