Error adding Textview by programming

Question

I’m having an error in creating Textviews by programming the application 'crasha' when arriving on the screen

Mainactivity.java

 package com.example.matheus.login;

import android.content.Intent;
import android.os.Bundle;
import android.support.design.widget.NavigationView;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.LinearLayout;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener {
    String login;
    String senha;
    String nome;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        Bundle extras = getIntent().getExtras();
        if(extras != null)
        {
            nome = extras.getString("nome");
        }



        super.onCreate(savedInstanceState);
        setContentView(R.layout.content_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.setDrawerListener(toggle);
        toggle.syncState();

        NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);
        TextView text = (TextView)navigationView.getHeaderView(0).findViewById(R.id.nome);
        text.setText(nome);

        String[] textArray = {"One", "Two", "Three", "Four"};
        DrawerLayout mFrame = (DrawerLayout) findViewById(R.id.container);
        for( int i = 0; i < textArray.length; i++ )
        {
            TextView textView = new TextView(this);
            textView.setText(String.valueOf(textArray[i]));
            mFrame.addView(textView);
        }
    }

    @Override
    public void onBackPressed() {
        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START);
        } else {
            super.onBackPressed();
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.

        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement


        return super.onOptionsItemSelected(item);
    }

    @SuppressWarnings("StatementWithEmptyBody")
    @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        // Handle navigation view item clicks here.
        int id = item.getItemId();

        if (id == R.id.nav_camera) {
            Intent intent = new Intent(MainActivity.this, MainActivity.class);
            intent.putExtra("nome", nome);
            MainActivity.this.startActivity(intent);
        } else if (id == R.id.nav_gallery) {
            Intent intent = new Intent(MainActivity.this, fechadas.class);
            intent.putExtra("nome", nome);
            MainActivity.this.startActivity(intent);
        } else if (id == R.id.nav_share) {
            Intent intent = new Intent(MainActivity.this, TelaLogin.class);

            MainActivity.this.startActivity(intent);
        }

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        drawer.closeDrawer(GravityCompat.START);
        return true;
    }
}

content_main.xml(Navigationview)

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout 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"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    tools:context="com.example.matheus.login.MainActivity"
    tools:showIn="@layout/app_bar_main">

    <LinearLayout
        android:id="@+id/container"
        android:layout_width="368dp"
        android:layout_height="495dp"
        tools:layout_editor_absoluteY="8dp"
        tools:layout_editor_absoluteX="8dp"
        android:orientation="vertical">


    </LinearLayout>
</android.support.v4.widget.DrawerLayout>

Crash error

     FATAL EXCEPTION: main
    Process: com.example.rohwedder.login, PID: 15173
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.rohwedder.login/com.example.matheus.login.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.LinearLayout.addView(android.view.View)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2665)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
    at android.app.ActivityThread.-wrap12(ActivityThread.java)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6119)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
    Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.LinearLayout.addView(android.view.View)' on a null object reference
    at com.example.matheus.login.MainActivity.onCreate(MainActivity.java:37)
    at android.app.Activity.performCreate(Activity.java:6679)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2618)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) 
    at android.app.ActivityThread.-wrap12(ActivityThread.java) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:154) 
    at android.app.ActivityThread.main(ActivityThread.java:6119) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 

at com.example.matheus.login.MainActivity.onCreate(MainActivity.java:37), refers to this line Frame.addView(textView);

Answer 1

You cannot define that name Frame with F uppercase, try another name, for example:

LinearLayout mFrame = (LinearLayout) findViewById(R.id.container);

And then:

 mFrame.addView(textView);

Answer 2

Try changing this line

DrawerLayout mFrame = (DrawerLayout) findViewById(R.id.container);

for this:

LinearLayout mFrame = (LinearLayout) findViewById(R.id.container);