This is the java definition
public java.io.Serializable getSerializable(java.lang.String key) { /* compiled code */ }
and this is how it is used it
val outlines = extras.getSerializable(OUTLINES_DATA)!! as ArrayList<OutlineContent>
And this generates
Unchecked cast: java.io.Serializable to java.util.ArrayList<com.silverkeytech.android_rivers.outliner.OutlineContent>