java - 在Android Studio上通过Intent将变量 "date"传递给另一个类，NULL OBJECT REFERENCE

Question

这里对 Android 开发相当陌生。我一直在修改代码并尝试获取用户在日历 View 上选择的日期，以将其传递给另一个类，该类将保存到数据库(一个允许用户为日历上的特定日期创建事件的应用程序)。我的问题是，当我通过它时，应用程序因空对象引用而崩溃。我似乎找不到导致空对象引用的原因以及需要更改的内容。任何帮助将不胜感激。

日历 Activity .java

package com.example.zaphk.studenthelperapplication3.calendar;

import android.content.Intent;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.widget.CalendarView;
import android.widget.Toast;

import com.example.zaphk.studenthelperapplication3.R;

public class CalendarActivity extends AppCompatActivity {
    public String date;
    private  static final String TAG = "CalendarActivity";
    private CalendarView mCalendarView;
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.calendar_layout);
        mCalendarView = (CalendarView) findViewById(R.id.calendarView);
        mCalendarView.setOnDateChangeListener(new CalendarView.OnDateChangeListener() {
            @Override
            public void onSelectedDayChange(CalendarView CalendarView, int year, int month, int dayOfMonth) {
                String date = year + "/" + month + "/"+ dayOfMonth ;
                Log.d(TAG, "onSelectedDayChange: yyyy/mm/dd:" + date);
                Intent intent = new Intent(CalendarActivity.this,CalendarEvent.class);
                intent.putExtra("date",date);
                startActivity(intent);

                Toast.makeText(CalendarActivity.this,date,Toast.LENGTH_SHORT).show();

            }
        });
    }
}

日历事件.java

package com.example.zaphk.studenthelperapplication3.calendar;

import android.content.DialogInterface;
import android.content.Intent;
import android.os.Bundle;
import android.support.design.widget.CoordinatorLayout;
import android.support.design.widget.FloatingActionButton;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.DefaultItemAnimator;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.support.v7.widget.Toolbar;
import android.text.TextUtils;
import android.view.LayoutInflater;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import java.util.ArrayList;
import java.util.List;

import com.example.zaphk.studenthelperapplication3.calendar.database.Calendar;
import com.example.zaphk.studenthelperapplication3.calendar.database.CalendarAdapter;
import com.example.zaphk.studenthelperapplication3.calendar.database.Calendar_DbHelper;
import com.example.zaphk.studenthelperapplication3.utils.MyDividerItemDecoration;
import com.example.zaphk.studenthelperapplication3.utils.RecyclerTouchListener;

import com.example.zaphk.studenthelperapplication3.R;


public class CalendarEvent extends AppCompatActivity {
    private CalendarAdapter mAdapter;
    private List<Calendar> notesList = new ArrayList<>();
    private CoordinatorLayout coordinatorLayout;
    private RecyclerView recyclerView;
    private TextView noNotesView;
    Intent intent = getIntent();
    public String date = intent.getStringExtra("date");


    private Calendar_DbHelper db;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_notes);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);




        coordinatorLayout = findViewById(R.id.coordinator_layout);
        recyclerView = findViewById(R.id.recycler_view);
        noNotesView = findViewById(R.id.empty_notes_view);

        db = new Calendar_DbHelper(this);

        notesList.addAll(db.getAllNotes());

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                showNoteDialog(false, null, -1);
            }
        });

        mAdapter = new CalendarAdapter(this, notesList);
        RecyclerView.LayoutManager mLayoutManager = new LinearLayoutManager(getApplicationContext());
        recyclerView.setLayoutManager(mLayoutManager);
        recyclerView.setItemAnimator(new DefaultItemAnimator());
        recyclerView.addItemDecoration(new MyDividerItemDecoration(this, LinearLayoutManager.VERTICAL, 16));
        recyclerView.setAdapter(mAdapter);

        toggleEmptyNotes();



        /**
         * On long press on RecyclerView item, open alert dialog
         * with options to choose
         * Edit and Delete
         * */
        recyclerView.addOnItemTouchListener(new RecyclerTouchListener(this,
                recyclerView, new RecyclerTouchListener.ClickListener() {
            @Override
            public void onClick(View view, final int position) {
            }

            @Override
            public void onLongClick(View view, int position) {
                showActionsDialog(position);
            }
        }));
    }



    /**
     * Inserting new note in db
     * and refreshing the list
     */
    private void createNote(String note) {

        // inserting note in db and getting
        // newly inserted note id
        long id = db.insertNote(note);

        // get the newly inserted note from db
        Calendar n = db.getNote(id);

        if (n != null) {
            // adding new note to array list at 0 position
            notesList.add(0, n);

