java.lang.NullPointerException：尝试在对 select sqlite 的空对象引用上调用虚拟方法

Question

我是android世界的新手。我有编码问题。这只是一个小错误，我不知道它不起作用，即使我改变了其他方法，但错误仍然是同样的错误。这里错误发生在 logcat:

java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.Cursor com.example.zellon.surveyapps.DatabaseHelper.getAData()' on a null object reference

我只想select数据库中的数据得到一个id但是不能因为上面的错误。

我将给出我为 select 来自数据库的数据编码的代码

surveyinstruction.java

package com.example.zellon.surveyapps;

import android.content.Intent;
import android.database.Cursor;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class surveyinstruction extends AppCompatActivity {
DatabaseHelper myDb;

protected void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    setContentView(R.layout.surveyinstruction);

    TextView user = (TextView) findViewById(R.id.user);

    Intent in = getIntent();

    String nameUser = in.getStringExtra("nameUser");
    String tarikhUser = in.getStringExtra("tarikhUser");

    user.setText("Selamat Datang " + nameUser);

    Button btnSeterusnya = (Button) findViewById(R.id.btnTerus);

    btnSeterusnya.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent intentTerus = new Intent(getApplicationContext(), surveymain.class);
            Cursor res = myDb.getAData();
            intentTerus.putExtra("id", res.getString(0));
            Log.e("ID ", res.getString(0));
            startActivity(intentTerus);
        }
    });
}
}

这里是数据库处理程序

DatabaseHelper.java

package com.example.zellon.surveyapps;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class DatabaseHelper extends SQLiteOpenHelper {
public static final String DATABASE_NAME = "survey.db";
public static final String TABLE_USER = "user";
public static final String COL_USER_1 = "ID";
public static final String COL_USER_2 = "NAMA";
public static final String COL_USER_3 = "TARIKH";
public static final String COL_USER_4 = "MARKAH";

public static final String TABLE_QUESTION = "question";

public static final String COL_QUES_1 = "ID";
public static final String COL_QUES_2 = "IDUSER";
public static final String COL_QUES_3 = "K1";
public static final String COL_QUES_4 = "K2";
public static final String COL_QUES_5 = "K3";
public static final String COL_QUES_6 = "K4";
public static final String COL_QUES_7 = "K5";
public static final String COL_QUES_8 = "A1";
public static final String COL_QUES_9 = "A2";
public static final String COL_QUES_10 = "A3";
public static final String COL_QUES_11 = "A4";
public static final String COL_QUES_12 = "A5";
public static final String COL_QUES_13 = "V1";
public static final String COL_QUES_14 = "V2";
public static final String COL_QUES_15 = "V3";
public static final String COL_QUES_16 = "V4";
public static final String COL_QUES_17 = "V5";
public static final String COL_QUES_18 = "D1";
public static final String COL_QUES_19 = "D2";
public static final String COL_QUES_20 = "D3";
public static final String COL_QUES_21 = "D4";
public static final String COL_QUES_22 = "D5";
public static final String COL_QUES_23 = "TOTK";
public static final String COL_QUES_24 = "TOTA";
public static final String COL_QUES_25 = "TOTV";
public static final String COL_QUES_26 = "TOTD";

public DatabaseHelper(Context context) {
    super(context, DATABASE_NAME, null, 1);
}

@Override
public void onCreate(SQLiteDatabase db) {
    db.execSQL("create table " + TABLE_USER + " (ID INTEGER PRIMARY KEY AUTOINCREMENT, NAMA TEXT, TARIKH TEXT, MARKAH INTEGER)");
    db.execSQL("create table " + TABLE_QUESTION + " (ID INTEGER PRIMARY KEY AUTOINCREMENT, IDUSER INTEGER, K1 TEXT, K2 TEXT, K3 TEXT, K4 TEXT, K5 TEXT, A1 TEXT, A2 TEXT, A3 TEXT, A4 TEXT, A5 TEXT, V1 TEXT, V2 TEXT, V3 TEXT, V4 TEXT, V5 TEXT, D1 TEXT, D2 TEXT, D3 TEXT, D4 TEXT, D5 TEXT, TOTK INTEGER, TOTA INTEGER, TOTV INTEGER, TOTD INTEGER)");
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_USER);
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_QUESTION);
    onCreate(db);
}

public boolean insertDataUser(String nama, String tarikh){
    SQLiteDatabase db = this.getWritableDatabase();
    ContentValues contentValues = new ContentValues();
    contentValues.put(COL_USER_2, nama);
    contentValues.put(COL_USER_3, tarikh);
    long result = db.insert(TABLE_USER, null, contentValues);

