深入解析Java 8 Lambdas

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"高清彩版 Java 8 Lambdas 由 Richard Warburton 编写，详细介绍了 Java 8 的新特性——Lambda 表达式。本书由 O'Reilly Media 出版，旨在教育、商业或销售推广使用。" 在 Java 8 中，Lambdas 是一个重大的语法革新，它引入了函数式编程的概念，使得代码更加简洁、可读性更强，特别是对于处理集合和并发编程。Lambda 表达式允许程序员以更直观的方式定义匿名函数，而无需创建单独的类或实现接口。这种功能对于简化回调、事件处理以及数据处理流程尤其有用。 Lambda 表达式的语法结构如下： ```java 参数列表 -> 表达式或代码块 ``` 例如，一个接受两个整数并返回它们之和的 Lambda 表达式可以写成 `(int a, int b) -> a + b`。Lambda 可以作为参数传递给方法，也可以被方法返回，这极大地提高了代码的灵活性。 Java 8 为了支持 Lambda，引入了函数式接口（Functional Interface）的概念。这些接口只有一个抽象方法，例如 `java.util.function.Function<T,R>` 接口，它接受一个类型 T 的参数并返回类型 R 的结果。Lambda 表达式可以直接赋值给这些接口类型的变量，从而实现了函数式编程中的“高阶函数”概念。此外，Java 8 还引入了流（Stream API），与 Lambda 配合使用，可以进行高效的数据处理。例如，可以使用 `Stream.of()` 创建流，然后通过 `filter()`, `map()`, `reduce()` 等方法对数据进行过滤、转换和聚合操作，最后通过 `collect()` 将结果收集到集合中。 Lambda 表达式与方法引用来实现的匿名内部类相比，有以下优势： 1. 更简洁的语法：Lambda 表达式通常比匿名内部类更短。 2. 更好的编译时检查：由于 Lambda 表达式的类型由上下文推断，编译器能提供更好的错误提示。 3. 更高的效率：Lambda 实例是内联的，因此在某些情况下，使用 Lambda 可能会有性能优势。在本书中，Richard Warburton 将深入探讨这些主题，并通过实例来解释如何在实际项目中应用 Lambda 表达式和 Stream API，以提高代码质量和效率。同时，他还可能讨论了 Lambda 在并行处理、反应式编程以及如何与其他 Java 8 新特性（如默认方法、日期和时间 API 等）结合使用的场景。 "高清彩版 Java 8 Lambdas" 是一本面向 Java 开发者的实用指南，它详细讲解了如何利用 Java 8 的 Lambda 表达式和相关特性来编写更优雅、更高效的代码。无论你是初学者还是经验丰富的开发者，这本书都能帮助你更好地理解和掌握 Java 8 的这一核心特性。

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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