Effective Java 学习笔记(二)

本文探讨了在Java中处理具有大量可选参数的类构造过程中的挑战,并介绍了如何使用Builder模式来简化这一过程。通过示例代码展示了重叠构造器、setter方法及Builder模式的优缺点。

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【第2条:遇到多个构造器参数时要考虑用构建器】

遇到参数较多时,静态工厂方法不容易扩展。那么一般来说重叠构造器可能是个好的做法,但是遇到参数多的情况下,需要重叠很多构造器。

代码如下:

public class Person {
	// 必须
	private String name;
	// 必须
	private String sex;
	// 可选
	private int age;
	// 可选
	private String tel_phone;
	// 可选
	private String mobile_phone;
	// 可选
	private String address;

	public Person(String name, String sex) {
		super();
		this.name = name;
		this.sex = sex;
	}

	public Person(String name, String sex, int age) {
		super();
		this.name = name;
		this.sex = sex;
		this.age = age;
	}

	public Person(String name, String sex, int age, String tel_phone) {
		super();
		this.name = name;
		this.sex = sex;
		this.age = age;
		this.tel_phone = tel_phone;
	}

	public Person(String name, String sex, int age, String tel_phone, String mobile_phone) {
		super();
		this.name = name;
		this.sex = sex;
		this.age = age;
		this.tel_phone = tel_phone;
		this.mobile_phone = mobile_phone;
	}

	public Person(String name, String sex, int age, String tel_phone, String mobile_phone, String address) {
		super();
		this.name = name;
		this.sex = sex;
		this.age = age;
		this.tel_phone = tel_phone;
		this.mobile_phone = mobile_phone;
		this.address = address;
	}
}
可以看到需要重叠多个构造器,那么如果参数增加到20多个呢。显然继续重叠不是一个好办法。

那么我们看看第二种setter的方式。

public class Person {
	// 必须
	private String name;
	// 必须
	private String sex;
	// 可选
	private int age;
	// 可选
	private String tel_phone;
	// 可选
	private String mobile_phone;
	// 可选
	private String address;

	public Person(String name, String sex) {
		super();
		this.name = name;
		this.sex = sex;
	}

	public Person(String name, String sex, int age) {
		super();
		this.name = name;
		this.sex = sex;
		this.age = age;
	}

	public Person(String name, String sex, int age, String tel_phone) {
		super();
		this.name = name;
		this.sex = sex;
		this.age = age;
		this.tel_phone = tel_phone;
	}

	public Person(String name, String sex, int age, String tel_phone, String mobile_phone) {
		super();
		this.name = name;
		this.sex = sex;
		this.age = age;
		this.tel_phone = tel_phone;
		this.mobile_phone = mobile_phone;
	}

	public Person(String name, String sex, int age, String tel_phone, String mobile_phone, String address) {
		super();
		this.name = name;
		this.sex = sex;
		this.age = age;
		this.tel_phone = tel_phone;
		this.mobile_phone = mobile_phone;
		this.address = address;
	}
	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public String getSex() {
		return sex;
	}

	public void setSex(String sex) {
		this.sex = sex;
	}

	public int getAge() {
		return age;
	}

	public void setAge(int age) {
		this.age = age;
	}

	public String getTel_phone() {
		return tel_phone;
	}

	public void setTel_phone(String tel_phone) {
		this.tel_phone = tel_phone;
	}

	public String getMobile_phone() {
		return mobile_phone;
	}

	public void setMobile_phone(String mobile_phone) {
		this.mobile_phone = mobile_phone;
	}

	public String getAddress() {
		return address;
	}

	public void setAddress(String address) {
		this.address = address;
	}

}
实例化的时候,如下代码

Person person = new Person();
    	person.setName("upxiaofeng");
    	person.setAge(27);
    	person.setSex("man");
    	person.setTel_phone("010-12345678");
    	person.setMobile_phone("17012345678");
    	person.setAddress("未知");
在《Effective Java》中指出的了可能出现的错误:因为在构造过程中,被分配到了几个调用中,有可能会导致JavaBean可能处于不一致的状态。

也就是说在使用settter的时候,参数多的时候,有可能会出现构造了不同的不是我们需要的对象。

至此,《Effective Java》中提出了Builder模式(构建模式)。代码如下:

public class Person {
	// 必须
	private String name;
	// 必须
	private String sex;
	// 可选
	private int age;
	// 可选
	private String tel_phone;
	// 可选
	private String mobile_phone;
	// 可选
	private String address;

	// 静态成员类
	public static class Builder {
		private String name;
		private String sex;
		private int age = 0;
		private String tel_phone = "默认";
		private String mobile_phone = "默认";
		private String address = "默认";

		public Builder(String name, String sex) {
			super();
			this.name = name;
			this.sex = sex;
		}

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

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

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

		public Builder address(String address) {
			this.address = address;
			return this;
		}
		public Person builder(){
			return new Person(this);
		}
	}

	private Person(Builder builder) {
		name = builder.name;
		sex = builder.sex;
		age = builder.age;
		tel_phone = builder.tel_phone;
		mobile_phone = builder.mobile_phone;
		address = builder.address;
	}

}
实例化的时候,代码如下:

Person person = new Person.Builder("upxiaofeng", "man").age(27).address("未知").builder();
这样看来代码更容易编写,且builder对参数加了约束,是否在当前的builder对象域当中。

但是builder模式无疑使增加了代码量,创建对象之前,必须先创建构建器。虽然创建构建起的开销在实践中可能不那么明显

但是在一些对性能要求很高的情况下,就可能成为问题。

用书中的一句话总结:

如果类的构造器或者静态工厂中具有多个参数,设计这种类中,Builder模式就是中不错的选中,特别是在大多数参数都是可选的时候。





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