    if(result == -1)
        return false;
    else
        return true;
}

public Cursor getCertainData(String namaUser, String tarikhUser){
    String selectQuery = "select * from " + TABLE_USER + " where NAMA like '" + namaUser + "' AND TARIKH like '" + tarikhUser + "'";
    SQLiteDatabase db = this.getWritableDatabase();
    Cursor res = db.rawQuery(selectQuery, null);
    return res;
}

public Cursor getAData(){
    SQLiteDatabase db = this.getWritableDatabase();
    String selectAQuery = "SELECT ID FROM " + TABLE_USER;
    Cursor re = db.rawQuery(selectAQuery, null);
    return re;
}

public boolean insertDataQues(int id, String k1, String k2, String k3, String k4, String k5, String a1, String a2, String a3, String a4, String a5, String v1, String v2, String v3, String v4, String v5, String d1, String d2, String d3, String d4, String d5, int totalk, int totala, int totalv, int totald){
    SQLiteDatabase dbQues = this.getWritableDatabase();
    ContentValues contentValues = new ContentValues();
    contentValues.put(COL_QUES_2, id);
    contentValues.put(COL_QUES_3, k1);
    contentValues.put(COL_QUES_4, k2);
    contentValues.put(COL_QUES_5, k3);
    contentValues.put(COL_QUES_6, k4);
    contentValues.put(COL_QUES_7, k5);

    contentValues.put(COL_QUES_8, a1);
    contentValues.put(COL_QUES_9, a2);
    contentValues.put(COL_QUES_10, a3);
    contentValues.put(COL_QUES_11, a4);
    contentValues.put(COL_QUES_12, a5);

    contentValues.put(COL_QUES_13, v1);
    contentValues.put(COL_QUES_14, v2);
    contentValues.put(COL_QUES_15, v3);
    contentValues.put(COL_QUES_16, v4);
    contentValues.put(COL_QUES_17, v5);

    contentValues.put(COL_QUES_18, d1);
    contentValues.put(COL_QUES_19, d2);
    contentValues.put(COL_QUES_20, d3);
    contentValues.put(COL_QUES_21, d4);
    contentValues.put(COL_QUES_22, d5);

    contentValues.put(COL_QUES_23, totalk);
    contentValues.put(COL_QUES_24, totala);
    contentValues.put(COL_QUES_25, totalv);
    contentValues.put(COL_QUES_26, totald);

    long result = dbQues.insert(TABLE_QUESTION, null, contentValues);

    if(result == -1)
        return false;
    else
        return true;
}
}

所以，在 surveyinstruction.java 的代码中，我只从 DatabaseHelper.java 和 select 数据库中的 ID 调用 getAData() 函数，但错误显示它是空对象引用。我不知道错误是什么。我希望有人能帮助我让它正常工作。谢谢。

Answer 1

您尚未初始化对 DatabaseHelper myDb; 的引用。这就是空指针异常的原因。

Answer 2

初始化数据库助手：

protected void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    setContentView(R.layout.surveyinstruction);

    DatabaseHelper db = new DatabaseHelper(this);

    //...
}

Answer 3

我对数据库中的 getId 也有类似的问题。我发现我没有从 mainAcitivity.java.

中获取数据库

    public adapter(Context mCtx, int listLayoutRes, List<employee> employeeList,SQLiteDatabase mDatabase) {
    super(mCtx, listLayoutRes, employeeList);

    this.mCtx = mCtx;
    this.listLayoutRes = listLayoutRes;
    this.employeeList = employeeList;
    this.mDatabase = mDatabase;
}

并使用 mainAcitivity 中的这个构造函数

    adpt= new adapter(this,R.layout.row,employeeList,mdatabase);

解决了我的问题。希望对您有所帮助。

Answer 4

这个怎么样，我已经声明了 Helper class，“NoteDB db = new NoteDB(this)” 但是 bug 错误表明 this 的空指针异常，“String title = db.getTitle”，“NoteDB db = new NoteDB(this)”行的声明没有用。

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

    Intent i = getIntent();
    long id = i.getLongExtra("ID",1);
    NoteDatabase db = new NoteDatabase(getApplicationContext());
    Note note = db.getNote(id);
    getSupportActionBar().setTitle(note.getTitle());
    Toast.makeText(this,"ID -> "+id,Toast.LENGTH_LONG).show();
  //  Toast.makeText(this,"Title -> "+note.getTitle(),Toast.LENGTH_LONG).show();
    FloatingActionButton fab = findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    });
}

java.lang.NullPointerException：尝试在对 select sqlite 的空对象引用上调用虚拟方法

java.lang.NullPointerException: Attempt to invoke virtual method on a null object reference to select sqlite

sqlite

nullpointerexception

null

android-sqlite

android-logcat