[历史归档] 本文原发布于 cstriker1407.info 个人博客，内容为历史存档，仅供参考。
发布时间： 2013-12-18　｜　标题：《Effective Java》读书笔记：保护性拷贝　｜　分类： 编程 / java / effective java　｜　标签： java · effective java · 保护性拷贝

最近在看《 Effective Java 》这本书，里面提到了编码时需要注意【 保护性拷贝（Rule39） 】，这里备份下实现的注意点，截图版权属于原作者。

github：

【 https://github.com/cstriker1407/think_in_java 】

首先列出基本类：

class Rule39Bean
{
	public int value;
	public Rule39Bean(int value)
	{
		this.value = value;
	}
	public Rule39Bean(Rule39Bean bean)
	{
		this.value = bean.value;
	}
}
abstract class Period
{
	public Period(Rule39Bean small, Rule39Bean big)
	{
	}
	protected Rule39Bean small;
	protected Rule39Bean big;
	public void check()
	{
		if (small.value < big.value)
		{
			System.out.println("Right!! small < big");
		}else
		{
			System.out.println("Wrong!! small >= big");
		}
	}
	public Rule39Bean getBig()
	{
		return big;
	}
}

使用方式A，有缺陷，未加任何保护：

class PeriodA extends Period
{
	public PeriodA(Rule39Bean small, Rule39Bean big)
	{
		super(small, big);
		this.small = small;
		this.big = big;
		if (small.value >= big.value)
		{
			throw new IllegalArgumentException("small >= big");
		}
	}
}

测试代码：

		{
			Rule39Bean small = new Rule39Bean(10);
			Rule39Bean big = new Rule39Bean(20);
			PeriodA periodA = new PeriodA(small, big);
			periodA.check();
			big.value = 5;//修改成功
			periodA.check();
		}

使用方式B，有缺陷，保护了构造函数，但是没有保护获取方法：

class PeriodB extends Period
{
	public PeriodB(Rule39Bean small, Rule39Bean big)
	{
		super(small, big);
		this.small = new Rule39Bean(small);
		this.big = new Rule39Bean(big);
		if (small.value >= big.value)
		{
			throw new IllegalArgumentException("small >= big");
		}
	}
}

测试代码：

		{
			Rule39Bean small = new Rule39Bean(10);
			Rule39Bean big = new Rule39Bean(20);
			PeriodB periodB = new PeriodB(small, big);
			periodB.check();
			big.value = 5;//修改失败
			periodB.check();
			periodB.getBig().value = 5;//修改成功
			periodB.check();
		}

使用方式C，在B的基础上增加了对获取方式的保护：

class PeriodC extends PeriodB
{
	public PeriodC(Rule39Bean small, Rule39Bean big)
	{
		super(small, big);
	}
	@Override
	public Rule39Bean getBig()
	{
		return new Rule39Bean(big);
	}
}

测试代码：

		{
			Rule39Bean small = new Rule39Bean(10);
			Rule39Bean big = new Rule39Bean(20);
			PeriodC periodC = new PeriodC(small, big);
			periodC.check();
			big.value = 5;//修改失败
			periodC.check();
			periodC.getBig().value = 5;//修改失败
			periodC.check();
		}

原书介绍：