I found that sometimes button showing text in upper case.

I have found the solution that we just need to define property textAllCaps in the xml file.

For example

 <Button
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:text="Hello"

                />

We see that button show "HELLO" text

Now after define textAllCaps as false the text all characters will not apper in upper case.

 <Button
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:text="Hello"
                android:textAllCaps="false"
                />

We see that button show "Hello" text