Android - crash on startup

Question

why my app crashes ll the time on startup? Im done

Main_Activity

import android.app.Activity;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBar;
import android.support.v7.app.AppCompatActivity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import java.io.IOException;
import java.net.InetAddress;
import java.net.UnknownHostException;


public class MainActivity extends AppCompatActivity
    implements NavigationDrawerFragment.NavigationDrawerCallbacks {

/**
 * Fragment managing the behaviors, interactions and presentation of the navigation drawer.
 */
private NavigationDrawerFragment mNavigationDrawerFragment;

/**
 * Used to store the last screen title. For use in {@link #restoreActionBar()}.
 */
private CharSequence mTitle;

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

    mNavigationDrawerFragment = (NavigationDrawerFragment)
            getSupportFragmentManager().findFragmentById(R.id.navigation_drawer);
    mTitle = getTitle();

    // Set up the drawer.
    mNavigationDrawerFragment.setUp(
            R.id.navigation_drawer,
            (DrawerLayout) findViewById(R.id.drawer_layout));





    //SERVER1

    boolean reachable = false;

    try {
        reachable = InetAddress.getByName("www.google.com").isReachable(2000);
    } catch (UnknownHostException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
        TextView view = (TextView) findViewById(R.id.textView2);
        view.setText("server down");
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
        TextView view = (TextView) findViewById(R.id.textView2);
        view.setText("server down");
    }

    if(reachable == true){
        TextView view = (TextView) findViewById(R.id.textView2);
        view.setText("server works");
    } else{

    }


    //SERVER2
    boolean reachable2 = false;


    try {
        reachable2 = InetAddress.getByName("www.wikipedia.com").isReachable(2000);
    } catch (UnknownHostException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
        TextView view = (TextView) findViewById(R.id.textView3);
        view.setText("server down");
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
        TextView view = (TextView) findViewById(R.id.textView3);
        view.setText("server down");
    }

    if(reachable2 == true){
        TextView view = (TextView) findViewById(R.id.textView3);
        view.setText("server works");
    } else{

    }


}

@Override
public void onNavigationDrawerItemSelected(int position) {

    Fragment myFragment = null;

    switch (position) {

        case 0:
            myFragment = new First_Fragment();
            break;

        case 1:
            myFragment = new Second_Fragment();
            break;

        case 2:
            myFragment = new Third_Fragment();
            break;
    }
    // update the main content by replacing fragments
    FragmentManager fragmentManager = getSupportFragmentManager();
    fragmentManager.beginTransaction()
            .replace(R.id.container, myFragment)
            .commit();
}

public void onSectionAttached(int number) {
    switch (number) {
        case 1:
            mTitle = getString(R.string.title_section1);
            break;
        case 2:
            mTitle = getString(R.string.title_section2);
            break;
        case 3:
            mTitle = getString(R.string.title_section3);
            break;
    }
}

public void restoreActionBar() {
    ActionBar actionBar = getSupportActionBar();
    actionBar.setDisplayShowTitleEnabled(true);
    actionBar.setTitle(mTitle);
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    if (!mNavigationDrawerFragment.isDrawerOpen()) {
        // Only show items in the action bar relevant to this screen
        // if the drawer is not showing. Otherwise, let the drawer
        // decide what to show in the action bar.
        getMenuInflater().inflate(R.menu.main, menu);
        restoreActionBar();
        return true;
    }
    return super.onCreateOptionsMenu(menu);
}

@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);
}

/**
 * A placeholder fragment containing a simple view.
 */
public static class PlaceholderFragment extends Fragment {
    /**
     * The fragment argument representing the section number for this
     * fragment.
     */
    private static final String ARG_SECTION_NUMBER = "section_number";

    /**
     * Returns a new instance of this fragment for the given section
     * number.
     */
    public static PlaceholderFragment newInstance(int sectionNumber) {
        PlaceholderFragment fragment = new PlaceholderFragment();
        Bundle args = new Bundle();
        args.putInt(ARG_SECTION_NUMBER, sectionNumber);
        fragment.setArguments(args);
        return fragment;
    }

    public PlaceholderFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container, false);
        return rootView;
    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        ((MainActivity) activity).onSectionAttached(
                getArguments().getInt(ARG_SECTION_NUMBER));
    }
}

}

first_layout

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:textAppearance="?android:attr/textAppearanceLarge"
    android:text="Large Text"
    android:id="@+id/textView2"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="107dp" />

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:textAppearance="?android:attr/textAppearanceLarge"
    android:text="Large Text"
    android:id="@+id/textView3"
    android:layout_centerVertical="true"
    android:layout_alignLeft="@+id/textView2"
    android:layout_alignStart="@+id/textView2" />

</RelativeLayout>

First_Fragment and Second_Fragment and Third_Fragment

import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;



public class First_Fragment extends Fragment {

View myView;

@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    myView = inflater.inflate(R.layout.first_layout, container, false);
    return myView;
}

}

It crashes every time on startup. I lost 2 hours of googling and chceking my code. Any solution? Im sorry if its noob mistake,I am beginner. Thank you!

Manifest

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="test.testapp">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme">
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

    </application>

</manifest>

LOGCAT

http://pastebin.com/f1vwCTST - logcat is here

Answer 1

User has not posted any stackTrace, but from my own experience I can tell that he must be facing the Null Pointer Exception at

((MainActivity) activity).onSectionAttached(getArguments().getInt(ARG_SECTION_NUMBER));

This is happening because getArguments is null. Question asker should attach the stackTrace as it would strengthen my claim. More experience person should look into it or should explain this behaviour.

Please change your code snippet From:

public static PlaceholderFragment newInstance(int sectionNumber) {
    PlaceholderFragment fragment = new PlaceholderFragment();
    Bundle args = new Bundle();
    args.putInt(ARG_SECTION_NUMBER, sectionNumber);
    fragment.setArguments(args);
    return fragment;
}

To

public static PlaceholderFragment newInstance(int sectionNumber) {

    Bundle args = new Bundle();
    args.putInt(ARG_SECTION_NUMBER, sectionNumber);
    PlaceholderFragment fragment = new PlaceholderFragment();
    fragment.setArguments(args);
    return fragment;
}

You can refer this link for more information: Android Fragment

IMPORTANT IMPORTANT Anyone who has downvoted this answer please check the above link or read the answer below.

I am posting my answer from that link

Adding to the answer of ρяσѕρєя K . While setting the argument one should maintain the sequence of execution of statements. For example in the answer sequence is:

Bundle bundle = new Bundle();
bundle.putString("edttext", "From Activity");
// set Fragmentclass Arguments
Fragmentclass fragobj = new Fragmentclass();
fragobj.setArguments(bundle);

But if you try to allocate Fragment first and then allocate an Bundle object and try to set it as argument to the allocated fragment, it might sometime be taken as null in onCreateView method inside the fragment. For example by using below code segment sometimes the getArguments inside onCreateView might return null.

Error prone code segment:

Fragmentclass fragobj = new Fragmentclass();
Bundle bundle = new Bundle();
bundle.putString("edttext", "From Activity");
// set Fragmentclass Arguments
fragobj.setArguments(bundle);

I was facing the same problem for quite some time but when I switched the order of statements my problem was solved. It would be great if someone could give a solid reason for this behaviour. Most probably it depends upon execution time. More experienced person should give reason behind it.