我试图按照练习在Android上创建一个简单的待办事项列表应用程序,但是当我运行项目时我得到了NullPointerException。我可以从LogCat看到我的代码中的第46行引起了问题,但我不明白为什么,这里' java文件:
package com.example.todolist;
//import android.R;
import java.util.ArrayList;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v7.app.ActionBarActivity;
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.widget.ArrayAdapter;
import android.widget.EditText;
import android.widget.ListView;
public class ToDoListActivity extends ActionBarActivity {
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//inflate the view
setContentView(R.layout.activity_to_do_list);
//get references to UI widgets
ListView myListView = (ListView)findViewById(R.id.myListView1);
final EditText myEditText = (EditText)findViewById(R.id.myEditText); //You are always allowed to initialize a final variable. The compiler makes sure that you can do it only once
//Create the Array List of to do items
final ArrayList<String> todoItems = new ArrayList<String>();
//Create the Array Adaptor 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);
//simple_list_item_1 is a reference to an built-in XML layout document that is part of the Android OS
//below is causing nullpointer exception
//Bind the ArrayAdapter to the list view
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) {
getSupportFragmentManager().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.to_do_list, 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_to_do_list,
container, false);
return rootView;
}
}
}
以下是2个XML布局文件,首先是activity_to_do_list.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="com.example.todolist.ToDoListActivity"
tools:ignore="MergeRootFrame" />
和fragment_to_do_list.xml:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
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"
/>
<ListView
android:id="@+id/myListView1"
android:layout_width="match_parent"
android:layout_height="wrap_content"
/>
</LinearLayout>
这是myListView.setAdapter(aa)行;这导致空指针异常,但我无法理解为什么因为aa已被实例化,据我所见。有人可以帮忙吗?
答案 0 :(得分:0)
您需要将大部分逻辑从活动的onCreate()
方法移动到片段的onCreateView()
。这是因为在您尝试在活动中找到ListView
时,片段的视图尚未膨胀。