= Solution
Retain `Data`, `AbstractData` and `StringData` from part (c). The following are the additional <software classes>. `Formatting` denotes the supplied formatting facility with its two static `format` overloads; its implementation is expressly not required. These calls preserve the distinction between a minimum width and a maximum width: no stored value is truncated.
``
abstract class NumericData extends AbstractData {
private int minDigits = 0;
protected NumericData(boolean leftj) { super(leftj); }
public final void setDigits(int n) { minDigits = Math.max(0, n); }
protected final int digits() { return minDigits; }
}
final class IntegerData extends NumericData {
private final int value;
public IntegerData(int n, boolean leftj) {
super(leftj);
value = n;
}
public String toString() {
return Formatting.format(value, digits());
}
}
final class RealData extends NumericData {
private final double value;
private int minPlaces = 0;
public RealData(double d, boolean leftj) {
super(leftj);
value = d;
}
public void setPlaces(int p) { minPlaces = Math.max(0, p); }
public String toString() {
return Formatting.format(value, digits(), minPlaces);
}
}
``
Zero is the default minimum, and a nonpositive setting removes that minimum. Formatting a negative value, or a value already longer than the minimum, is delegated to the stated formatter contract. The inherited `length()` calls the current `toString()`, so a change to either formatting setting immediately changes the measured column width. For example, after `setDigits(3)`, an `IntegerData` containing 42 renders `042` and has length 3; after `setPlaces(2)`, a `RealData` containing 2.7 renders `2.70` and has length at least 4.
Unlike `StringData`, these numeric <software objects> are mutable because their displayed state can change. Their numeric values and alignment remain fixed. The useful invariant is
$$
\boxed{\text{the current formatting settings determine both text and its reported length}.}
$$
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