My app is crashing when I run the AVD and click a button that attempts to save username and password on the database

Question

I'm using Android Studio with the SQLite tool to try to build a simple register screen, but when I run the AVD and click the my register button (It's called cadastrar), the app crashes.

I have searched Stack Overflow and other sites for a solution and found similar questions but couldn't really grasp the logic behind the explanation (Im new to programming, so I mostly watch youtube tutorials and do a lot of copy paste while trying to understand the code).

This is DbHelper.java:

package com.example.gerenciadorestoqueoficial;

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

public class DbHelper extends SQLiteOpenHelper {
    public static final String DATABASE_NAME = "register.db";
    public static final String TABLE_NAME = "cadastrarusuario";
    public static final String COL_1 = "ID";
    public static final String COL_2 = "nome";
    public static final String COL_3 = "password";

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

    @Override
    public void onCreate(SQLiteDatabase sqLiteDatabase) {
        sqLiteDatabase.execSQL("CREATE TABLE cadastrarusuario (ID INTEGER PRIMARY KEY AUTOINCREMENT, nome TEXT, password TEXT)");
    }

    @Override
    public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {
        sqLiteDatabase.execSQL(" DROP TABLE IF EXISTS " + TABLE_NAME );
        onCreate(sqLiteDatabase);
    }

    public long adicionarUsuario(String usuario, String password){
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues contentValues = new ContentValues();
        contentValues.put("usuario", usuario);
        contentValues.put("password", password);
        long res = db.insert("cadastrarusuario", null, contentValues);
        db.close();
        return res;
    }
    public boolean checkUser(String nome, String password){
        String[] columns = { COL_1 };
        SQLiteDatabase db = getReadableDatabase();
        String selection = COL_1 + "=?" + " e " + COL_3 + "=?";
        String[] selectionArgs = {nome, password};
        Cursor cursor = db.query(TABLE_NAME,columns,selection,selectionArgs,null,null,null);
        int count = cursor.getCount();
        cursor.close();
        db.close();

        if(count>0)
            return true;
        else
            return false;
    }
}

This is Cadastro.Java:

package com.example.gerenciadorestoqueoficial;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class Cadastro extends AppCompatActivity {


    EditText cadastroNome;
    EditText cadastroPassword;
    EditText cnfCadastroPassword;
    Button cadastrar;
    TextView verLogin;
    DbHelper db;


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


        cadastroNome = (EditText)findViewById(R.id.etCadastroNome);
        cadastroPassword = (EditText)findViewById(R.id.etCadastroPassword);
        cnfCadastroPassword =(EditText)findViewById(R.id.etConfirmarPassword);
        cadastrar = (Button)findViewById(R.id.btnCadastrar);
        verLogin = (TextView)findViewById(R.id.tvLoginCadastro);

        verLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent loginIntent = new Intent(Cadastro.this,MainActivity.class);
                startActivity(loginIntent);
            }
        });

        cadastrar.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                String usuario = cadastroNome.getText().toString().trim();
                String password = cadastroPassword.getText().toString().trim();
                String cnfPassword = cnfCadastroPassword.getText().toString().trim();

                if(password.equals(cnfPassword)){
                    long val = db.adicionarUsuario(usuario, password);
                    if(val > 0 ){
                    Toast.makeText(Cadastro.this, "Usuário cadastrado", Toast.LENGTH_SHORT).show();
                    Intent irParaLogin = new Intent(Cadastro.this, MainActivity.class);
                    startActivity(irParaLogin);
                }else{
                        Toast.makeText(Cadastro.this, "Erro no registro", Toast.LENGTH_SHORT).show();
                    }
                    Toast.makeText(Cadastro.this, "A senha não é igual nos dois campos", Toast.LENGTH_SHORT).show();
                }
            }
        });

        }

    }

And this is the error I get:

    --------- beginning of crash
09-21 02:29:43.649 5617-5617/com.example.gerenciadorestoqueoficial E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.gerenciadorestoqueoficial, PID: 5617
    java.lang.NullPointerException: Attempt to invoke virtual method 'long com.example.gerenciadorestoqueoficial.DbHelper.adicionarUsuario(java.lang.String, java.lang.String)' on a null object reference
        at com.example.gerenciadorestoqueoficial.Cadastro$2.onClick(Cadastro.java:53)
        at android.view.View.performClick(View.java:4780)
        at android.view.View$PerformClick.run(View.java:19866)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5254)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
09-21 02:29:49.585 5617-5637/com.example.gerenciadorestoqueoficial D/EGL_emulation: eglMakeCurrent: 0xaec34ac0: ver 2 0 (tinfo 0xaec393a0)
09-21 02:34:43.736 5617-5617/com.example.gerenciadorestoqueoficial I/Process: Sending signal. PID: 5617 SIG: 9

Answer 1

You must have to initializes Your DbHelper db in onCreate() method.

DbHelper db =new DbHelper (this);

or

DbHelper db =new DbHelper (getContext());