Niubi Hong Hong Lambda expressions, simple and elegant is productivity!

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Author: Sevenvidia

https://www.zhihu.com/question/20125256/answer/324121308

What is Lambda?

We know, for a Java variable, we can assign it to a "value."

If you want to "block of code" assigned to a Java variable, how to do it?

For example, I want the right piece of code that is assigned to a variable named aBlockOfCode of Java:

Before Java 8, this can not be done. But after the advent of Java 8, using the Lambda features, you can do.

Of course, this is not a very concise wording. Therefore, in order to make this assignment more elegant, we can remove some useless statement.

In this way, we will succeed in the very elegant "block of code" assigned to a variable. The "piece of code," or "this function is assigned to a variable" is a Lambda Expression.

But there is still a problem, is what type of variable aBlockOfCode should be?

In Java 8 inside, all types of Lambda is an interface, Lambda expressions itself, which is "part of the code," the need to achieve this interface. I think this is a key to understanding Lambda, can be summarized, Lambda expression itself is to implement an interface. Direct say may still be a little annoying, we continue to look at an example. We give the above plus a aBlockOfCode type:

This interface is only one type of interface functions to be implemented, we call it "function interface." In order to avoid subsequent increase in human interface functions in this interface led to its multiple interface functions need to be implemented into a "non-function interface", we can add a statement @FunctionalInterface, so that others will not be able to add on the inside in the above the new interface function of:

In this way, we get a complete statement of Lambda expressions:

Lambda expressions What is the role?

The most intuitive effect is to make the code becomes incredibly easy.

We can compare Lambda expressions and traditional Java implementation of the same interface:

这两种写法本质上是等价的。但是显然，Java 8中的写法更加优雅简洁。并且，由于Lambda可以直接赋值给一个变量，我们就可以直接把Lambda作为参数传给函数, 而传统的Java必须有明确的接口实现的定义，初始化才行：

有些情况下，这个接口实现只需要用到一次。传统的Java 7必须要求你定义一个“污染环境”的接口实现MyInterfaceImpl，而相较之下Java 8的Lambda, 就显得干净很多。

Lambda结合FunctionalInterface Lib, forEach, stream()，method reference等新特性可以使代码变的更加简洁！

直接上例子。

假设Person的定义和List<Person>的值都给定。

现在需要你打印出guiltyPersons List里面所有LastName以"Z"开头的人的FirstName。

原生态Lambda写法：定义两个函数式接口，定义一个静态函数，调用静态函数并给参数赋值Lambda表达式。

这个代码实际上已经比较简洁了，但是我们还可以更简洁么？

当然可以。在Java 8中有一个函数式接口的包，里面定义了大量可能用到的函数式接口（java.util.function (Java Platform SE 8 )）。

所以，我们在这里压根都不需要定义NameChecker和Executor这两个函数式接口，直接用Java 8函数式接口包里的Predicate<T>和Consumer<T>就可以了——因为他们这一对的接口定义和NameChecker/Executor其实是一样的。

第一步简化 - 利用函数式接口包：

静态函数里面的for each循环其实是非常碍眼的。这里可以利用Iterable自带的forEach()来替代。forEach()本身可以接受一个Consumer<T> 参数。

第二步简化 - 用Iterable.forEach()取代foreach loop：

由于静态函数其实只是对List进行了一通操作，这里我们可以甩掉静态函数，直接使用stream()特性来完成。stream()的几个方法都是接受Predicate<T>，Consumer<T>等参数的（java.util.stream (Java Platform SE 8 )）。你理解了上面的内容，stream()这里就非常好理解了，并不需要多做解释。

第三步简化 - 利用stream()替代静态函数：

对比最开始的Lambda写法，这里已经非常非常简洁了。但是如果，我们要求变一下，变成print这个人的全部信息，及p -> System.out.println(p); 那么还可以利用Method reference来继续简化。所谓Method reference, 就是用已经写好的别的Object/Class的method来代替Lambda expression。格式如下：