How can I prevent views from overlapping in relative layout?

Best Fix:

android:layout_toStartOf="@id/item_on_the_right"

Could you instead contain those two TextViews within a horizontal LinearLayout (which itself would still be a child of your RelativeLayout). Then, assign appropriate weight attributes to each of the two TextViews within that LinearLayout.

To get same result using RelativeLayout please use android:layout_toLeftOf="@+id/right_text". It will position the right edge of this view to the left of the given anchor view ID.

Please follow this example:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:paddingLeft="@dimen/margin_material_small"
    android:paddingRight="@dimen/margin_material_small">
    <TextView
        android:id="@+id/long_text"
        style="@style/Base.TextAppearance.AppCompat.Title"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true"
        android:layout_toLeftOf="@+id/right_text"
        android:singleLine="true"
        tools:text="Some text field that will definitely overlap with another one" />
    <TextView
        android:id="@+id/right_text"
        style="@style/Base.TextAppearance.AppCompat.Display1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentRight="true"
        android:layout_centerVertical="true"
        android:singleLine="true"
        tools:text="10.34" />
    <TextView
        android:id="@+id/subtext"
        style="@style/Base.TextAppearance.AppCompat.Medium"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/long_text"
        android:layout_alignParentLeft="true"
        android:layout_toLeftOf="@+id/right_text"
        android:singleLine="true"
        tools:text="Some other text slightly smaller"/>
</RelativeLayout>

Result of above layout: