英文来源于：Java in a Nutshell, 6th Edition

　　　　　　　　　　　Definition of a Lambda Expression A lambda expression is essentially a function that does not have a name, and can be treated as a value in the language. As Java does not allow code to run around on its own outside of classes, in Java, this means that a lambda is an anonymous method that is defined on some class (that is possibly unknown to the developer). 　　　　　　　　　　　　　　　　　　　　　　　　---Java in a Nutshell, 6th Edition

其实正则表达式就是没有名字的函数，（参数，返回值，函数体），在java　语言中视为一个值． 因为java的函数必须在类中定义，这就意味着，java语言中的函数表达式即为在某类中定义的匿名方法．

函数表达式在java视为一个值，即：Runnable r = () -> System.out.println("Hello World");　形式．