mutableperiod.java

effective-java中文英文版,大师人物写的

// Page 226

import java.util.*;
import java.io.*;

public class MutablePeriod {
    // A period instance
    public final Period period;

    // period's start field, to which we shouldn't have access
    public final Date start;

    // period's end field, to which we shouldn't have access
    public final Date end;

    public MutablePeriod() {
        try {
            ByteArrayOutputStream bos =
                new ByteArrayOutputStream();
            ObjectOutputStream out =
                new ObjectOutputStream(bos);

            // Serialize a valid Period instance
            out.writeObject(new Period(new Date(), new Date()));

            /*
             * Append rogue "previous object refs" for internal
             * Date fields in Period. For details, see "Java 
             * Object Serialization Specification," Section 6.4.
             */
            byte[] ref = { 0x71, 0, 0x7e, 0, 5 }; // Ref #5
            bos.write(ref); // The start field
            ref[4] = 4;     // Ref # 4
            bos.write(ref); // The end field

            // Deserialize Period and "stolen" Date references
            ObjectInputStream in = new ObjectInputStream(
            new ByteArrayInputStream(bos.toByteArray()));
            period = (Period) in.readObject();
            start  = (Date)   in.readObject();
            end    = (Date)   in.readObject();
        } catch (Exception e) {
            throw new RuntimeException(e.toString());
        }
    }

    public static void main(String[] args) {
        MutablePeriod mp = new MutablePeriod();
        Period p = mp.period;
        Date pEnd = mp.end;

        // Let's turn back the clock
        pEnd.setYear(78);
        System.out.println(p);

        // Bring back the 60's!
        pEnd.setYear(69);
        System.out.println(p);
    }
}