= Solution
Make defensive copies on both entry and exit, while keeping the supplied `Dimension` class unchanged:
``
class Frame(d: Dimension) {
require(d != null)
private val saved = new Dimension(d.width, d.height)
private val savedArea = saved.width * saved.height
def area: Int = savedArea
def dimension: Dimension = new Dimension(saved.width, saved.height)
def isPortrait: Boolean = saved.height >= saved.width
}
``
The private stored dimensions have no aliases exposed to the caller and are never changed. Mutating the constructor argument affects only the caller's object; mutating a returned `Dimension` affects only a fresh copy. Consequently \b[the stored area is calculated once and stays equal to the stored dimensions' product]. This <defensive copying> is sufficient because `Dimension` contains only primitive fields; a more deeply nested mutable structure could require a <deep copy> or an immutable representation.
The multiplication above deliberately retains the supplied class's `Int` arithmetic and accessor type. If an exact mathematical area outside that range is required, compute once with `saved.width.toLong * saved.height.toLong` and expose `Long` instead; avoiding aliases and avoiding <integer overflow> are separate concerns.
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