java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“ android.view.View android.app.Activity.findViewById（int）”

Question

我的Android项目中总是出现错误。

java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“ android.view.View
android.app.Activity.findViewById（int）”

在我的MainActivity中，我正在调用FragmentStart类。

FragmentStart.class

import android.app.Activity;

import android.content.Intent;

import android.os.Bundle;

import android.support.v4.app.Fragment;

import android.view.LayoutInflater;

import android.view.View;

import android.view.ViewGroup;

import android.widget.AdapterView;

import android.widget.ArrayAdapter;

import android.widget.Button;

import android.widget.ListView;



public class FragmentStart extends Fragment {



    //Define Listview

    private ListView startList;

    private String[] stringList;

    Activity activity = getActivity();



    public FragmentStart() {

        //

    }





    @Override

    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        View rootView = inflater.inflate(R.layout.fragment_start, container, false);



        // Create ListView start

        startList = (ListView) rootView.findViewById(R.id.startList);

        stringList = getResources().getStringArray(R.array.startList);



        if (activity != null) {

            ArrayAdapter<String> objAdapter = new ArrayAdapter<String>(activity, android.R.layout.simple_list_item_1, stringList);

            startList.setAdapter(objAdapter);

            startList.setOnItemClickListener(new AdapterView.OnItemClickListener() {



                @Override

                public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

                    switch (startList.getPositionForView(view)) {



                        case 0:

                            // ...

                            break;



                    }

                }

            });

        }

        // Create ListView end



        //New Entry

        Button add = (Button) activity.findViewById(R.id.add);

        add.setOnClickListener(new View.OnClickListener() {

            public void onClick(View view) {

                Intent myIntent = new Intent(view.getContext(), addItem.class);

                startActivityForResult(myIntent, 0);

            }



        });



        return rootView;

    }

}

Answer 1

除了@Selvin指出的内容外，它看起来像这样：

Button add = (Button) activity.findViewById(R.id.add);

…应更改为：

Button add = (Button) rootView.findViewById(R.id.add);