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Výčtový typ pro starší Javu

Už několikrát jsem přemýšlel nad tím, že bych měl na stránky občas přidávat i věci týkající se mé práce, tedy hlavně javy. Ještě musím vymyslet, jak vkládat kód hezky s barvičkama, ale to přijde ;).

Včera se mě ptal jeden bývalý spolužák, jak udělat správně výčtový typ ve starší Javě. Tak jsem mu poslal následující příklad, který jsem okoukal v knize Java Efektivně.

Poté, co si kód prošel, prohlásil:

ty jo, zase jsem o neco chytrejsi
ted jsem se naucil, jak napsat vec, co jsme driv delali pomoci konstant na dva radky, tak jak ji napsat na radku 100!
uplne mam pocit, ze jsem postoupil do dalsiho levelu ;)
import java.io.Serializable;
import java.util.HashMap;
import java.util.Map;

/**
* Implementace vyctoveho typu v jave 1.4 a starsi.
* @author Josef Cacek
*/
public final class AppState implements Serializable {

  private static final long serialVersionUID = -4346433248901502211L;

  private String name;
  private static Map allStates = new HashMap();

  public static final AppState NEW = new AppState("NEW");
  public static final AppState ASSIGNED = new AppState("ASSIGNED");
  public static final AppState INPROGRESS = new AppState("INPROGRESS");
  public static final AppState CLOSED = new AppState("CLOSED");

  /**
   * vypnout default contructor
   */
  private AppState() {
    throw new IllegalStateException();
  }

  /**
   * Kontruktor, ktery potrebujem.
   * @param aName jmeno stavu
   */
  private AppState(final String aName) {
    if (aName==null) {
      throw new NullPointerException("State name can't be null.");
    }
    name = aName.toUpperCase();
    if (allStates.containsKey(name)) {
      //kdybychom se zapomneli
      //a pri copy paste nezmenili parametr konstruktoru
      throw new IllegalArgumentException("State with name '"
          + name + "' already exists.");
    }
    allStates.put(name, this);
  }

  /**
   * Vraci soucasny stav
   * @return stav jako String
   */
  public String getName() {
    return name;
  }

  /* (non-Javadoc)
   * @see java.lang.Object#toString()
   */
  public String toString() {
    return getName();
  }

  /**
   * Vraci stavovy objekt pro dane jmeno
   * @param aName jmeno stavu
   * @return stav
   */
  public static AppState valueOf(final String aName) {
    if (! isState(aName)) {
      throw new IllegalArgumentException("Wrong state name - '"
          + aName + "'.");
    }
    return (AppState) allStates.get(aName.toUpperCase());
  }

  /**
   * Vraci jmeno stavu v parametru
   * @param aState stav
   * @return jmeno stavu (nebo null, jestlize parametr je null)
   */
  public static String toString(final AppState aState) {
    return aState==null?null:aState.toString();
  }

  /**
   * Vraci true jestlize existuje stav s danym jmenem.
   * @param aName jmeno stavu
   * @return true, kdyz existuje stav s danym jmenem
   */
  public static boolean isState(final String aName) {
    return aName!=null && allStates.containsKey(aName.toUpperCase());
  }
}

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