PATTERNS IN JAVAMatters of state

PATTERNS IN JAVA
Matters of state

Kevlin Henney

Listing 3. Implementation of the clock's life cycle using a Stateless Object for the mode.


public class Clock
{
  public synchronized void changeMode()
    { mode.changeMode(this); }
  public synchronized void increment()
    { mode.increment(this); }
  public synchronized void cancel()
    { mode.cancel(this); }
  private interface Mode
  {
    void changeMode(Clock context);
    void increment(Clock context);
    void cancel(Clock context);
  }
  private static class DisplayingTime implements Mode ...
  private static abstract class SettingTime implements Mode
...
  private static class SettingHours extends SettingTime ...
  private static class SettingMinutes extends SettingTime
  {
    public void changeMode(Clock context)
    { context.mode = displayingTime; }
    public void increment(Clock context)
    { ++context.minutes; }
  }
  private int hours, minutes;
  private Mode mode = settingHours;
  private static final Mode
    displayingTime = new DisplayingTime(),
    settingHours    = new SettingHours(),
    settingMinutes = new SettingMinutes();
}

About the Author

Kevlin Henney is a Principal Technologist with QA Training in the UK.

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