java.lang.IllegalStateExeption: Could not find a m

问题:

I am new to Android. I have started doing with ActivityLifeCycle app. In this I have 3 activity classes. From first activity I want to go to second activity class using intents when a button is clicked in the 1st activity. But it is giving error. And I have imported correct android.view.View package.

The same question was asked by someone else earlier, but I didn't get the solution, here is the error-prone code.

activity_main.xml:

<Button
    android:id="@+id/button1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/textView1"
    android:layout_marginTop="22dp"
    android:layout_toRightOf="@+id/textView1"
    android:onClick="startActivityB"
    android:clickable="true"
    android:text="startb" />

<Button
    android:id="@+id/button2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/button1"
    android:layout_below="@+id/button1"
    android:layout_marginTop="31dp"
    android:onClick="finishA"
    android:clickable="true"
    android:text="FinishA" />

<Button

MainActivity.java: (and this is the 1st activity)

protected void startActivityB(View v) {

    Intent intent = new Intent(getApplicationContext(), Activity_B.class);
    startActivity(intent);
}
protected void finishA(View v)
{
    MainActivity.this.finish();
}

This are the errors I got in logcat:

D/Avtivity_A(1333): onStart()of Activity_A started
D/Avtivity_A(1333): onResume()Activity_A started
D/AndroidRuntime(1333): Shutting down VM
W/dalvikvm(1333): threadid=1: thread exiting with uncaught exception (group=0x4001d800)
E/AndroidRuntime(1333): FATAL EXCEPTION: main
E/AndroidRuntime(1333): java.lang.IllegalStateException: Could not find a method
     finishA(View) in the activity class com.example.lifecycle.MainActivity for
     onClick handler on view class android.widget.Button with id 'button2'
E/AndroidRuntime(1333):   at android.view.View$1.onClick(View.java:2059)
E/AndroidRuntime(1333):   at android.view.View.performClick(View.java:2408)
E/AndroidRuntime(1333):   at android.view.View$PerformClick.run(View.java:8816)
E/AndroidRuntime(1333):   at android.os.Handler.handleCallback(Handler.java:587)
E/AndroidRuntime(1333):   at android.os.Handler.dispatchMessage(Handler.java:92)
E/AndroidRuntime(1333):   at android.os.Looper.loop(Looper.java:123)
E/AndroidRuntime(1333):   at android.app.ActivityThread.main(ActivityThread.java:4627)
E/AndroidRuntime(1333):   at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime(1333):   at java.lang.reflect.Method.invoke(Method.java:521)
E/AndroidRuntime(1333):   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
E/AndroidRuntime(1333):   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
E/AndroidRuntime(1333):   at dalvik.system.NativeStart.main(Native Method)

回答1:

In your xml for the activity you defined an onClick handler, and this name is the function name of the handler. I think you didn't implement the function.

i.E.

in the XML you have :

android:onClick="finishA"

and in the class you must implement a function:

public void finishA(View view)
{
}

Not sure if this is the cause, but it looks like it. If not, then post the XML and the class (the relevant parts of it).

Update

In your question the error references button2 and your XML is about button1, so you should show us the correct files.

Update

Your onClick handler is protected but it needs to be public as in the sample I showed above. When I test this in my app I get the same error when I make it protected.

回答2:

In your code you have called a function somewhere (Not in the code you have provided in your question)

java.lang.IllegalStateException: Could not find a method finishA(View) in the activity class com.example.lifecycle.MainActivity for onClick handler on view class android.widget.Button with id 'button2'

It says that in your mainActivity the function which is implemented on button2 click not the button1 which you shown in your code. it may Contains a line which says a call to finishA(View v) and when the compiler search for the same it is missing in the declaration of the class.

After Update to question

as suggested by Devolus You have used protected access modifier for your function finishA() you should use public for the same one instead of protected.

If you use protected as an access modifier then it makes the function available only to the class and the subclasses of the same class. Because of which when you are calling the finishA() from your activity B it is not able to find out the finishA() function.

The Protected variables and methods allow the class itself to access them, classes inside of the same package to access them, and subclasses of that class to access them

Where as using public access modifier the function will be available to each class which implements an object of the class Activity-A

You can read more at : http://www.java-made-easy.com/java-access-modifiers.html

and this question on SO will be helpful

You can also go on to use protected access modifier in your function if your bothe Activity-Aand Activity-B classes are in same package , by creating an object of class Activity-A in Activity-B class (note: if you are going to use class name without creating an object you will have to use protected static while declaring the function.)

With

protected void finishA(View v)
{
    MainActivity.this.finish();
}

Call the method in Activity-B class as(it will work only If the both classes are in same package)

ActivityA obj = new ActivityA ();
obj.finishA(View v);

With

 public void finishA(View v)
    {
        MainActivity.this.finish();
    }

Call the method in Activity-B class as

ActivityA obj = new ActivityA ();
obj.finishA(View v);

With

 public static void finishA(View v)
    {
        MainActivity.this.finish();
    }

Call the method in Activity-B class as

ActivityA.finishA(View v);

with

 protected static void finishA(View v)
    {
        MainActivity.this.finish();
    }

Call the method in Activity-B class as(it will work only If the both classes are in same package)

ActivityA.finishA(View v);