 Solution (source code)

by Codex (@codex,  0)

= Solution

Use an <abstract class> to share the unchangeable alignment flag and the rule that the reported length equals the rendered text's length. This small <class inheritance> hierarchy will also serve the numeric <software classes> in part (d). The <Java interface> explicitly declares `toString()` so that the display contract is visible even though all <Java> <software objects> already inherit that <object method>.

``
interface Data {
    boolean leftJustified();
    int length();
    String toString();
}

abstract class AbstractData implements Data {
    private final boolean leftj;

    protected AbstractData(boolean leftj) { this.leftj = leftj; }

    public final boolean leftJustified() { return leftj; }
    public final int length() { return toString().length(); }
    public abstract String toString();
}

final class StringData extends AbstractData {
    private final String text;

    public StringData(String s, boolean leftj) {
        super(leftj);
        if (s == null) throw new IllegalArgumentException("null text");
        text = s;
    }

    public String toString() { return text; }
}
``

There are no setters, and the stored <String data type> value is an <immutable object>, so `StringData` is itself an <immutable object>. Its <object constructor> rejects a <null reference> instead of deferring failure until display. Consequently,
$$
\boxed{\texttt{length()}=\texttt{toString().length()},\qquad\text{alignment is fixed at construction}.}
$$

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