这是我的Java。
当我尝试运行它时，给我一个空指针异常
在以下行：
back.setOnClickListener（new View.OnClickListener（）
我调试了，确实Button“返回”为空。
但为什么？
修改
它在Gingerbread手机上运行，而不是在Nexus 7上运行
public class QuotesActivity extends Activity implements MyInterface {
private String url2;
public static ArrayList<Stock> lt;
public static ArrayList<Stock> favQ;
public MyAdapter myAdap;
public Intent intent;
public ListView lv;
public FavManager fm;
private AdView adView;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.search);
AdRequest request = new AdRequest();
// request.addTestDevice(AdRequest.TEST_EMULATOR);
request.setGender(AdRequest.Gender.MALE);
boolean tabletSize = getResources().getBoolean(R.bool.isTablet);
if (tabletSize) {
// Create the adView
adView = new AdView(this, AdSize.SMART_BANNER, "dgfdfgdfgdfgd");
} else {
// Create the adView
adView = new AdView(this, AdSize.BANNER, "sfsdfsdfsdfsfs");
}
// Lookup your LinearLayout assuming it's been given
// the attribute android:id="@+id/mainLayout"
LinearLayout layout = (LinearLayout) findViewById(R.id.ad);
// Add the adView to it
layout.addView(adView);
// Initiate a generic request to load it with an ad
adView.loadAd(request);
lt = new ArrayList<Stock>();
favQ = new ArrayList<Stock>();
fm = new FavManager(getApplicationContext());
favQ.clear();
favQ.addAll(fm.ritiraFav());
PreferenceManager.setDefaultValues(this, R.xml.preferences, false);
lt.clear();
// ////////////
try {
InputStream in = getResources().openRawResource(R.raw.markets3);
if (in != null) {
InputStreamReader tmp = new InputStreamReader(in);
BufferedReader reader = new BufferedReader(tmp);
String str;
String str1 = null;
String str2 = null;
boolean flag = true;
while ((str = reader.readLine()) != null) {
if (flag) {
str1 = str;
flag = false;
} else {
str2 = str;
lt.add(new Stock(str1, str2, "0", "0"));
flag = true;
}
}
in.close();
}
}
catch (Throwable t) {
// Log.e("", "Exception: " + t.toString());
}
// ///////
Collections.sort(lt, new Comparator<Stock>() {
public int compare(Stock s1, Stock s2) {
return s1.nome.compareToIgnoreCase(s2.nome);
}
});
lv = (ListView) findViewById(android.R.id.list);
lv.setScrollbarFadingEnabled(false);
Button favourites = (Button) findViewById(R.id.favoritesbtn);
favourites.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent(QuotesActivity.this,
QuotesActivityFavourites.class);
startActivity(intent);
}
});
Button back = (Button) findViewById(R.id.buttonFinish);
back.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
finish();
}
});
这是XML：
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >
<RelativeLayout
android:id="@+id/RequestFocusLayout"
android:layout_width="0dp"
android:layout_height="0dp"
android:focusable="true"
android:focusableInTouchMode="true" />
<LinearLayout
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:background="@drawable/gradient_box"
android:orientation="vertical"
android:textColor="#FFFFFF" >
<RelativeLayout
android:layout_width="fill_parent"
android:layout_height="20dip" >
<TextView
android:id="@+id/lista"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerInParent="true"
android:text="All stocks on US Markets"
android:textSize="15dip" />
</RelativeLayout>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="50dp"
android:orientation="horizontal" >
<TextView
android:id="@+id/nome"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_gravity="center_vertical"
android:layout_weight="2"
android:paddingLeft="5dp"
android:text="Name"
android:textColor="@android:color/white"
android:textSize="10sp" />
<Button
android:id="@+id/googsearch"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_gravity="center_vertical"
android:layout_margin="0dp"
android:layout_weight="4"
android:background="@drawable/button"
android:drawableLeft="@drawable/actionsearch"
android:focusable="false"
android:padding="0dp"
android:text="Search"
android:textColor="@android:color/white"
android:textSize="7dp" />
<View
android:id="@+id/View"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_gravity="center_vertical"
android:layout_weight="2" />
<TextView
android:id="@+id/textView1"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_gravity="center_vertical"
android:layout_weight="2"
android:text="Symbol"
android:textColor="@android:color/white"
android:textSize="8sp" />
<TextView
android:id="@+id/prezzo"
android:layout_width="0px"
android:layout_height="wrap_content"
android:layout_gravity="center_vertical"
android:layout_weight="2"
android:text="Price"
android:textColor="@android:color/white"
android:textSize="8sp" />
<TextView
android:id="@+id/freccia"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_gravity="center_vertical"
android:layout_weight="1"
android:text=""
android:textColor="@android:color/white"
android:textSize="15sp" />
<TextView
android:id="@+id/cambiamento"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_gravity="center_vertical"
android:layout_weight="2"
android:text="Change %"
android:textColor="@android:color/white"
android:textSize="8dip" />
<TextView
