无法使用 SQLite 将数据插入数据库，出现空指针异常 [重复]

Question

我有两个活动课。一个有一个浮动按钮，另一个有一个简单的按钮。不知何故，当我在运行插入查询时出错时。 我得到的错误是：

java.lang.NullPointerException: Attempt to invoke virtual method 'void com.sidharth.android.navigationdrawer.controller.DatabaseHelper.insertNote(java.lang.String, java.lang.String)' on a null object reference

at com.sidharth.android.navigationdrawer.view.CreateNoteActivity.insert(CreateNoteActivity.java:60)

MainActivity.java

import android.content.Intent;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.view.View;
import android.support.design.widget.NavigationView;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;

import com.sidharth.android.navigationdrawer.R;

public class  MainActivity extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener {

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

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(MainActivity.this, CreateNoteActivity.class);
                startActivity(intent);

            }
        });

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.addDrawerListener(toggle);
        toggle.syncState();

        NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);
    }

    @Override
    public void onBackPressed() {
        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START);
        } else {
            super.onBackPressed();
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    @SuppressWarnings("StatementWithEmptyBody")
    @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        // Handle navigation view item clicks here.
        int id = item.getItemId();

        if (id == R.id.nav_camera) {
            // Handle the camera action
        } else if (id == R.id.nav_gallery) {

        } else if (id == R.id.nav_slideshow) {

        } else if (id == R.id.nav_manage) {

        } else if (id == R.id.nav_share) {

        } else if (id == R.id.nav_send) {

        }

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        drawer.closeDrawer(GravityCompat.START);
        return true;
    }
}

CreateNoteActivity.java

import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;

import com.sidharth.android.navigationdrawer.R;
import com.sidharth.android.navigationdrawer.controller.DatabaseHelper;

public class CreateNoteActivity extends AppCompatActivity {

    EditText noteTitle;
    EditText noteContent;
    Button btnSav;

    DatabaseHelper db ;
    private String title ;
    private String content ;

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

       /* Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);*/

        noteTitle = (EditText) findViewById(R.id.dialog_title);
        Log.d("noteTitle first",noteTitle+"");

        noteContent = (EditText) findViewById(R.id.note);
        Log.d("noteContent first",noteContent+"");

        btnSav = (Button) findViewById(R.id.btnSave);
        btnSav.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                title = noteTitle.getText().toString();
                content = noteContent.getText().toString();

                Log.d("Title inside button",title+"");
                Log.d("Content inside button",content+"");
                insert(title,content);

            }
        });
    }

    public void insert(String title, String content){
        Log.d("title before insert ", title);
        Log.d("content before insert ", content);
        db.insertNote(title,content);

        Log.d("note id - ", "");
        Log.d("title inside insert ", title);
        Log.d("content inside insert ", content);
        //Log.d("return from insert ",id+"");
    }

}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:openDrawer="start">

    <include
        layout="@layout/app_bar_main"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

    <android.support.design.widget.NavigationView
        android:id="@+id/nav_view"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:fitsSystemWindows="true"
        app:headerLayout="@layout/nav_header_main"
        app:menu="@menu/activity_main_drawer" />

</android.support.v4.widget.DrawerLayout>

app_bar_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".view.MainActivity">

    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/AppTheme.AppBarOverlay">

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?attr/colorPrimary"
            app:popupTheme="@style/AppTheme.PopupOverlay" />

    </android.support.design.widget.AppBarLayout>


    <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|end"
        android:layout_margin="@dimen/fab_margin"
        app:srcCompat="@android:drawable/ic_dialog_email" />

</android.support.design.widget.CoordinatorLayout>

note_dialog.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:paddingLeft="@dimen/activity_margin"
    android:paddingRight="@dimen/activity_margin"
    android:paddingTop="@dimen/activity_margin">

    <EditText android:id="@+id/dialog_title"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="@dimen/dimen_10"
        android:fontFamily="sans-serif-medium"
        android:lineSpacingExtra="8sp"
        android:hint="@string/lbl_new_note_title"
        android:textColor="@color/colorAccent"
        android:textSize="@dimen/lbl_new_note_title"
        android:textStyle="normal" />

    <EditText
        android:id="@+id/note"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="@dimen/dimen_10"
        android:background="@android:color/transparent"
        android:gravity="top"
        android:hint="@string/hint_enter_note"
        android:inputType="textCapSentences|textMultiLine"
        android:lines="4"
        android:textColorHint="@color/hint_enter_note"
        android:textSize="@dimen/input_new_note" />

    <Button
        android:id="@+id/btnSave"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@color/colorAccent"
        android:text="save" />


</LinearLayout>

AndroidManifest

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.sidharth.android.navigationdrawer">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity
            android:name=".view.MainActivity"
            android:label="@string/app_name"
            android:theme="@style/AppTheme.NoActionBar">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".view.CreateNoteActivity"></activity>
    </application>

</manifest>

Note.java

public class Note {

    //table name
    public static final String TABLE_NAME = "notes";

    //column name
    public static final String COLUMN_ID = "id";
    public static final String COLUMN_TITLE = "title";
    public static final String COLUMN_CONTENT = "content";

    //pojo class
    private int id;
    private String title;
    private String content;

    public static final String CREATE_TABLE = "CREATE TABLE " +TABLE_NAME + "("
            + COLUMN_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, "
            + COLUMN_TITLE + " TEXT, "
            + COLUMN_CONTENT + " TEXT " + ");" ;

    public Note() {
    }

    public Note(int id, String title, String content) {
        this.id = id;
        this.title = title;
        this.content = content;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }
}

DatabaseHelper.java

public void insertNote(String title, String content){
        Log.d("inside insert method ", " hiiii ");
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues values = new ContentValues();

        values.put(Note.COLUMN_TITLE,title);
        values.put(Note.COLUMN_CONTENT,content);

        db.insert(Note.TABLE_NAME,null,values);
        db.close();
        //Log.d("id in insert", id+"");
        //return id;
    }

Answer 1

您收到该错误的原因是您尝试访问活动不知道的类。要让您的活动了解 POJO 类，只需在您的 onCreate() 方法中初始化 Note POJO。像这样：

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

         //Initialize Note pojo
         note = new Note();
         ...

注意：请确保在使用前正确初始化 pojo。