I'm new to android.Please look through this. When I click send, the app stops

Question

I was learning from this. Idk what's wrong but it shows "Your app has stopped" as soon as I click send. Please tell me what's wrong. All i did was just copy their lines and this is not posting unless I type out more info so I think this is enough to make it posted. If I don't type and meaningless thing after this, this means this has posted. Thanks

Main Java code:

package gangster.doc;
import android.content.Intent;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.EditText;

public class Main extends AppCompatActivity {
public final static String EXTRA_MESSAGE = "lolsachi.me";

// Called when the user clicks the Send button
public void sendMessage(View view) {
    // Do something in response to button
    Intent intent = new Intent(this, DisplayMessageActivity.class);
    EditText editText = (EditText) findViewById(R.id.edit_message);
    String message = editText.getText().toString();
    intent.putExtra(EXTRA_MESSAGE, message);
    startActivity(intent);
}

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

    FloatingActionButton fab = (FloatingActionButton) 
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();
        }
    });
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    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
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}
} 

2nd Activity file:

package gangster.doc;
 import android.content.Intent;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.widget.RelativeLayout;
import android.widget.TextView;

  public class DisplayMessageActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    Intent intent = getIntent();
    String message = intent.getStringExtra(Main.EXTRA_MESSAGE);
    TextView textView = new TextView(this);
    textView.setTextSize(40);
    textView.setText(message);
    RelativeLayout layout = (RelativeLayout) findViewById(R.id.content);
    layout.addView(textView);
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_display_message);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    FloatingActionButton fab = (FloatingActionButton) 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();
        }
    });
    getSupportActionBar().setDisplayHomeAsUpEnabled(true);
}

}

content_main.xml file :

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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:orientation="horizontal"
android:layout_width="match_parent"
android:layout_height="match_parent"
app:layout_behavior="@string/appbar_scrolling_view_behavior"
tools:showIn="@layout/activity_main"
android:layout_weight="1">

<EditText
    android:layout_weight="1"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:id="@+id/edit_message"
    android:hint="@string/edit_message"/>
<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/button_send"
    android:onClick="sendMessage"/>


</LinearLayout>

Content_display_message file :

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
app:layout_behavior="@string/appbar_scrolling_view_behavior"
tools:context="gangster.doc.DisplayMessageActivity"
tools:showIn="@layout/activity_display_message"
android:id="@+id/content">

</RelativeLayout>

Answer 1

Since you are a newbie let me explain the issue that you are trying to initialize a view which is yet not inflated.

 RelativeLayout layout = (RelativeLayout) findViewById(R.id.content);

The view associated with R.id.content is in the activity_display_message.xml. So in order to use any view from activity_display_message.xml , you have to inflate activity_display_message.xml in your activity first using -

setContentView(R.layout.activity_display_message);

Also,

super.onCreate(savedInstanceState);

should be the very first statement in onCreate() of any Activity.

Answer 2

You just have to change some lines

public class DisplayMessageActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_display_message);
    Intent intent = getIntent();
    String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);
    TextView textView = new TextView(DisplayMessageActivity.this);
    textView.setTextSize(40);
    textView.setText(message);
    RelativeLayout layout = (RelativeLayout) findViewById(R.id.content);
    layout.addView(textView);

    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    FloatingActionButton fab = (FloatingActionButton) 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();
        }
    });
    getSupportActionBar().setDisplayHomeAsUpEnabled(true);
}

}

Answer 3

Edit your code :

super.onCreate(savedInstanceState);
setContentView(R.layout.activity_display_message);

should be right after

@Override
protected void onCreate(Bundle savedInstanceState) {