值对象（Value Object）模式

针对一些数据对象，我们更强调的是这个对象的值而不是这个对象本身的时候，就可以使用值对象模式。意思就是两个对象判定相等的时候应该是两个对象的"值"相等，而不是它俩必须是同一个对象。在编写值对象模式的时候，应该注意以下几点（以java为例）：

1、必须重写equals()、hashCode()，且重写的原则是两个对象的"值"相等的化，equal()和hashCode()必然相等。

2、值对象一般不可修改，因此成员变量应用final修饰。

3、应该提供getters方法获取成员变量的值。

4、两个对象使用equal()比较而不是==

类图：

代码：

/**
 * HeroStat is a value object
 * 
 * {@link http://docs.oracle.com/javase/8/docs/api/java/lang/doc-files/ValueBased.html}
 */
public class HeroStat {

  // Stats for a hero

  private final int strength;
  private final int intelligence;
  private final int luck;

  // All constructors must be private.
  private HeroStat(int strength, int intelligence, int luck) {
    super();
    this.strength = strength;
    this.intelligence = intelligence;
    this.luck = luck;
  }

  // Static factory method to create new instances.
  public static HeroStat valueOf(int strength, int intelligence, int luck) {
    return new HeroStat(strength, intelligence, luck);
  }

  public int getStrength() {
    return strength;
  }

  public int getIntelligence() {
    return intelligence;
  }

  public int getLuck() {
    return luck;
  }

  /*
   * Recommended to provide a static factory method capable of creating an instance from the formal
   * string representation declared like this. public static HeroStat parse(String string) {}
   */

  // toString, hashCode, equals

  @Override
  public String toString() {
    return "HeroStat [strength=" + strength + ", intelligence=" + intelligence
        + ", luck=" + luck + "]";
  }

  @Override
  public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + intelligence;
    result = prime * result + luck;
    result = prime * result + strength;
    return result;
  }

  @Override
  public boolean equals(Object obj) {
    if (this == obj) {
      return true;
    }
    if (obj == null) {
      return false;
    }
    if (getClass() != obj.getClass()) {
      return false;
    }
    HeroStat other = (HeroStat) obj;
    if (intelligence != other.intelligence) {
      return false;
    }
    if (luck != other.luck) {
      return false;
    }
    if (strength != other.strength) {
      return false;
    }
    return true;
  }

  // The clone() method should not be public. Just don't override it.

}

public class App {

  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
  
  /**
   * This practice creates three HeroStats(Value object) and checks equality between those.
   */
  public static void main(String[] args) {
    HeroStat statA = HeroStat.valueOf(10, 5, 0);
    HeroStat statB = HeroStat.valueOf(10, 5, 0);
    HeroStat statC = HeroStat.valueOf(5, 1, 8);

    LOGGER.info(statA.toString());

    LOGGER.info("Is statA and statB equal : {}", statA.equals(statB));
    LOGGER.info("Is statA and statC equal : {}", statA.equals(statC));
  }
}