Layout weight with matchparent attribute

Question

I have been often told to use 0dp on views while using weight in XML like this :

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <Button
        android:id="@+id/a1"
        android:layout_width="wrap_content"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:text="1" />

</LinearLayout>

but there is a problem with this code which is when i use a view like Button, i can't force it to take the exact weight im giving to it.

 <LinearLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <Button
        android:id="@+id/a1"
        android:layout_width="wrap_content"
        android:layout_height="0dp"
        android:layout_weight="25"
        android:text="1" />

    <Button
        android:id="@+id/a2"
        android:layout_width="wrap_content"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:text="2" />

</LinearLayout>

in the code written above, the 2nd button will never be exactly 1/26 because the button itself has some margin and padding by default.

but when i use match_parent for their height, it forces them to be exactly 1/26 and it works perfectly.

but i can't understand why the 1st button gets to be 1/26 and it seems like they exchange their weight, and it gets more complicated when i use 3 views.

1. is there a better way of achieving this goal ?
2. and why weight acts different while using match_parent ?

Answer 1

the spacing in the Button is not padding or margin, but it was a background. if you want to remove the spacing you should change the background of the Button

it is recommended to use android:layout_height="0dp" because the docs of layout_weight said :

Indicates how much of the extra space in the LinearLayout is allocated to the view associated with these LayoutParams.

It said "extra space" not "space". So the right height should be 0dp + "extra space calculated"

here some sample code

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:weightSum="6"
    xmlns:android="http://schemas.android.com/apk/res/android">

    <Button
        android:id="@+id/a1"
        android:layout_width="wrap_content"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:background="@color/red"
        android:text="1" />

    <Button
        android:id="@+id/a2"
        android:layout_width="wrap_content"
        android:layout_height="0dp"
        android:layout_weight="2"
        android:background="@color/blue"
        android:text="2" />

    <Button
        android:id="@+id/a3"
        android:layout_width="wrap_content"
        android:layout_height="0dp"
        android:layout_weight="3"
        android:background="@color/yellow"
        android:text="3" />

</LinearLayout>

and the result