Android : How to create custom dialog

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:background="@color/black"
    android:orientation="vertical">

<ImageView
    android:id="@+id/image"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:src="@mipmap/ic_launcher"
    android:layout_margin="5dp" />

<TextView
    android:id="@+id/text"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="@string/app_name"
    android:textColor="@color/white"/>

 <Button
    android:id="@+id/ButtonOK"
    android:layout_width="50dp"
    android:layout_height="wrap_content"
    android:text="OK"
    android:layout_margin="5dp"/>
</LinearLayout>

@Override
public void onBackPressed() {
 CustomDialog();

}

private void CustomDialog(){

// custom dialog
final Dialog dialog = new Dialog(this);
dialog.setContentView(R.layout.custom_dialog);
// ---- Button ----
Button b_ok = dialog.findViewById(R.id.ButtonOK);

b_ok.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {

 dialog.cancel();
}
});
}