Java 空指针异常
我正在疯狂地尝试找出这个问题。当我单击我的活动时,它会崩溃,出现NullPointerException。我尝试过调试,但无法进入相关活动。我尝试过删除数据库,但仍然崩溃。我尝试过删除ListView,仍然崩溃。我尝试过修改数据库,仍然崩溃 出于某种奇怪的原因,当我在“playbutton”上移除按钮及其onClickListener时,我能够进入活动,活动中的一切都正常工作。有人能帮我解释一下为什么这个按钮会使应用程序在活动之前崩溃,而不是在onclick上崩溃吗Java 空指针异常,java,android,android-listview,nullpointerexception,Java,Android,Android Listview,Nullpointerexception,我正在疯狂地尝试找出这个问题。当我单击我的活动时,它会崩溃,出现NullPointerException。我尝试过调试,但无法进入相关活动。我尝试过删除数据库,但仍然崩溃。我尝试过删除ListView,仍然崩溃。我尝试过修改数据库,仍然崩溃 出于某种奇怪的原因,当我在“playbutton”上移除按钮及其onClickListener时,我能够进入活动,活动中的一切都正常工作。有人能帮我解释一下为什么这个按钮会使应用程序在活动之前崩溃,而不是在onclick上崩溃吗 04-05 23:15:15
04-05 23:15:15.886: E/AndroidRuntime(11445): FATAL EXCEPTION: main
04-05 23:15:15.886: E/AndroidRuntime(11445): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.fullfrontalgames.numberfighter/com.fullfrontalgames.numberfighter.PlayAFriend}: java.lang.NullPointerException
04-05 23:15:15.886: E/AndroidRuntime(11445): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2136)
04-05 23:15:15.886: E/AndroidRuntime(11445): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2174)
04-05 23:15:15.886: E/AndroidRuntime(11445): at android.app.ActivityThread.access$700(ActivityThread.java:141)
04-05 23:15:15.886: E/AndroidRuntime(11445): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1267)
04-05 23:15:15.886: E/AndroidRuntime(11445): at android.os.Handler.dispatchMessage(Handler.java:99)
04-05 23:15:15.886: E/AndroidRuntime(11445): at android.os.Looper.loop(Looper.java:137)
04-05 23:15:15.886: E/AndroidRuntime(11445): at android.app.ActivityThread.main(ActivityThread.java:5059)
04-05 23:15:15.886: E/AndroidRuntime(11445): at java.lang.reflect.Method.invokeNative(Native Method)
04-05 23:15:15.886: E/AndroidRuntime(11445): at java.lang.reflect.Method.invoke(Method.java:511)
04-05 23:15:15.886: E/AndroidRuntime(11445): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:792)
04-05 23:15:15.886: E/AndroidRuntime(11445): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:555)
04-05 23:15:15.886: E/AndroidRuntime(11445): at dalvik.system.NativeStart.main(Native Method)
04-05 23:15:15.886: E/AndroidRuntime(11445): Caused by: java.lang.NullPointerException
04-05 23:15:15.886: E/AndroidRuntime(11445): at com.fullfrontalgames.numberfighter.PlayAFriend.onCreate(PlayAFriend.java:54)
04-05 23:15:15.886: E/AndroidRuntime(11445): at android.app.Activity.performCreate(Activity.java:5058)
04-05 23:15:15.886: E/AndroidRuntime(11445): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
04-05 23:15:15.886: E/AndroidRuntime(11445): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2100)
活动
package com.fullfrontalgames.numberfighter;
import com.fullfrontalgames.numberfighter.DBAdapter;
import com.fullfrontalgames.numberfighter.R;
import android.app.Activity;
import android.content.Intent;
import android.database.Cursor;
import android.os.Bundle;
import android.support.v4.widget.SimpleCursorAdapter;
import android.view.View;
import android.widget.Button;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.TextView;
public class PlayAFriend extends Activity{
DBAdapter DBAdapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.playafriend);
final DBAdapter db = new DBAdapter(this);
DBAdapter = db.open();
ListView FriendLV = (ListView) findViewById(android.R.id.list);
Button playbutton = (Button) findViewById(R.id.playbutton);
final TextView friend = (TextView) findViewById(R.id.textview_friends);
Cursor friendslist = db.GetAllFriends();
String[] from = new String[] {"FRIENDS"}; // your column/columns here
int[] to = new int[] {R.id.textview_friends};
@SuppressWarnings("deprecation")
