Android Null指针异常

Question

我在尝试将我的意图加载到一个处理我的数据库数据列表视图的类时，收到“ NullPointerException”错误。 我是列表视图方面的新手，所以希望有人可以告诉我我哪里出问题了！

我在想，空指针是由于在第一行posistion光标是在没有数据被人发现，但我认为看logcat的，它的一个问题，我的XML布局“项”？

这是我的ListView类：

package com.example.sqliteexample;



import android.app.Activity;
import android.app.ListActivity;
import android.database.Cursor;
import android.os.Bundle;
import android.support.v4.widget.SimpleCursorAdapter;
import android.widget.ListView;

public class SQLView extends Activity {

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    HotOrNot H = new HotOrNot(this, null, null);

    setContentView(R.layout.layout);
     ListView listContent = (ListView)findViewById(R.id.contentList);

     HotOrNot Content = new HotOrNot(this, null, null);

    Cursor cursor = Content.getData();

    startManagingCursor(cursor);

    @SuppressWarnings("static-access")
    String [] from = new String [] {H.KEY_NAME, H.KEY_HOTNESS};
    int [] to = new int [] {R.id.txtName, R.id.txtAge};

    @SuppressWarnings("deprecation")
    SimpleCursorAdapter cursorAdapter = new SimpleCursorAdapter(this, R.layout.entries, cursor, from, to);

    listContent.setAdapter(cursorAdapter);

我的数据库处理程序类中的“ getData”方法：

public Cursor getData() {

        String [] columns = new String[]{KEY_ROWID, KEY_NAME, KEY_HOTNESS};
        Cursor c = ourDatabase.query(DATABASE_TABLE, columns, null, null, null, null, null);


        return c;

HotOrNot DB处理程序类：

package com.example.sqliteexample;

import android.app.ListActivity;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.widget.ListView;

public class HotOrNot {

public static final String KEY_ROWID = "_id";
public static final String KEY_NAME = "persons_name";
public static final String KEY_HOTNESS = "persons_hotness";

private static String DATABASE_NAME = "HotOrNotdb";
private static String DATABASE_TABLE = "peopleTable";
private static int DATABASE_VERSION = 30;

private static DBHelper ourHelper;
private static Context ourContext;
private static SQLiteDatabase ourDatabase;

ListView listview;


public void DB_NAME(String DBName)
{
    DATABASE_NAME = DBName;

}

public String returnDB_NAME()
{
    return DATABASE_NAME;
}


public void DB_tableNAME(String DBtName)
{
    DATABASE_TABLE = DBtName;

}

public String returnDB_tNAME()
{
    return DATABASE_TABLE;
}


public void DB_NAME(int DBVersion)
{
    DATABASE_VERSION = DBVersion;

}

public int returnDB_VERSION()
{
    return DATABASE_VERSION;
}



// class constructor for context. when the object is constructed in the main programme
// the context of that class i.e 'this' is sent to this constructor to set the object context.
// null values set on SQLiteDatabase and DBhelper as there is nothing to pass from the called
// objects.
public HotOrNot (Context c, SQLiteDatabase newSQL, DBHelper BD)
{
    ourContext = c;
    ourDatabase = newSQL;
    ourHelper = BD;

}







private static class DBHelper extends SQLiteOpenHelper
{
    public DBHelper(Context context) {
        super(context, KEY_NAME, null, DATABASE_VERSION);

    }

    @Override
    public void onCreate(SQLiteDatabase db) {


        db.execSQL("CREATE TABLE " + DATABASE_TABLE + " (" +
                KEY_ROWID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
                KEY_NAME + " TEXT NOT NULL, " +
                KEY_HOTNESS + " TEXT NOT NULL);"

                );


    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

        db.execSQL("DROP TABLE IF EXISTS " + DATABASE_TABLE);
        onCreate(db);
    }
}



    public HotOrNot open() throws SQLException
    {

        ourHelper = new DBHelper(ourContext);
        ourDatabase = ourHelper.getWritableDatabase();
        return this;

    }

    public void close()
    {
        ourHelper.close();

    }

    public long createEntry(String name, String hotness) {


        ContentValues cv = new ContentValues();
        cv.put(KEY_NAME, name);
        cv.put(KEY_HOTNESS, hotness);
        return ourDatabase.insert(DATABASE_TABLE, null, cv);

    }

    public Cursor getData() {

        String [] columns = new String[]{KEY_ROWID, KEY_NAME, KEY_HOTNESS};
        Cursor c = ourDatabase.query(DATABASE_TABLE, columns, null, null, null, null, null);
        //String result = "";

        //int iRow = c.getColumnIndex(KEY_ROWID);
        //int iName = c.getColumnIndex(KEY_NAME);
        //int ihotness = c.getColumnIndex(KEY_HOTNESS);

