java - 空指针异常

Question

我快要疯了，试图弄清楚这个问题。当我单击我的 Activity 时，它会因 NullPointerException 崩溃。我已经尝试过调试，但无法进入有问题的 Activity 。我已经尝试删除数据库，仍然崩溃。我尝试删除ListView，仍然崩溃。我尝试修改数据库，仍然崩溃。

出于某种奇怪的原因，当我删除“播放按钮”上的按钮及其 onClickListener 时，我能够进入该 Activity ，并且该 Activity 中的所有内容都有效。有人可以帮助我解释为什么此按钮会导致应用程序崩溃吗？ Activity 而不是 onclick 上？

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)

Activity

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();
                }
            });





    }

    }

玩 friend 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"
    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

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical" >

    <FrameLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:padding="5dp"
        android:background="@drawable/searchimageview"
        android:orientation="vertical" >

        <Button
        android:id="@+id/playbutton"
        android:layout_width="60dp"
        android:layout_height="60dp"
        android:layout_gravity="right"
        android:background="@drawable/playbutton" />

<TextView 
    android:id="@+id/textview_friends"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:textColor="@color/black"
    android:textStyle="bold"
    android:textSize="40dp"
    android:layout_gravity="center" />


    </FrameLayout>



</LinearLayout>

Answer 1

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。