java.lang.Nullpointerexception: Attempt to invoke virtual method

Question

My program gave this problem when I tried to create a stack (database), to store strings on my Android app

package com.project.meuapp2;

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


public class Principal extends AppCompatActivity {
    private Button btn;
    private EditText texto;
    BancodeDados DB;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_principal);
        texto = (EditText) findViewById(R.id.texto);
        btn = (Button) findViewById(R.id.btn);
        texto.setText("Insira um dado");
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String y = texto.getText().toString();
                DB.InsereInicio(y);
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.main, menu);
        return super.onCreateOptionsMenu(menu);
    }

    @Override
    public boolean onOptionsItemSelected (MenuItem item){
        Intent i = null;

        switch(item.getItemId()) {
            case R.id.mnTela1:
                i = new Intent(this, pagina2.class);
                Toast.makeText(this, "item1", Toast.LENGTH_LONG).show();
                startActivity(i);
                break;
            case R.id.mnTela2:
                Toast.makeText(this, "item2", Toast.LENGTH_LONG).show();
                break;
        }
        return super.onOptionsItemSelected(item);
    }
}

package com.project.meuapp2;

public class BancodeDados {
    elemento primeiro = new elemento();
    elemento ultimo = new elemento();

    public void InsereInicio(String x){
        elemento novodadoinicio = new elemento();
        novodadoinicio.dado = x;
        novodadoinicio.proximo = primeiro;
        if(primeiro==null){
            ultimo = novodadoinicio;
        }
        primeiro = novodadoinicio;
    }

    public void InsereFinal(String x){
        elemento novodadoultimo = new elemento();
        novodadoultimo.dado = x;
        novodadoultimo.proximo = primeiro;
        if(ultimo==null){
            primeiro = novodadoultimo;
        }
        ultimo = novodadoultimo;
    }

    public String removeInicio(){
        String x;
        x = primeiro.dado;
        primeiro = primeiro.proximo;
        return x;
    }
}

Log of Android Studio

03-27 18:14:28.017 5239-5239/com.project.meuapp2 E/Androidruntime: FATAL EXCEPTION: main Process: com.project.meuapp2, PID: 5239 java.lang.Nullpointerexception: Attempt to invoke virtual method 'void com.project.meuapp2.BancodeDados.Insereinicio(java.lang.String)' on a null Object Reference at com.project.meuapp2.Home$1.onClick(Home.java:30) at android.view.View.performClick(View.java:5198) at android.view.View$Performclick.run(View.java:21147) at android.os.Handler.handleCallback(Handler.java:739) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:148) at android.app.Activitythread.main(Activitythread.java:5417) at java.lang.reflect.Method.invoke(Native Method) at com.android.Internal.os.Zygoteinit$Methodandargscaller.run(Zygoteinit.java:726) at com.android.Internal.os.Zygoteinit.main(Zygoteinit.java:616)

Note: I’ve tried initializing the variable primeiro, only that it accuses the same error.

Answer 1

Possibly this solves the specific problem (there may still be others):

public class Principal extends AppCompatActivity {
    private Button btn;
    private EditText texto;
    BancodeDados DB = new BancodeDados(); // <===================== aqui

I put in the Github for future reference.

You need to have an instance created in order to access your members. Only declaring the variable does not help. Initialize solves this exception but it may be that the instance needs to be configured appropriately to work as desired.