Class By定义了页面元素的定位和支持哪些页面元素（至少我是这么理解的），使用及其简单：Used in conjunction with the web methods. Examples are By.id(String id) and By.cssSelector(String selector)，下面只举一个CssSelector的具体源码来分析下是如何实现的：
public abstract class By {
......
/** * Select a WebElement by its css selector. * @param selectors the css selector of the web element * @return the CssSelector object */
public static By cssSelector(final String selectors) { return new CssSelector(selectors);
}
......
static class CssSelector extends By { private final String selector;
public CssSelector(String selector) { this.selector = selector; }
@Override public String getValue(){ return selector; } }
这里先定义了一个抽象类By, cssSelector的数据类型是By, 传了一个常量selectors, 并且需要返回一个selector常量, 在下面是内部new的方法的具体实现，分析下在Java技术上用到的一些东西：
1）首先，定义了一个抽象类By，为什么是定义一个抽象类而不是一个接口？ 因为在抽象类中可以写具体的实现，如果写成接口则以后每个类都需要具体实现
Which should you use, abstract classes or interfaces?
Consider using abstract classes if any of these statements apply to your situation:You want to share code among several closely related classes.
You expect that classes that extend your abstract class have many common methods or fields, or require access modifiers other than public (such as protected and private).
You want to declare non-static or non-final fields. This enables you to define methods that can access and modify the state of the object to which they belong.
Consider using interfaces if any of these statements apply to your situation:You expect that unrelated classes would implement your interface. For example, the interfaces Comparable
and Cloneable
are implemented by many unrelated classes.
You want to specify the behavior of a particular data type, but not concerned about who implements its behavior.
You want to take advantage of multiple inheritance of type.
2）方法返回数据类型为定义的抽象类本身By：做了一定的规定，返回的一定是By定义的相关数据类型，
3）静态内部类继承了By这个抽象类：静态内部类，不需要实例化即可使用
4）实现了具体的方法： public CssSelector(String selector) { this.selector = selector;
5）重写了getValue方法：原先是默认为空，现在是返回该selector的内存引用。
