builder模式也叫建造者模式，将一个复杂对象的构建与他的表示分离，使用者可以一步一步的构建一个复杂的对象

User user = User.builder().id("1").name("cat").age(9).build();

User类中创建一个静态内部类 Builder
Builder 类中，包含User类的全部属性
Builder 类中，每个属性创建赋值方法，并返回 Builder 对象
        

public class User {
    // final 不可变 ，构造函数设值
	public final String id;
	public final String name;
	public final Integer age;

	public User(String id, String name, Integer age) {
		this.id = id;
		this.name = name;
		this.age = age;
	}

	public static Builder builder() {
		return new Builder();
	}

	public static class Builder {
		private String id;
		private String name;
		private Integer age;

		public Builder id(String id) {
			this.id = id;
			return this;
		}

		public Builder name(String name) {
			this.name = name;
			return this;
		}

		public Builder age(Integer age) {
			this.age = age;
			return this;
		}

		public User build() {
			return new User(this.id, this.name, this.age);
		}

	}

	public String toString() {
		return id + "," + name + "," + age;
	}
}