java - Listview按钮点击抛出空指针异常

Question

在我的 UpdateSupervitionActivity 类中，调用 CallAdapter() 方法来生成 listview。 listview 的每个item 都有一个用于删除item 的按钮。加载 listview 后，当我尝试通过单击 listview 按钮从 listview 中删除项目时，它会抛出 nullpointer 异常，并且应用程序崩溃。

具有 CallAdapter() 方法的 Java 类:

package bd.edu.bubt.regup;

public class UpdateSupervitionActivity extends AppCompatActivity {

EditText fac_code, s_intake, s_section;
ListView listView;
Button search, addlist, update;
Spinner shift;

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

    fac_code = (EditText) findViewById(R.id.sh_code);
    s_intake = (EditText) findViewById(R.id.intake);
    s_section = (EditText) findViewById(R.id.section);
    listView = (ListView) findViewById(R.id.faclistview);
    shift = (Spinner) findViewById(R.id.shift);
    addlist = (Button) findViewById(R.id.addlist);
    search = (Button) findViewById(R.id.search);
    update = (Button) findViewById(R.id.update);
}

public void CallAdapter(Context context)
    {
        adapterlist.clear();

        if(daylist.size() == 0)
        {
            //no intake list for day shift

            if(evelist.size() == 0)
            {
                //no intake list for evening shift
            }
            else if(evelist.size() > 0)
            {
                for (int i=0;i<evelist.size();i++)
                {
                    String s = evelist.get(i);
                    int p = s.indexOf("-");

                    String intake = s.substring(0,p);
                    String section = s.substring(p+1,s.length());

                    adapterlist.add(new UpdateSupervisionItem(intake, section, "Evening"));
                }
            }
        }
        else if(daylist.size() > 0)
        {
            for (int i=0;i<daylist.size();i++)
            {
                String s = daylist.get(i);
                int p = s.indexOf("-");

                String intake = s.substring(0,p);
                String section = s.substring(p+1,s.length());

                adapterlist.add(new UpdateSupervisionItem(intake, section, "Day"));
            }

            if(evelist.size() == 0)
            {
                //no intake list for evening shift
            }
            else if(evelist.size() > 0)
            {
                for (int i=0;i<evelist.size();i++)
                {
                    String s = evelist.get(i);
                    int p = s.indexOf("-");

                    String intake = s.substring(0,p);
                    String section = s.substring(p+1,s.length());

                    adapterlist.add(new UpdateSupervisionItem(intake, section, "Evening"));
                }
            }
        }

        UpdateSupervisionAdapter updateSupervisionAdapter = new UpdateSupervisionAdapter(context, R.layout.update_supervision_list_view_layout, adapterlist);
        listView.setAdapter(updateSupervisionAdapter);
    }
}

适配器类:

package bd.edu.bubt.regup;

public class UpdateSupervisionAdapter extends ArrayAdapter<UpdateSupervisionItem> {

    ArrayList<UpdateSupervisionItem> adapterlist = new ArrayList<>();
    Context context;

    public UpdateSupervisionAdapter(Context context, int textViewResourceId, ArrayList<UpdateSupervisionItem> objects) {

        super(context, textViewResourceId, objects);
        this.context = context;
        adapterlist = objects;
    }

    @Override
    public int getCount() {

        return super.getCount();
    }


    @Override
    public View getView(final int position, View convertView, ViewGroup parent){

        View v = convertView;
        LayoutInflater inflater = (LayoutInflater) getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        v = inflater.inflate(R.layout.update_supervision_list_view_layout, null);

        TextView intake = (TextView) v.findViewById(R.id.intake);
        TextView section = (TextView) v.findViewById(R.id.section);
        TextView shift = (TextView) v.findViewById(R.id.shift);
        final Button remove = (Button) v.findViewById(R.id.remove);

        remove.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                Animation animation = AnimationUtils.loadAnimation(getContext(), R.anim.blink_anim);
                remove.startAnimation(animation);

                UpdateSupervisionItem updateSupervisionItem = getItem(position);

                String in_take = updateSupervisionItem.getIntake();
                String sec_tion = updateSupervisionItem.getSection();
                String shi_ft = updateSupervisionItem.getShift();

                UpdateSupervitionActivity updateSupervitionActivity = new UpdateSupervitionActivity();

                if(TextUtils.equals(shi_ft,"Day"))
                {
                    updateSupervitionActivity.daylist.remove(in_take+ "-" +sec_tion);
                    updateSupervitionActivity.CallAdapter(context);
                }
                else if(TextUtils.equals(shi_ft,"Evening"))
                {
                    updateSupervitionActivity.evelist.remove(in_take+ "-" +sec_tion);
                    updateSupervitionActivity.CallAdapter(context);
                }
            }
        });

        intake.setText("Intake: " +adapterlist.get(position).getIntake());
        section.setText("Section: " +adapterlist.get(position).getSection());
        shift.setText("Shift: " +adapterlist.get(position).getShift());

        return v;
    }

}

错误日志:

FATAL EXCEPTION: main
                  Process: bd.edu.bubt.regup, PID: 31219
                  java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
                      at bd.edu.bubt.regup.UpdateSupervitionActivity.CallAdapter(UpdateSupervitionActivity.java:352)
                      at bd.edu.bubt.regup.UpdateSupervisionAdapter$1.onClick(UpdateSupervisionAdapter.java:70)
                      at android.view.View.performClick(View.java:5637)
                      at android.view.View$PerformClick.run(View.java:22429)
                      at android.os.Handler.handleCallback(Handler.java:751)
                      at android.os.Handler.dispatchMessage(Handler.java:95)
                      at android.os.Looper.loop(Looper.java:154)
                      at android.app.ActivityThread.main(ActivityThread.java:6119)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

Answer 1

出现这个错误的原因是listview没有通过id找到。看起来您的适配器首先是从另一个 Activity 初始化的，这是您的 ListView 布局的真实上下文。有两种方法可以解决这个问题:1)您应该通过

在更新类中初始化您的 ListView

ListView listView = context.findViewById(R.id.faclistview)

2) 您可以将 CallAdapter 移动到适配器的实际上下文 Activity 中

更新尝试一下:

ListView listView = (ListView) ((ActivityWhereAdapterIsForstInitialized) context).findViewById(R.id.faclistview)