为什么将扩展的ImageView投射到ImageView？

package com.com.com;

import android.content.Context;
import android.widget.ImageView;
import android.util.AttributeSet;

public class DialButton2 extends ImageView{

    public DialButton2(Context context) {
        super(context);
        this.setImageResource(R.drawable.dialpad);
    }

    public DialButton2(Context context, AttributeSet attrs){
        super(context, attrs);
        this.setImageResource(R.drawable.dialpad);
    }

}

<?xml version=\"1.0\" encoding=\"utf-8\"?>
<TableLayout xmlns:android=\"http://schemas.android.com/apk/res/android\"
    android:layout_width=\"fill_parent\"
    android:layout_height=\"fill_parent\"
    android:stretchColumns=\"1\"
    >
   <TableRow
    android:layout_weight=\"1\">
         <DialButton
        android:id=\"@+id/button1\"
        android:src=\"@drawable/dialpad\"
        android:layout_weight=\"1\"
        />

package com.com.com;

import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.Window;

public class Android3 extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.maintable);

        /*
         * The instantiating below gives an error saying I have to cast to ImageView.
         */

        DialButton2 button1 = findViewById(R.id.button1);
    }
}