java - Android 空指针异常

Question

当我尝试将 Intent 加载到处理数据库中数据 ListView 的类时，我收到“NullPointerException”。对于 ListView ，我是个新手，所以希望有人能告诉我哪里出了问题!

我认为空指针是由于在光标所在的第一行位置没有找到数据而导致的，但我认为查看logcat，这是一个问题我的 XML 布局“条目”？

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 数据库处理程序类:

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 ListView 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>

日志猫:

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

您只是忘记调用open()。

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

(但请阅读 Java 命名约定，状态变量应以小写字母开头。)