设计模式6：建造者模式

但凡用过若干安卓第三方库的人，都应该对Builder DP（Design Pattern）也就是建造者模式不会感到陌生，例如Okhttp。
在我看来，建造者模式就是使用链式赋值，比一个个参数地set省力得多。

代码：
现在我们需要建造一个英雄（Hero），有职业（Profession）、名字（Name）、发型（HairType）、发色（HairColor）、护甲（Armor）、武器（Weapon）等属性。
假如不用建造者模式的话，我们怎么创建一个英雄实例呢？自然只能一个参数一个参数地使用set方法。
使用builder，单行即可：

/**
 * 
 * Hero, the class with many parameters.
 * 
 */
public final class Hero {

  private final Profession profession;
  private final String name;
  private final HairType hairType;
  private final HairColor hairColor;
  private final Armor armor;
  private final Weapon weapon;

  private Hero(Builder builder) {
    this.profession = builder.profession;
    this.name = builder.name;
    this.hairColor = builder.hairColor;
    this.hairType = builder.hairType;
    this.weapon = builder.weapon;
    this.armor = builder.armor;
  }

  public Profession getProfession() {
    return profession;
  }

  public String getName() {
    return name;
  }

  public HairType getHairType() {
    return hairType;
  }

  public HairColor getHairColor() {
    return hairColor;
  }

  public Armor getArmor() {
    return armor;
  }

  public Weapon getWeapon() {
    return weapon;
  }

  @Override
  public String toString() {

    StringBuilder sb = new StringBuilder();
    sb.append("This is a ")
            .append(profession)
            .append(" named ")
            .append(name);
    if (hairColor != null || hairType != null) {
      sb.append(" with ");
      if (hairColor != null) {
        sb.append(hairColor).append(' ');
      }
      if (hairType != null) {
        sb.append(hairType).append(' ');
      }
      sb.append(hairType != HairType.BALD ? "hair" : "head");
    }
    if (armor != null) {
      sb.append(" wearing ").append(armor);
    }
    if (weapon != null) {
      sb.append(" and wielding a ").append(weapon);
    }
    sb.append('.');
    return sb.toString();
  }

  /**
   * 
   * The builder class.
   * 
   */
  public static class Builder {

    private final Profession profession;
    private final String name;
    private HairType hairType;
    private HairColor hairColor;
    private Armor armor;
    private Weapon weapon;

    /**
     * Constructor
     */
    public Builder(Profession profession, String name) {
      if (profession == null || name == null) {
        throw new IllegalArgumentException("profession and name can not be null");
      }
      this.profession = profession;
      this.name = name;
    }

    public Builder withHairType(HairType hairType) {
      this.hairType = hairType;
      return this;
    }

    public Builder withHairColor(HairColor hairColor) {
      this.hairColor = hairColor;
      return this;
    }

    public Builder withArmor(Armor armor) {
      this.armor = armor;
      return this;
    }

    public Builder withWeapon(Weapon weapon) {
      this.weapon = weapon;
      return this;
    }

    public Hero build() {
      return new Hero(this);
    }
  }
}

可以看到，set方法都不见了，取而代之的是一个builder类。一般而言，builder类就像是建造者模式的一个标志，假如不是用建造者模式，还是别取这个名字，以免误导。反而言之，建造者模式使用builder作为建造类就是理所当然的事情了，约定俗成。