android:id="@+id/aggiorna"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_gravity="bottom"
android:layout_margin="0dp"
android:layout_weight="2"
android:focusable="false"
android:gravity="center"
android:padding="0dp"
android:text="Click to\nupdate"
android:textColor="@android:color/white"
android:textSize="8dp" />
</LinearLayout>
</LinearLayout>
<ListView
android:id="@+id/android:list"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_weight="1" >
</ListView>
<LinearLayout
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:background="@drawable/gradient_box"
android:orientation="vertical"
android:textColor="#FFFFFF" >
<LinearLayout
android:id="@+id/ad"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:minHeight="50dp" />
<RelativeLayout
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:orientation="horizontal" >
<Button
android:id="@+id/preferencebutton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentRight="true"
android:layout_alignParentTop="true"
android:background="@drawable/button"
android:drawableLeft="@drawable/actionsettings"
android:paddingLeft="5dp"
android:paddingRight="5dp"
android:text="Preferences\n&\nHelp"
android:textColor="@android:color/white"
android:textSize="8sp" />
<Button
android:id="@+id/favoritesbtn"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_alignParentTop="true"
android:background="@drawable/button"
android:drawableLeft="@drawable/ratingfavorite"
android:paddingLeft="0dp"
android:paddingRight="5dp"
android:text="Your\nPortfolio"
android:textColor="@android:color/white"
android:textSize="8sp" />
<Button
android:id="@+id/buttonFinish"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:background="@drawable/button"
android:drawableLeft="@drawable/contentundo"
android:paddingLeft="0dp"
android:paddingRight="5dp"
android:text="Return"
android:textColor="@android:color/white"
android:textSize="8sp" />
</RelativeLayout>
</LinearLayout>
这是LogCat：
09-17 19:18:41.372: E/AndroidRuntime(25221): FATAL EXCEPTION: main
09-17 19:18:41.372: E/AndroidRuntime(25221): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.marketsinyourpocket/com.marketsinyourpocket.QuotesActivity}: java.lang.NullPointerException
09-17 19:18:41.372: E/AndroidRuntime(25221): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
09-17 19:18:41.372: E/AndroidRuntime(25221): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
09-17 19:18:41.372: E/AndroidRuntime(25221): at android.app.ActivityThread.access$600(ActivityThread.java:141)
09-17 19:18:41.372: E/AndroidRuntime(25221): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
09-17 19:18:41.372: E/AndroidRuntime(25221): at android.os.Handler.dispatchMessage(Handler.java:99)
09-17 19:18:41.372: E/AndroidRuntime(25221): at android.os.Looper.loop(Looper.java:137)
09-17 19:18:41.372: E/AndroidRuntime(25221): at android.app.ActivityThread.main(ActivityThread.java:5103)
09-17 19:18:41.372: E/AndroidRuntime(25221): at java.lang.reflect.Method.invokeNative(Native Method)
09-17 19:18:41.372: E/AndroidRuntime(25221): at java.lang.reflect.Method.invoke(Method.java:525)
09-17 19:18:41.372: E/AndroidRuntime(25221): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
09-17 19:18:41.372: E/AndroidRuntime(25221): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
09-17 19:18:41.372: E/AndroidRuntime(25221): at dalvik.system.NativeStart.main(Native Method)
09-17 19:18:41.372: E/AndroidRuntime(25221): Caused by: java.lang.NullPointerException
09-17 19:18:41.372: E/AndroidRuntime(25221): at com.marketsinyourpocket.QuotesActivity.onCreate(QuotesActivity.java:136)
09-17 19:18:41.372: E/AndroidRuntime(25221): at android.app.Activity.performCreate(Activity.java:5133)
09-17 19:18:41.372: E/AndroidRuntime(25221): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
09-17 19:18:41.372: E/AndroidRuntime(25221): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
09-17 19:18:41.372: E/AndroidRuntime(25221): ... 11 more
答案 0 :(得分：2)
这一行：
Button back = (Button) findViewById(R.id.buttonFinish);
不一定会返回按钮。如果可以找到所述按钮，它将返回按钮。否则，您将获得一个空引用，这将导致下一行的错误。
检查按钮是否存在，并使用您传递给findViewById方法的ID。如果这是您真正打算用来获取按钮的方法。
答案 1 :(得分：1)
您的XML无效 - 您的根元素为RelativeLayout，但结尾标记为</LinearLayout>。因此，您的按钮资源可能无法编译到您的应用中。
在您最新的XML中，有4个开放<LinearLayout>标记，只有3个结束标记。
答案 2 :(得分：1)
经过几次编辑后，我仍然无法判断你粘贴的是你的整个XML文件，所以我打算假设它是。
您的代码不匹配。我采取了你的布局并剥去了绒毛以显示结构：
<LinearLayout> // L1 Open
<RelativeLayout/> // R1
<LinearLayout> // L2 Open
<RelativeLayout> // R2 Open
<TextView/>
</RelativeLayout> // R2 Close
<LinearLayout> // L3 Open
<TextView/>
<Button/>
<View/>
<TextView/>
<TextView/>
<TextView/>
<TextView/>
<TextView/>
</LinearLayout> // L3 Close
</LinearLayout> // L2 Close
<ListView/>
<LinearLayout> // L4 Open
<LinearLayout/> // L5
<RelativeLayout> // R3 Open
<Button/>
<Button/>
<Button/>
</RelativeLayout> // R3 Close
</LinearLayout> // L4 Close
// No L1 Close!
您需要另一个结束</LinearLayout>标记。否则，它会搞乱解析，你会得到一些奇怪的结果。
如果您粘贴的内容不，实际上是您的整个XML文件，那么我强烈建议您将来从头开始粘贴整个文件。它将消除一些猜测和挫败感。
不管怎样，我强烈推荐Build->Clean。好吧，我建议在更改/res文件夹中的任何之后，但是如果你出现一个奇怪的空指针或类投射异常，我会建议加倍。