java NullPointerException：尝试对空对象引用调用虚拟方法？
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NullPointerException: Attempt to invoke virtual method on a null object reference?
提问by Carlton
I get a NUllPointerException when trying to invoke a method on a button:
尝试调用按钮上的方法时出现 NUllPointerException：
private FButton fButton1;
...
...
fButton1 = (FButton) findViewById(R.id.FButton1);
fButton1.setButtonColor(ContextCompat.getColor(this, fbutton_color_turquoise)); //Exception here
I am using this library hereand I don't get why I get the exception?
ERROR LOG:
错误日志：
FATAL EXCEPTION: main
Process: com.user.app, PID: 11209
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.user.app/com.user.app.activities.StartUpScreen}: java.lang.NullPointerException: Attempt to invoke virtual method 'void info.hoang8f.widget.FButton.setButtonColor(int)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void info.hoang8f.widget.FButton.setButtonColor(int)' on a null object reference
at com.user.app.activities.StartUpScreen.onCreate(StartUpScreen.java:44)
at android.app.Activity.performCreate(Activity.java:6237)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)?
at android.app.ActivityThread.-wrap11(ActivityThread.java)?
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)?
at android.os.Handler.dispatchMessage(Handler.java:102)?
at android.os.Looper.loop(Looper.java:148)?
at android.app.ActivityThread.main(ActivityThread.java:5417)?
at java.lang.reflect.Method.invoke(Native Method)?
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)?
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)?
my layout.xml where FButton is:
我的 layout.xml 其中 FButton 是：
<info.hoang8f.widget.FButton
fbutton:buttonColor="@color/fbutton_color_turquoise"
fbutton:shadowColor="@color/fbutton_color_green_sea"
fbutton:shadowEnabled="true"
fbutton:shadowHeight="5dp"
android:text="Celcius"
android:textColor="#FFF"
fbutton:cornerRadius="5dp"
android:layout_marginTop="8dp"
app:layout_constraintTop_toBottomOf="@+id/imageButton3"
android:layout_marginLeft="8dp"
app:layout_constraintLeft_toLeftOf="parent"
android:layout_height="wrap_content"
android:layout_marginStart="8dp"
android:layout_width="match_parent"
android:id="@+id/FButton1"
app:layout_constraintRight_toLeftOf="@+id/FButton2"
android:layout_marginRight="8dp"
android:layout_marginEnd="8dp"
app:layout_constraintHorizontal_bias="1.0" />
My ids.xml file that I had to create, since android did not recoginze the id:
我必须创建的 ids.xml 文件，因为 android 没有识别 id：
<?xml version="1.0" encoding="utf-8"?>
<resources>
<item name="FButton1" type="id" />
</resources>
回答by Vyacheslav
It seems your fButton1is null.
Just check where you call findViewById and check your xml file whether this element really exists
看来你fButton1是空的。只需检查您调用 findViewById 的位置并检查您的 xml 文件该元素是否真的存在
回答by gautam kumar
Your error is in setting Button color, use this to change the color of Button programmatically.
您的错误在于设置按钮颜色，使用它以编程方式更改按钮的颜色。
setBackgroundColor(getResources().getColor(R.color.holo_light_green))
One thing more, Starting from Android Support Library-23 getColor method has been deprecated, so instead use this
还有一件事，从 Android Support Library-23 开始，不推荐使用 getColor 方法，所以改用这个
ContextCompat.getColor(context, R.color.your_color);
Thus to support all versions use this,
因此为了支持所有版本使用这个，
public static final int getColor(Context context, int id)
{
final int version = Build.VERSION.SDK_INT;
if (version >= 23) {
return ContextCompatApi23.getColor(context, id);
}
else {
return context.getResources().getColor(id);
}
}
