Android自定义对话框类按钮NPE

Question

I am trying to create a custom dialog class with a custom layout, but, for reasons unknown to be, I am getting a null pointer exception when trying to access the buttons for the layout. Here is the XML for the dialog:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/enableTopRelativeLayout"
android:layout_width="460dp"
android:layout_height="wrap_content"
android:background="@drawable/dialog_background"
android:paddingBottom="15dp"
android:paddingLeft="15dp"
android:paddingRight="15dp"
android:paddingTop="15dp" >

<ImageView
    android:id="@+id/imageView1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentLeft="true"
    android:layout_centerVertical="true"
    android:src="@drawable/ic_launcher" />

<RelativeLayout
    android:id="@+id/enableInnerRelativeLayout"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_toRightOf="@id/imageView1" >

    <TextView
        android:id="@+id/enableMessage"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerInParent="false"
        android:paddingBottom="10dp"
        android:text="Start service?"
        android:textAppearance="?android:attr/textAppearanceMedium" />

     <TextView
        android:id="@+id/enableStrut"
        android:layout_width="25dp"
        android:layout_height="wrap_content"
        android:layout_below="@id/enableMessage"
        android:layout_centerHorizontal="true"
        android:text=" " />

    <Button
        android:id="@+id/noenable"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/enableMessage"
        android:layout_toLeftOf="@id/enableStrut"
        android:background="@drawable/button_background"
        android:text="No"
        android:textColorLink="@color/color2"
        android:textStyle="bold" />

    <Button
        android:id="@+id/enable"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/enableMessage"
        android:layout_toRightOf="@id/enableStrut"
        android:background="@drawable/button_background"
        android:text="Yes"
        android:textColorLink="@color/color2"
        android:textStyle="bold" />

</RelativeLayout>

Here is the code for the class:

public class DialogStartService extends Dialog implements android.view.View.OnClickListener{
public Button yes;
public Button no;
public TextView tv;

public DialogStartService(Context context) {
    super(context);
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    setContentView(R.layout.start_service_layout);

    getWindow().setBackgroundDrawable(new ColorDrawable(android.graphics.Color.TRANSPARENT));

    tv = (TextView) findViewById(R.id.enableMessage);

    yes = (Button) findViewById(R.id.enable);
    no = (Button) findViewById(R.id.noenable);

    yes.setOnClickListener(this);
    no.setOnClickListener(this);

}

@Override
public void onClick(View v) {       

    dismiss();
}

}

I instantiate the dialog with this code:

DialogStartService enableDialog = new DialogStartService(this);

    Button enable = (Button) enableDialog.findViewById(R.id.enable);
    enable.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            // Do stuff

        }

    });

    Button noenable = (Button) enableDialog.findViewById(R.id.noenable);
    noenable.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            // Do stuff
        }

    });


    enableDialog.show();

The stack trace indicates the NPE occurs on this line:

enable.setOnClickListener(new OnClickListener() {

My question for the experts is... why is this code causing a NPE, and what is the best way to fix it? Thank you for any input you have.

Answer 1

According to documentation of findViewById() method:

Finds a child view with the given identifier. Returns null if the specified child view does not exist or the dialog has not yet been fully created (for example, via show() or create()).

In your code you call findViewById() before show() - that's why Button enable is null.