java - 打开其他布局时，应用已停止

Question

我正在创建一个用于计划的应用程序，当我选择一个日期并按select键以移至另一个布局以为此日期创建任务时，该应用程序将停止。

这是我的主要代码，效果很好:

package com.example.calendarview;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.CalendarView;
import android.widget.TextView;

public class CalendarActivity extends AppCompatActivity {

    CalendarView calendarView;
    TextView dateSelected;
    Button selectButton;
    Intent scheduleActivityIntent;
    String date;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_calendar);

        selectButton = (Button) findViewById(R.id.selectButton);
        selectButton.setOnClickListener(new CalendarView.OnClickListener(){

            public void onClick(View v) {
                System.out.println("Button Clicked");

                scheduleActivityIntent = new Intent(getApplicationContext(), ScheduleActivity.class);
                startActivity(scheduleActivityIntent);
            }
        });

        calendarView = (CalendarView) findViewById(R.id.calendarView);
        dateSelected = (TextView) findViewById(R.id.dateSelected);
        calendarView.setOnDateChangeListener(new CalendarView.OnDateChangeListener() {
            @Override
            public void onSelectedDayChange(@androidx.annotation.NonNull CalendarView view, int year, 
int month, int dayOfMonth) {
                date = dayOfMonth + "/" + (month+1) + "/" + year;
                dateSelected.setText(date);
            }
        });

     }
}

这是我的第二项 Activity ，我在其中使用ListView来显示当前任务，我敢肯定这很好-

package com.example.calendarview;

import android.os.Bundle;
import android.util.Log;
import android.widget.ListView;
import java.util.ArrayList;

public class ScheduleActivity extends CalendarActivity {

    private static final String TAG = "ScheduleActivity";

    ListView taskList;
    Task haircut;
    Task work;
    ArrayList<Task> taskListAdapter;
    ScheduleAdapter adapter;

    String aTaskName = "Haircut";
    String aTaskDate = "21/01/2020";
    String aTaskTime = "14:00";
    int aTaskTimeFinish = 1;
    int aTaskUrgency = 2;
    String aTaskDescription = "Go get a haircut";

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_schedule);

        Log.d(TAG, "onCreate: Started");
        taskList = (ListView) findViewById(R.id.schedule);

        // Need to create object insert
        haircut = new Task(aTaskName, aTaskDate, aTaskTime, aTaskTimeFinish, aTaskUrgency, aTaskDescription);
        //Should be made as a function in order to insert user's tasks

        //Creating Array adapter
        taskListAdapter = new ArrayList<>();
        taskListAdapter.add(haircut);

        adapter = new ScheduleAdapter(getApplicationContext(), R.layout.activity_schedule, taskListAdapter);

        taskList.setAdapter(adapter);
    }
}

这是我的ListView适配器，我很确定这是有问题的，但是我并没有真正了解问题以及如何解决它-

package com.example.calendarview;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.TextView;
import androidx.annotation.NonNull;
import java.util.ArrayList;

public class ScheduleAdapter extends ArrayAdapter <Task>
{
    private static final String TAG = "ScheduleAdapter";
    Context mContext;

    Task task;
    View cView;
    TextView tvName;
    TextView tvTime;
    TextView tvUrgency;
    LayoutInflater inflater;

    //Default constructor for the adapter

    public ScheduleAdapter(@NonNull Context context, int mResource, @NonNull ArrayList<Task> tasks) {
        super(context,0,  tasks);
    }

    @android.support.annotation.NonNull
    @Override
    public View getView(int position, @Nullable View convertView, @android.support.annotation.NonNull ViewGroup parent) {

        cView = convertView;
        inflater = LayoutInflater.from(mContext);

        task = getItem(position);
        if(cView == null) {
            cView = LayoutInflater.from(getContext()).inflate(R.layout.schedule_items, parent, false);
        }

        tvName = cView.findViewById(R.id.task_name);
        tvTime = cView.findViewById(R.id.task_time);
        tvUrgency = cView.findViewById(R.id.task_urgency);

        tvName.setText(task.getTaskName());
        tvTime.setText(task.getTaskTime());
        tvUrgency.setText(task.getTaskUrgency());

        return cView;
    }
}

任务是一个基本对象；

    private String taskName;
    private String taskDate;
    private String taskTime;
    private int taskTimeFinish;
    private int taskUrgency;
    private String taskDescription;

我得到这个错误-

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.calendarview, PID: 7247
    java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
        at android.view.LayoutInflater.from(LayoutInflater.java:229)
        at com.example.calendarview.ScheduleAdapter.getView(ScheduleAdapter.java:35)
        at android.widget.AbsListView.obtainView(AbsListView.java:2372)
        at android.widget.ListView.makeAndAddView(ListView.java:2052)
        at android.widget.ListView.fillDown(ListView.java:786)
        at android.widget.ListView.fillFromTop(ListView.java:847)
        at android.widget.ListView.layoutChildren(ListView.java:1826)
        at android.widget.AbsListView.onLayout(AbsListView.java:2171)
        at android.view.View.layout(View.java:19590)
        at android.view.ViewGroup.layout(ViewGroup.java:6053)
        at android.support.constraint.ConstraintLayout.onLayout(ConstraintLayout.java:1915)
        at android.view.View.layout(View.java:19590)
        at android.view.ViewGroup.layout(ViewGroup.java:6053)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at android.view.View.layout(View.java:19590)
        at android.view.ViewGroup.layout(ViewGroup.java:6053)
        at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1791)
        at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1635)
        at android.widget.LinearLayout.onLayout(LinearLayout.java:1544)
        at android.view.View.layout(View.java:19590)
        at android.view.ViewGroup.layout(ViewGroup.java:6053)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at android.view.View.layout(View.java:19590)
        at android.view.ViewGroup.layout(ViewGroup.java:6053)
        at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1791)
        at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1635)
        at android.widget.LinearLayout.onLayout(LinearLayout.java:1544)
        at android.view.View.layout(View.java:19590)
        at android.view.ViewGroup.layout(ViewGroup.java:6053)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at com.android.internal.policy.DecorView.onLayout(DecorView.java:758)
        at android.view.View.layout(View.java:19590)
        at android.view.ViewGroup.layout(ViewGroup.java:6053)
        at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2484)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2200)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1386)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6733)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:911)
        at android.view.Choreographer.doCallbacks(Choreographer.java:723)
        at android.view.Choreographer.doFrame(Choreographer.java:658)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:897)
        at android.os.Handler.handleCallback(Handler.java:789)
        at android.os.Handler.dispatchMessage(Handler.java:98)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6541)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
Process 7247 terminated.

Answer 1

变更委托(delegate)人:

 public ScheduleAdapter(@NonNull Context context, int mResource, @NonNull 
ArrayList<Task> tasks) {
super(context,0,  tasks);
}

我认为您必须在委托(delegate)方中执行此操作:mContext = context;

因为您的mcontext为null，所以您必须将 Activity 的上下文分配给适配器上下文