深入理解Java 8 Lambdas与流处理

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"Java 8 Lambdas" 是一本由Richard Warburton编写的关于Java 8新特性，尤其是Lambda表达式及其应用的书籍。书中详细介绍了如何利用Lambda表达式进行函数式编程，以及如何在Java 8中使用流(Streams)、函数式接口、集合库的高级功能、并行处理和并发等概念。 1. **Lambda表达式**：Lambda表达式是Java 8引入的一种新语法，用于简洁地表示匿名函数。你的第一个Lambda表达式可能是一个简单的参数列表和一个冒号后跟随的代码块。Lambda可以根据上下文推断类型，这称为类型推断。它们常与功能性接口结合使用，这些接口只有一个抽象方法，例如Runnable或Comparator。 2. **流(Streams)**：流提供了一种内部迭代的方式处理集合，使得代码更简洁且可读性更强。流的操作包括映射(map)，过滤(filter)，扁平化(flatMap)，最大值(max)和最小值(min)，以及reduce操作，用于将流的元素聚合。通过组合这些操作，可以对数据进行复杂的转换和处理。 3. **库和默认方法**：Java 8的库对Lambda表达式的支持体现在许多内置接口上，如Collections和Arrays。@FunctionalInterface注解用于明确接口是函数式接口。默认方法允许在接口中添加新方法而不破坏已有的实现，它引入了类似多重继承的功能，但避免了类的多继承问题。 4. **高级集合和Collectors**：Collectors是用于转换和聚合流的强大工具，它们可以用于收集流到其他集合、转换元素、分组数据或执行其他形式的聚合。方法引用是另一种强大的特性，可以将方法作为函数进行传递。 5. **数据并行性**：Java 8的Stream API支持并行处理，使得数据处理可以利用多核处理器的优势。需要注意的是，并行处理并不总是能提升性能，需要考虑任务的性质和数据的分布。 6. **测试、调试和重构**：使用Lambda表达式时，测试和调试策略需要相应调整。例如，可以使用peek操作来插入调试语句，同时要小心Lambda的副作用。 7. **设计和架构原则**：Lambda表达式促进了设计模式的实现，如命令模式、策略模式、观察者模式和模板方法模式。此外，Lambda也支持SOLID原则，如单一职责原则、开放封闭原则和依赖倒置原则。 8. **Lambda启用的并发**：Java 8引入了非阻塞I/O和反应式编程的概念，如Futures和CompletableFuture，它们简化了异步编程，减少了回调地狱（Callback Hell）的问题。 9. **走向未来**：随着技术的发展，Java 8的Lambda表达式和其他功能将继续影响软件设计和开发实践。这本书旨在帮助开发者理解和熟练掌握Java 8中的Lambda表达式，通过丰富的示例和练习，促进从传统面向对象编程向函数式编程的转变，提高代码的简洁性和效率。

and time associated with developing enterprise business applications, and any barrier

to developing easy-to-use and efficient libraries should be removed.

Abstraction is a concept that is familiar to us all from object-oriented programming.

The difference is that object-oriented programming is mostly about abstracting over

data, while functional programming is mostly about abstracting over behavior. The real

world has both of these things, and so do our programs, so we can and should learn

from both influences.

There are other benefits to this new abstraction as well. For many of us who aren’t writing

performance-critical code all the time, these are more important wins. You can write

easier-to-read code—code that spends time expressing the intent of its business logic

rather than the mechanics of how it’s achieved. Easier-to-read code is also easier to

maintain, more reliable, and less error-prone.

You don’t need to deal with the verbosity and readbility issues surrounding anonymous

inner classes when writing callbacks and event handlers. This approach allows pro‐

grammers to work on event processing systems more easily. Being able to pass functions

around easily also makes it easier to write lazy code that initializes values only when

necessary.

In addition, the language changes that enable the additional collection methods, de

fault methods, can be used by everyday programmers who are maintaining their own

libraries.

It’s not your grandfather’s Java any longer, and that’s a good thing.

What Is Functional Programming?

Functional programming is a term that means different things to different people. At

the heart of functional programming is thinking about your problem domain in terms

of immutable values and functions that translate between them.

The communities that have developed around different programming languages each

tend to think that the set of features that have been incorporated into their language are

the key ones. At this stage, it’s a bit too early to tell how Java programmers will define

functional programming. In a sense, it’s unimportant; what we really care about is writ‐

ing good code rather than functional code.

In this book, I focus on pragmatic functional programming, including techniques that

can be used and understood by most developers and that help them write programs that

are easier to read and maintain.

2 | Chapter 1: Introduction

Anonymous inner classes were designed to make it easier for Java programmers to pass

around code as data. Unfortunately, they don’t make it easy enough. There are still four

lines of boilerplate code required in order to call the single line of important logic. Look

how much gray we get if we color out the boilerplate:

button.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent event) {

System.out.println("button clicked");

}

});

Boilerplate isn’t the only issue, though: this code is fairly hard to read because it obscures

the programmer’s intent. We don’t want to pass in an object; what we really want to do

is pass in some behavior. In Java 8, we would write this code example as a lambda

expression, as shown in Example 2-2.

Example 2-2. Using a lambda expression to associate behavior with a button click

button.addActionListener(event -> System.out.println("button clicked"));

Instead of passing in an object that implements an interface, we’re passing in a block of

code—a function without a name. event is the name of a parameter, the same parameter

as in the anonymous inner class example. -> separates the parameter from the body of

the lambda expression, which is just some code that is run when a user clicks our button.

Another difference between this example and the anonymous inner class is how we

declare the variable event. Previously, we needed to explicitly provide its type—Actio

nEvent event. In this example, we haven’t provided the type at all, yet this example still

compiles. What is happening under the hood is that javac is inferring the type of the

variable event from its context—here, from the signature of addActionListener. What

this means is that you don’t need to explicitly write out the type when it’s obvious. We’ll

cover this inference in more detail soon, but first let’s take a look at the different ways

we can write lambda expressions.

Although lambda method parameters require less boilerplate code

than was needed previously, they are still statically typed. For the sake

of readability and familiarity, you have the option to include the type

declarations, and sometimes the compiler just can’t work it out!

How to Spot a Lambda in a Haystack

There are a number of variations of the basic format for writing lambda expressions,

which are listed in Example 2-3.

6 | Chapter 2: Lambda Expressions

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