App is crashing before opening

Question

I'm working on an app and I've been working on a "post" service where you can write a post and display it on a feed screen made of listview. Now everything seem to be fine but I don't know why it's crashing before even starting. Here is the code:

Post class:

package com.example.ali.test1;

import android.app.Activity;
import android.content.Intent;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.text.Editable;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.EditText;
import android.widget.ListView;

import java.util.ArrayList;


public class Post extends ActionBarActivity {
        EditText input;

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

        // android:id="@+id/input"
        input = (EditText) findViewById(R.id.input);
        // android:id="@+id/list"


        startActivityForResult(new Intent(Post.this , MainActivity.class), 1);
    }


    public void addToList(View v) {
        Editable text = input.getText();
        Intent intent = new Intent();
        intent.putExtra("result", text);
        setResult(Activity.RESULT_OK, intent);
        finish();
    }

    }

in this class I have an edit text and abutton once the button is pressed I'm supposed to get the text in edit text and post in the listView of the other class

Main:

package com.example.ali.test1;

import android.app.Activity;
import android.content.Intent;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ArrayAdapter;
import android.widget.ListView;

import java.util.ArrayList;


public class MainActivity extends ActionBarActivity   {


    ListView list;
    ArrayAdapter<String> adapter;

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

        adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, new ArrayList<String>());
        list.setAdapter(adapter);

    }

    protected void onActivityResult(int requestCode, int resultCode, Intent data) {


            if (requestCode == 1) if (resultCode == Activity.RESULT_OK) {
                String result = data.getStringExtra("result");
                adapter.add(result);
                adapter.notifyDataSetChanged();



            }
        }



    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();
        int id2 = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        else if (id2 == R.id.action_cart) {
            startActivity(new Intent(MainActivity.this, Post.class));
        }
        return super.onOptionsItemSelected(item);
    }
}

And the xml files are:

Post:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context="com.example.ali.test1.Post">


    <EditText
        android:id="@+id/input"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
       />
    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Add to list"
        android:onClick="addToList"
        android:layout_below="@id/input"/>


</RelativeLayout>

Main:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context="com.example.ali.test1.MainActivity">



    <ListView
    android:id="@+id/list1"
    android:layout_width="match_parent"
    android:layout_height="200dp"
    android:layout_weight="1" />


</RelativeLayout>

any help would be appreciated I've been stuck here for too long. Thank You.

Answer 1

First, sorry for my english.

There is problem with android app going to background, where OS release memory for other functions on OS or other application, when it need's memory. There release your object and you have NullPointerException on resume.

How fix this? OnResume event check object value and if it is null, create object.

How test this? Kill process in Android Device Monitor

Answer 2

You are trying to add adapter to a listview which is null, that's why you must be getting NullPointerException. You need to initialize it before you can use it.

You need to add

list= (ListView) findViewById(R.id.mylistview); // replace R.id.mylistview with the id of your listview in layout activity_main

after

setContentView(R.layout.activity_main);

in onCreate method of MainActivity

Answer 3

Change your MainActivity .Because you not initialize Listview properly.

 ListView list;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    list= (ListView) findViewById(R.id.list1);
    adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, new ArrayList<String>());
    list.setAdapter(adapter);

}