        //for(c.moveToFirst(); !c.isAfterLast(); c.moveToNext())
        //{
            //result = result + c.getString(iRow) + "" + c.getString(iName) + "" + c.getString(ihotness);

        //}

        //c.close();

        return c;
    }

    public String getName(long l) {

        String [] columns = new String[]{KEY_ROWID, KEY_NAME, KEY_HOTNESS};
        Cursor c = ourDatabase.query(DATABASE_TABLE, columns, KEY_ROWID + "=" + l, null, null, null, null);
        if(c != null)
        {
            // move to the selected row
        c.moveToFirst();
        String name = c.getString(1);
        return name;
        }
        return null;

    }


    public String getHotness(long l) {

        String [] columns = new String[]{KEY_ROWID, KEY_NAME, KEY_HOTNESS};
        Cursor c = ourDatabase.query(DATABASE_TABLE, columns, KEY_ROWID + "=" + l, null, null, null, null);
        if(c != null)
        {
            // move to the selected row
        c.moveToFirst();
        String hotness = c.getString(2);
        return hotness;

    }
        return null;


    }

    public void updateEntry(long newl, String nameEdit, String hotnessEdit) {
    ContentValues cvUpdate = new ContentValues();

    cvUpdate.put(KEY_NAME, nameEdit);
    cvUpdate.put(KEY_HOTNESS, hotnessEdit);

    ourDatabase.update(DATABASE_TABLE, cvUpdate, KEY_ROWID + "=" + newl, null);


    }

    public void delID(long l) {
        ourDatabase.delete(DATABASE_TABLE, KEY_ROWID + "=" + l, null);

    }

}

我的列表视图的XML布局：

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

<TextView
    android:id="@+id/txtName"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Name:" />

<TextView
    android:id="@+id/txtAge"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Age:" />

</LinearLayout>

listview XML布局：

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

<ListView
    android:id="@+id/contentList"
    android:layout_width="match_parent"
    android:layout_height="wrap_content" >
</ListView>

</LinearLayout>

Logcat：

01-11 00:23:26.655: D/AndroidRuntime(272): Shutting down VM
01-11 00:23:26.655: W/dalvikvm(272): threadid=1: thread exiting with uncaught exception (group=0x4001d800)
01-11 00:23:26.675: E/AndroidRuntime(272): FATAL EXCEPTION: main
01-11 00:23:26.675: E/AndroidRuntime(272): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.sqliteexample/com.example.sqliteexample.SQLView}: java.lang.NullPointerException
01-11 00:23:26.675: E/AndroidRuntime(272):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
01-11 00:23:26.675: E/AndroidRuntime(272):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
01-11 00:23:26.675: E/AndroidRuntime(272):  at android.app.ActivityThread.access$2300(ActivityThread.java:125)
01-11 00:23:26.675: E/AndroidRuntime(272):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
01-11 00:23:26.675: E/AndroidRuntime(272):  at android.os.Handler.dispatchMessage(Handler.java:99)
01-11 00:23:26.675: E/AndroidRuntime(272):  at android.os.Looper.loop(Looper.java:123)
01-11 00:23:26.675: E/AndroidRuntime(272):  at android.app.ActivityThread.main(ActivityThread.java:4627)
01-11 00:23:26.675: E/AndroidRuntime(272):  at java.lang.reflect.Method.invokeNative(Native Method)
01-11 00:23:26.675: E/AndroidRuntime(272):  at java.lang.reflect.Method.invoke(Method.java:521)
01-11 00:23:26.675: E/AndroidRuntime(272):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
01-11 00:23:26.675: E/AndroidRuntime(272):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
01-11 00:23:26.675: E/AndroidRuntime(272):  at dalvik.system.NativeStart.main(Native Method)
01-11 00:23:26.675: E/AndroidRuntime(272): Caused by: java.lang.NullPointerException
01-11 00:23:26.675: E/AndroidRuntime(272):  at com.example.sqliteexample.HotOrNot.getData(HotOrNot.java:143)
01-11 00:23:26.675: E/AndroidRuntime(272):  at com.example.sqliteexample.SQLView.onCreate(SQLView.java:24)
01-11 00:23:26.675: E/AndroidRuntime(272):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
01-11 00:23:26.675: E/AndroidRuntime(272):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)

Answer 1

您只是忘记了调用open() 。

HotOrNot Content = new HotOrNot(this, null, null);
Content.open();
Cursor cursor = Content.getData();

（但是请阅读有关Java命名约定的信息，这些状态变量应以小写字母开头。）

Answer 2

public HotOrNot (Context c, SQLiteDatabase newSQL, DBHelper BD)
{
    ourContext = c;
    ourDatabase = newSQL;
    ourHelper = BD;

}

在这里，您要将onCreate中的null传递给SQLiteDatabase

HotOrNot H = new HotOrNot(this, null, null);