Builder 模式

问题

当我们拥有一个类，它有几个必需成员变量和几个可选成员变量，如何构建这个类的实例，才能保证构建过程可读和安全？

现有方法的痛处

• 重叠构造器：

  1. 必须提供大量的构造器，不然会出现填写本不想设置的参数的情况
  2. 从方法的调用很难看出每个参数的含义

• JavaBeans 模式的 setter 方法：
  整个构造过程被划分到多个调用之中，因此

  1. 无法通过仅仅观察构造器参数了解类的成员
  2. 需要做额外的工作保证其线程安全

Builder 模式

假设 Player 类的必需成员为 height, weight，可选成员为 strength, speed, skill，则该类的 Builder 模式如下：

// Builder Patternpublic class Player {    private final double height;    private final double weight;    private final double strength;    private final double speed;    private final double skill;    public static class Builder {        // Required parameters        private final double height;        private final double weight;        // Optional parameters - initialized to default values        private double strength = 0;        private double speed    = 0;        private double skill    = 0;        public Builder(double height, double weight) {            this.height = height;            this.weight = weight;        }        public Builder strength(double val) {            strength = val;            return this;        }        public Builder speed(double val) {            speed = val;            return this;        }        public Builder skill(double val) {            skill = val;            return this;        }        public Player build() {            return new Player(this);        }    }    private Player(Builder builder) {        height      = builder.height;        weight      = builder.weight;        strength    = builder.strength;        speed       = builder.speed;        skill       = builder.skill;    }}

调用如下：

Player player = new Player.Builder(198, 120)                        .speed(80)                        .skill(95)                        .build();