ListAdapter cursorAdapter = new SimpleCursorAdapter(this, R.layout.list_items, friendslist, from, to,0);
FriendLV.setAdapter(cursorAdapter);
playbutton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
String Defender = friend.getText().toString();
db.GetFriend(Defender);
startActivity(new Intent ("com.fullfrontalgames.numberfighter.Fightattacker"));
db.close();
}
});
}
}
PlayafriendXML
<?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"
android:background="@drawable/nfbackground"
android:orientation="vertical" >
<ImageView
android:id="@id/titlebar"
android:layout_width="wrap_content"
android:layout_height="66dp"
android:src="@drawable/nftitlebar" />
<ListView
android:id="@android:id/list"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical" >
</ListView>
</LinearLayout>
列表项xml
请尝试此代码
public class Test extends Activity{
DBAdapter DBAdapter;
ListView FriendLV;
Button playbutton;
final TextView friend;
@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.playafriend);
final DBAdapter db = new DBAdapter(this);
db.open();
Cursor friendslist = db.GetAllFriends();
if(friendslist.getCount()!=0){
FriendLV = (ListView) findViewById(android.R.id.list);
playbutton = (Button) findViewById(R.id.playbutton);
friend = (TextView) findViewById(R.id.textview_friends);
String[] from = new String[] {"FRIENDS"}; // your column/columns here
int[] to = new int[] {R.id.textview_friends};
@SuppressWarnings("deprecation")
ListAdapter cursorAdapter = new SimpleCursorAdapter(this, R.layout.list_items, friendslist, from, to,0);
FriendLV.setAdapter(cursorAdapter);
playbutton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
String Defender = friend.getText().toString();
db.GetFriend(Defender);
startActivity(new Intent ("com.fullfrontalgames.numberfighter.Fightattacker"));
db.close();
}
});
}
}
}
如果您遇到任何问题,请告诉我。如果使用listview,您的活动应该扩展ListActivity请添加完整的stacktrace。您可能不应该让这些行DBAdapter DBAdapter;DBAdapter=db.open();而是db.open();应该足够了。无论如何,friend或db都是null。请用错误消息发布logcat。至少我想说我们已经回答了您的空指针问题。如果您的
ListView
在布局文件中,那么您应该将其id定义为@android:id/list
,您应该在java文件中通过将其初始化为ListView FriendLV=(ListView)findViewById来访问它(R.id.list);
。这样试试吧。我的建议如下:让你的活动成为一个列表活动;按照Grishu说的做:ListView FriendLV=(ListView)findViewById(R.id.list);在playafriend布局中添加一个。在线性布局下将playafriend布局中添加PlayaButton(从列表项布局中删除);向列表中添加一个McClickListener,并在其中添加代码以管理单击项目列表(因此您可能不需要播放按钮)。如果你想让播放按钮出现在每个列表项中,那么我建议你在谷歌上搜索可点击的列表项。我的空点已解决,但我尝试了此代码按钮不起作用。我只需要对此采取不同的方法,或者改为制作一个新的监听器,然后放弃播放按钮。不,这没有什么区别。播放按钮显示出来了除了我在列表下面的那一个之外,我没有再点击他们。实际上你必须面对什么问题?这个问题的标题是关于一个空点异常,我不知道,但是从问题的结果来看,空点已经解决了,现在我没有一个按钮可以在这个列表视图设置上使用,因为这导致了nullpoint.在您的listview中,如果每一行都包含按钮,那么您必须在android中创建自定义适配器。如下所示。
public class Test extends Activity{
DBAdapter DBAdapter;
ListView FriendLV;
Button playbutton;
final TextView friend;
@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.playafriend);
final DBAdapter db = new DBAdapter(this);
db.open();
Cursor friendslist = db.GetAllFriends();
if(friendslist.getCount()!=0){
FriendLV = (ListView) findViewById(android.R.id.list);
playbutton = (Button) findViewById(R.id.playbutton);
friend = (TextView) findViewById(R.id.textview_friends);
String[] from = new String[] {"FRIENDS"}; // your column/columns here
int[] to = new int[] {R.id.textview_friends};
@SuppressWarnings("deprecation")
ListAdapter cursorAdapter = new SimpleCursorAdapter(this, R.layout.list_items, friendslist, from, to,0);
FriendLV.setAdapter(cursorAdapter);
playbutton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
String Defender = friend.getText().toString();
db.GetFriend(Defender);
startActivity(new Intent ("com.fullfrontalgames.numberfighter.Fightattacker"));
db.close();
}
});
}
}
}