java NullPointerException：尝试在空对象引用上调用虚拟方法“void android.widget.CheckBox.setOnClickListener，仅在平板电脑上”
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/36293241/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
NullPointerException: Attempt to invoke virtual method 'void android.widget.CheckBox.setOnClickListener on a null object reference, ONLY on tablets
提问by Carrie
I saw many similar questions online, and tried several solutions. But none fixed my issue. One important thing I want to mention is that, my code works well on android Phones, it only crashes on android Tablets. I have no idea what's going on.
我在网上看到很多类似的问题，并尝试了几种解决方案。但没有人解决我的问题。我想提的一件重要事情是，我的代码在 android 手机上运行良好，它只在 android 平板电脑上崩溃。我不知道是怎么回事。
This is the stack:
这是堆栈：
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.xxxx.xxxx.android/com.xxxx.xxxx.android.LoginScreenActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.CheckBox.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2808)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2873)
at android.app.ActivityThread.access0(ActivityThread.java:181)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1482)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:6145)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.CheckBox.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.xxxx.xxxx.android.LoginScreenActivity.onCreate(LoginScreenActivity.java:89)
at android.app.Activity.performCreate(Activity.java:6374)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2752)
... 10 more
This is the LoginScreenActivity:
这是登录屏幕活动：
public class LoginScreenActivity extends Activity implements OnClickListener
{
private Context context;
private CheckBox showPassword;
@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.login_screen);
context = getApplicationContext();
showPassword = (CheckBox)findViewById(R.id.checkbox_show_password);
//below is line 89 of LoginScreenActivity, exception happened here.
showPassword.setOnClickListener(new OnClickListener()
{
@Override
public void onClick(View view)
{
//.................
}
});
}
}
Here's the xml file:
这是xml文件：
<?xml version="1.0" encoding="utf-8"?>
<ScrollView
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fillViewport="true">
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="wrap_content">
<CheckBox
android:id="@+id/checkbox_show_password"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Show Password"/>
</RelativeLayout>
</ScrollView>
回答by OneCricketeer
There are two problems that are apparent.
有两个问题很明显。
The obvious: NullPointerException
显而易见的： NullPointerException
The solution: All instances of login_screen.xmlwill need a Checkboxwith android:id="@+id/checkbox_show_password"in order for findViewById(R.id.checkbox_show_password)to not return null.
解决方案： 的所有实例login_screen.xml都需要一个Checkboxwithandroid:id="@+id/checkbox_show_password"才能findViewById(R.id.checkbox_show_password)不返回空值。
This is worth mentioning because in the comments it was stated that there are multiple res/layout-*folders and the error only happens on tablets, so that is important.
这是值得一提的，因为在评论中指出有多个res/layout-*文件夹并且错误只发生在平板电脑上，所以这很重要。
Second problem:
第二个问题：
While any Viewcan use an OnClickListener, you have a CheckBox, not a Button, so you'll probably want to know when the checkbox was checked, not just clicked.
虽然任何人View都可以使用 an OnClickListener，但您使用的是CheckBox，而不是Button，因此您可能想知道复选框何时被选中，而不仅仅是单击。
In that case, use an OnCheckedChangeListener
在这种情况下，请使用 OnCheckedChangeListener
showPassword.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
@Override
public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
if (isChecked){
// do something
}else {
//do something else
}
}
});
回答by Kostas Drak
Well with a checkbox you cannot have an onClickListener. Please remove the Interface (implements) also. You may want to try this:
好吧，有了复选框，你就不能有一个onClickListener. 也请移除接口 ( implements)。你可能想试试这个：
showPassword.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
@Override
public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
if (showPassword.isChecked()){
//do something
}else {
//do something else
}
}
});
Hope it helps!!!
希望能帮助到你！！！
