WHAT

双重否定的条件语句，可以使用一个肯定的条件语句代替

记住：双重否定 = 肯定

本次重构涉及到的基本重构手法：

Extract Method（提炼方法）
Inline Method（内联方法）
Substitute Algorithm（替换算法）

WHEN

双重否定，虽然机器能够很好理解并正确执行，但是不符合人的自然语言习惯，且令人迷惑，降低代码可读性

HOW

重构前的代码

package remove.doubles.negative;

public class SomeClass {
	
	private boolean found = false;

	public boolean isNotFound() {
		return !found;
	}

	public void someMethod() {
		if (!isNotFound()) { // double negative
			// do something
		}
	}
	
	public void someMethod2() {
		if (isNotFound()) { // negative
			// do something else
		}
	}
	
}

重构步骤

提炼肯定条件方法：选中否定条件方法中的肯定部分（如果现在没有，替换算法，杜撰一个），使用快捷键（Alt + Shitf + M）或者上下文菜单（Refactor -> Extract Method ... ），弹出” Extract Method“对话框
方法名取名为：isFound，并选择Access Modifier为public，提取函数之后的代码

package remove.doubles.negative;

public class SomeClass {
	
	private boolean found = false;

	public boolean isNotFound() {
		return !isFound();
	}

	public boolean isFound() {// extracted positive conditional method
		return found;
	}

	public void someMethod() {
		if (!isNotFound()) { // double negative
			// do something
		}
	}
	
	public void someMethod2() {
		if (isNotFound()) { // negative
			// do something else
		}
	}
	
}

针对每一个双重否定条件，将双重否定改为肯定
- 先内联否定条件方法：在调用处，选择否定条件，使用快捷键（Alt + Shit + I）或者上下文菜单（Refactor -> Inline...），弹出“Inline Method”对话框，选择“Only the selected invocation”
- 重构后的代码

package remove.doubles.negative;

public class SomeClass {
	
	private boolean found = false;

	public boolean isNotFound() {
		return !isFound();
	}

	public boolean isFound() {// extracted positive conditional method
		return found;
	}

	public void someMethod() {
		if (!!isFound()) { // double negative, after inline method 
			// do something
		}
	}
	
	public void someMethod2() {
		if (isNotFound()) { // negative
			// do something else
		}
	}
	
}

- 替换算法，把双重否定改为肯定

package remove.doubles.negative;

public class SomeClass {
	
	private boolean found = false;

	public boolean isNotFound() {
		return !isFound();
	}

	public boolean isFound() {// extracted positive conditional method
		return found;
	}

	public void someMethod() {
		if (isFound()) { // changed double negative to positive
			// do something
		}
	}
	
	public void someMethod2() {
		if (isNotFound()) { // negative
			// do something else
		}
	}
	
}

最后，如果否定条件方法isNotFound()，没有调用，使用隐藏方法（Hidden Method），或者只有少数几个调用，使用内联方法：Inline method
- 选中 isNotFound方法，使用快捷键（Alt + Shit + I）或者上下文菜单（Refactor -> Inline...），弹出“Inline Method”对话框，选择“All invocations”，并选择“Delete method declaration”

重构后的代码

package remove.doubles.negative;

public class SomeClass {
	
	private boolean found = false;

	public boolean isFound() {// extracted positive conditional method
		return found;
	}

	public void someMethod() {
		if (isFound()) { // changed double negative to positive
			// do something
		}
	}
	
	public void someMethod2() {
		if (!isFound()) { // changed negative to use not positive
			// do something else
		}
	}
	
}