构造者模式（builder）

在工作中我们经常遇到参数比较多的构造函数，构造函数的参数可能很快就会失控，而且可能很难理解每个参数。如果将来你想添加参数，或者改变参数，这个参数列表还可能更快增长，很容易出错。例如生成下面的“英雄”角色，选择职业、性别、头发颜色等，角色的生成就变成了一个一步一步的过程，当所有的选择都准备好了，这个过程就完成了。

public Hero(Profession profession, String name, HairType hairType, HairColor hairColor, Armor armor, Weapon weapon) {
}

我们可以利用java中的构造者设计模式进行重构，首先有我们想要的“英雄”。

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;
  }
}

然后是建造者：

 public static class Builder {
    private final Profession profession;
    private final String name;
    private HairType hairType;
    private HairColor hairColor;
    private Armor armor;
    private Weapon weapon;

    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);
    }
  }

然后调用方法：

var mage = new Hero.Builder(Profession.MAGE, "Riobard")
.withHairColor(HairColor.BLACK)
.withWeapon(Weapon.DAGGER).build();

类图：

builder.png

Real world examples

• java.lang.StringBuilder
• java.nio.ByteBuffer as well as similar buffers such as FloatBuffer, IntBuffer and so on.
• java.lang.StringBuffer
• All implementations of java.lang.Appendable
• Apache Camel builders
• Apache Commons Option.Builder