            // refreshing the list
            mAdapter.notifyDataSetChanged();

            toggleEmptyNotes();
        }
    }

    /**
     * Updating note in db and updating
     * item in the list by its position
     */
    private void updateNote(String note, int position) {
        Calendar n = notesList.get(position);
        // updating note text
        n.setNote(note);

        // updating note in db
        db.updateNote(n);

        // refreshing the list
        notesList.set(position, n);
        mAdapter.notifyItemChanged(position);

        toggleEmptyNotes();
    }

    /**
     * Deleting note from SQLite and removing the
     * item from the list by its position
     */
    private void deleteNote(int position) {
        // deleting the note from db
        db.deleteNote(notesList.get(position));

        // removing the note from the list
        notesList.remove(position);
        mAdapter.notifyItemRemoved(position);

        toggleEmptyNotes();
    }

    /**
     * Opens dialog with Edit - Delete options
     * Edit - 0
     * Delete - 0
     */
    private void showActionsDialog(final int position) {
        CharSequence colors[] = new CharSequence[]{"Edit", "Delete"};

        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        builder.setTitle("Choose option");
        builder.setItems(colors, new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                if (which == 0) {
                    showNoteDialog(true, notesList.get(position), position);
                } else {
                    deleteNote(position);
                }
            }
        });
        builder.show();
    }


    /**
     * Shows alert dialog with EditText options to enter / edit
     * a note.
     * when shouldUpdate=true, it automatically displays old note and changes the
     * button text to UPDATE
     */
    private void showNoteDialog(final boolean shouldUpdate, final Calendar note, final int position) {
        LayoutInflater layoutInflaterAndroid = LayoutInflater.from(getApplicationContext());
        View view = layoutInflaterAndroid.inflate(R.layout.note_dialog, null);

        AlertDialog.Builder alertDialogBuilderUserInput = new AlertDialog.Builder(CalendarEvent.this);
        alertDialogBuilderUserInput.setView(view);

        final EditText inputNote = view.findViewById(R.id.note);
        TextView dialogTitle = view.findViewById(R.id.dialog_title);
        dialogTitle.setText(!shouldUpdate ? getString(R.string.lbl_new_note_title) : getString(R.string.lbl_edit_note_title));

        if (shouldUpdate && note != null) {
            inputNote.setText(note.getNote());
        }
        alertDialogBuilderUserInput
                .setCancelable(false)
                .setPositiveButton(shouldUpdate ? "update" : "save", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialogBox, int id) {

                    }
                })
                .setNegativeButton("cancel",
                        new DialogInterface.OnClickListener() {
                            public void onClick(DialogInterface dialogBox, int id) {
                                dialogBox.cancel();
                            }
                        });

        final AlertDialog alertDialog = alertDialogBuilderUserInput.create();
        alertDialog.show();

        alertDialog.getButton(AlertDialog.BUTTON_POSITIVE).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // Show toast message when no text is entered
                if (TextUtils.isEmpty(inputNote.getText().toString())) {
                    Toast.makeText(CalendarEvent.this, "Enter note!", Toast.LENGTH_SHORT).show();
                    return;
                } else {
                    alertDialog.dismiss();
                }

                // check if user updating note
                if (shouldUpdate && note != null) {
                    // update note by it's id
                    updateNote(inputNote.getText().toString(), position);
                } else {
                    // create new note
                    createNote(inputNote.getText().toString());
                }
            }
        });
    }

    /**
     * Toggling list and empty notes view
     */
    private void toggleEmptyNotes() {
        // you can check notesList.size() > 0

        if (db.getNotesCount() > 0) {
            noNotesView.setVisibility(View.GONE);
        } else {
            noNotesView.setVisibility(View.VISIBLE);
        }
    }

    public String getDate(){
        return date;
    }
}

Logcat 错误:

2019-10-06 21:49:33.346 26474-26474/com.example.zaphk.studenthelperapplication3 E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.zaphk.studenthelperapplication3, PID: 26474
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.zaphk.studenthelperapplication3/com.example.zaphk.studenthelperapplication3.calendar.CalendarEvent}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Intent.getStringExtra(java.lang.String)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2762)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2943)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1630)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6626)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:811)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Intent.getStringExtra(java.lang.String)' on a null object reference
        at com.example.zaphk.studenthelperapplication3.calendar.CalendarEvent.<init>(CalendarEvent.java:41)
        at java.lang.Class.newInstance(Native Method)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1196)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2752)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2943) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1630) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6626) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:811) 

Answer 1

你应该放

Intent intent = getIntent();
public String date = intent.getStringExtra("date");

在onCreate()方法内

如这个stackoverflow answer Intent 是在创建 Activity 对象时设置的。在创建 Activity 之前您无法访问它

编辑

你可以像这样添加一个 setter/getter

    Intent intent;
    public String date ;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_notes);
        intent =  = getIntent();
        date = intent.getStringExtra("date")
    }
    public String getDate(){
        return date;
    }