Question

我是Java和Android的新手当我尝试将我的Android应用程序连接到SQLite时，每次在应用程序调用数据库帮助程序类上的onCreate方法时崩溃应用程序时都会产生空点错误。

我无法在代码中找到错误，所以请有人帮助我。

MainActivity.java代码：

package com.example.sdreddy.dbfinal;

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


public class MainActivity extends AppCompatActivity {
DbNeed dbn;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Button dd=(Button)findViewById(R.id.button);

        dd.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                TextView tv = (TextView) findViewById(R.id.textView);
                try {
                    dbn.Addd("SDREDDY");

                    String zz = dbn.getdata();
                    tv.setText(zz);
                }
                catch (Exception ee){
                    tv.setText((CharSequence) ee.toString());
                }
            }
        });
    }
}

DbNeed.java代码

package com.example.sdreddy.dbfinal;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

import java.util.Random;
import java.util.concurrent.CountDownLatch;

/**
 * Created by SDREDDY on 26-04-2018.
 */

public class DbNeed extends SQLiteOpenHelper {
    public static final String dbname="Database.db";
    public static final String tblname="MyTable";

    public DbNeed(Context context) {
        super(context, dbname, null, 1);
    }

    @Override
    public void onCreate(SQLiteDatabase sqLiteDatabase) {
        sqLiteDatabase.execSQL("create table if not exists "+tblname+" (id text,value text)");

    }

    @Override
    public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {
        sqLiteDatabase.execSQL("drop  table if exists "+tblname);
        onCreate(sqLiteDatabase);

    }
    public void Addd(String Name){
        SQLiteDatabase sqLiteDatabase=this.getWritableDatabase();
        ContentValues cv=new ContentValues();
        Random r=new Random();
        String idz=String.valueOf(r.nextInt());
        cv.put("id",idz);
        cv.put("name",Name);
        sqLiteDatabase.insert(tblname,null,cv);
        sqLiteDatabase.close();
    }
    public String getdata(){
        SQLiteDatabase sqLiteDatabase=this.getReadableDatabase();
        String qu="SELECT * FROM "+tblname;
        Cursor crz=sqLiteDatabase.rawQuery(qu,null);
        sqLiteDatabase.close();
        String sim=crz.getString(0);
        return sim;


    }
}

activity_main.xml中

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout 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="com.example.sdreddy.dbfinal.MainActivity">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <EditText
            android:id="@+id/editText"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:ems="10"
            android:inputType="textPersonName"
            android:text="Name" />

        <Button
            android:id="@+id/button"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Button" />

        <TextView
            android:id="@+id/textView"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="TextView"
            android:textSize="30sp" />
    </LinearLayout>

</android.support.constraint.ConstraintLayout>

Log cat of my Code

Answer 1

在行setContentView(R.layout.activity_main);之后添加以下内容： -

    dbn = new DbNeed(this);

这将实例化（设置） dbn ，它在此阶段仅被声明，因此为空。

Android Sqlite Nullpoint Exception --- java.lang.NullPointerExceptio

1 个答案: