第 50 条：必要时进行保护性拷贝

必要时进行保护性拷贝.jpeg

// Broken "immutable" time period class
public final class Period {
    private final Date start;
    private final Date end;
    /**
    * @param start the beginning of the period
    * @param end the end of the period; must not precede start
    * @throws IllegalArgumentException if start is after end
    * @throws NullPointerException if start or end is null
    */
    public Period(Date start, Date end) {
        if (start.compareTo(end) > 0)
            throw new IllegalArgumentException(start + " after " + end);
        this.start = start;
        this.end = end;
    }
    public Date start() {
        return start;
    }
    public Date end() {
        return end;
    }
    ... // Remainder omitted
}

// Attack the internals of a Period instance
Date start = new Date();
Date end = new Date();
Period p = new Period(start, end);
end.setYear(78); // Modifies internals of p!

// Repaired constructor - makes defensive copies of parameters
public Period(Date start, Date end) {
    this.start = new Date(start.getTime());
    this.end = new Date(end.getTime());
    if (this.start.compareTo(this.end) > 0)
        throw new IllegalArgumentException(this.start + " after " + this.end);
}

// Second attack on the internals of a Period instance
Date start = new Date();
Date end = new Date();
Period p = new Period(start, end);
p.end().setYear(78); // Modifies internals of p!

// Repaired accessors - make defensive copies of internal fields
public Date start() {
    return new Date(start.getTime());
}
public Date end() {
    return new Date(end.getTime());
}