android:layout_weight= "0.0"

The android:layout_weight is the weight of the space occupied by the view. 0.0 is a more specific indication that it is necessary to occupy the space required and not to participate in the division of space. If both the one and the three are 0.0 (that is, the upper and lower controls are 0.0), the system has reserved the top and bottom positions for them, and both Occupy 1, indicating that the remainder of the allocated space is left, because there are only a few controls left and all of them are given. 0.0 is a useful way to make sure that a button is left at the bottom of the ListView, no matter how long the list is, the user does not need to pull to the end to see the button, making sure that the button must appear below the screen.

android:layout_weight= "0.0"