不幸的是，“App”已停止Android Studio

Question

I am new to developing android. And I got after writing my code and running the debugger: "Unfortunately App has stopped"

my MainActivity.java

package instaroggie.nl.content;

import android.app.Activity;
import android.app.ActionBar;
import android.app.Fragment;
import android.os.Bundle;
import android.view.KeyEvent;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.os.Build;
import android.widget.ArrayAdapter;
import android.widget.EditText;
import android.widget.ListView;

import java.util.ArrayList;
import java.util.List;

public class MainActivity extends Activity {

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

    ListView myListView = (ListView) findViewById(R.id.myListView);
    final EditText myEditText = (EditText) findViewById(R.id.myEditText);

    //create the Array List of to do items
    final ArrayList<String> todoItems = new ArrayList<String>();

    //create the Array Adapter to bind the array to the List View
    final ArrayAdapter<String> aa;

    aa = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, todoItems);
    myListView.setAdapter(aa);

    myEditText.setOnKeyListener(new View.OnKeyListener() {
        public boolean onKey(View v, int keyCode, KeyEvent event) {
            if (event.getAction() == KeyEvent.ACTION_DOWN)
               if ((keyCode == KeyEvent.KEYCODE_DPAD_CENTER) ||
                        (keyCode == KeyEvent.KEYCODE_ENTER)) {
                    todoItems.add(0, myEditText.getText().toString());
                   aa.notifyDataSetChanged();
                   myEditText.setText("");
                   return true;
                }
            return false;
        }
    });

    if (savedInstanceState == null) {
        getFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {

    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.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();
    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 {

    public PlaceholderFragment() {
    }

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

}

My activity_main.xml:

 <FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    tools:ignore="MergeRootFrame" />

and my fragment_main.xml:

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

<EditText
    android:id="@+id/myEditText"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:hint="@string/addItemHint"
    android:contentDescription="@string/addItemContentDescription"/>

<ListView
    android:id="@+id/myListView"
    android:layout_width="match_parent"
    android:layout_height="wrap_content" />

</LinearLayout>

Does anyone know where my problem is?

Answer 1

setContentView(R.layout.activity_main);
ListView myListView = (ListView) findViewById(R.id.myListView);

activity_main.xml does not have listview with id myListView . findviewById looks for a view with the id mentioned in the current inflated layout. If your activity_main.xml does not have listview with id myListView and you try to initialize ListView in activity you get NPE.

<ListView
android:id="@+id/myListView" // is in fragment_main.xml

Try

FragmentTransaction fragTran = getSupportFragmentManager().beginTransaction();
fragTran.replace(R.id.containere, new PlaceholderFragment());
fragTran.commit();

Inflate fragment_main.xml and initialize your views in onCreateView of fragment.

ListView myListView;
EditText myEditText;
@Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container, false);
        myListView = (ListView) rootView.findViewById(R.id.myListView);
        myEditText = (EditText) rootView.findViewById(R.id.myEditText);
        ...// rest of the code
        return rootView;
    }