Scala速成指南：Cay S. Horstmann原著

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"Scala for the Impatient 是一本针对渴望快速学习Scala编程语言的读者编写的完整指南。本书由Cay S. Horstmann撰写，详细介绍了Scala的基础和高级特性，旨在帮助开发者迅速掌握这门多范式编程语言。" 在《Scala for the Impatient》这本书中，作者Cay S. Horstmann深入浅出地讲解了Scala的核心概念和语法，旨在让那些时间紧迫的开发者能够迅速理解和应用Scala。Scala是一种结合了面向对象和函数式编程特性的强类型编程语言，它运行在Java虚拟机（JVM）上，因此可以充分利用Java生态系统的资源。书中的内容可能包括以下几个关键知识点： 1. **基本语法**：Scala的基础语法与Java有许多相似之处，但也有许多不同。例如，它支持匿名函数、模式匹配和高阶函数，使得代码更加简洁和可读。 2. **类型系统**：Scala的类型系统非常强大，包括类、特质、枚举、隐式转换等。它允许定义抽象类型和类型别名，以提高代码的灵活性和安全性。 3. **面向对象编程**：Scala支持类、对象和继承，但它的面向对象设计更为灵活，如使用特质来实现多重继承和行为混入。 4. **函数式编程**：Scala的函数是第一类公民，可以作为参数传递，也可以作为返回值。它支持柯里化、尾递归优化和惰性求值等函数式特性。 5. **集合库**：Scala的集合库是其一大亮点，提供了丰富的数据结构如List、Set、Map，以及高效的操作，如flatMap、filter和map等。 6. ** Actors模型**：Scala内置对Akka框架的支持，使得编写并发和分布式系统变得简单，利用Actors模型实现线程安全的通信。 7. **模式匹配**：Scala的模式匹配功能允许开发者优雅地处理数据结构和解构对象，简化复杂逻辑。 8. **元编程**：通过反射和宏，Scala允许在运行时或编译时进行元编程，以实现代码生成和动态类型行为。 9. ** Scala与其他技术集成**：由于运行在JVM上，Scala可以无缝集成Java库，并能与Spring、Hadoop等Java生态系统中的框架和工具配合使用。 10. **案例研究**：书中可能包含实际项目中的例子，以帮助读者理解如何在实践中应用Scala。《Scala for the Impatient》是一本适合有一定编程经验并希望快速掌握Scala语言的读者的书籍。通过阅读本书，读者将能够了解Scala的强大功能，并能够在实际项目中有效地使用它。

San Francisco, 2012

scala> 0.5 * res0

res1: Double = 21.0

scala> "Hello, " + res0

res2: java.lang.String = Hello, 42

As you can see, the interpreter also displays the type of the result—in our examples, Int, Double, and

java.lang.String.

You can call methods. Depending on how you launched the interpreter, you may be able to use tab

completion for method names. Try typing res2.to and then hit the Tab key. If the interpreter offers

choices such as

Click here to view code image

toCharArray toLowerCase toString toUpperCase

this means tab completion works. Type a U and hit the Tab key again. You now get a single

completion:

res2.toUpperCase

Hit the Enter key, and the answer is displayed. (If you can’t use tab completion in your environment,

you’ll have to type the complete method name yourself.)

Also try hitting the ↑ and ↓ arrow keys. In most implementations, you will see the previously issued

commands, and you can edit them. Use the ←, →, and Del keys to change the last command to

res2.toLowerCase

As you can see, the Scala interpreter reads an expression, evaluates it, prints it, and reads the next

expression. This is called the read-eval-print loop, or REPL.

Technically speaking, the scala program is not an interpreter. Behind the scenes, your input is quickly

compiled into bytecode, and the bytecode is executed by the Java virtual machine. For that reason,

most Scala programmers prefer to call it “the REPL”.

Tip

The REPL is your friend. Instant feedback encourages experimenting, and you will feel good

whenever something works.

It is a good idea to keep an editor window open at the same time, so you can copy and paste

successful code snippets for later use. Also, as you try more complex examples, you may want

to compose them in the editor and then paste them into the REPL.

1.2 Declaring Values and Variables

Instead of using the names res0, res1, and so on, you can define your own names:

scala> val answer = 8 * 5 + 2

answer: Int = 42

You can use these names in subsequent expressions:

scala> 0.5 * answer

res3: Double = 21.0

A value declared with val is actually a constant—you can’t change its contents:

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scala> answer = 0

<console>:6: error: reassignment to val

To declare a variable whose contents can vary, use a var:

var counter = 0

counter = 1 // OK, can change a var

In Scala, you are encouraged to use a val unless you really need to change the contents. Perhaps

surprisingly for Java or C++ programmers, most programs don’t need many var variables.

Note that you need not specify the type of a value or variable. It is inferred from the type of the

expression with which you initialize it. (It is an error to declare a value or variable without

initializing it.)

However, you can specify the type if necessary. For example,

val greeting: String = null

val greeting: Any = "Hello"

Note

In Scala, the type of a variable or function is always written after the name of the variable or

function. This makes it easier to read declarations with complex types.

As I move back and forth between Scala and Java, I find that my fingers write Java

declarations such as String greeting on autopilot, so I have to rewrite them as greeting: String.

This is a bit annoying, but when I work with complex Scala programs, I really appreciate that I

don’t have to decrypt C-style type declarations.

Note

You may have noticed that there were no semicolons after variable declarations or

assignments. In Scala, semicolons are only required if you have multiple statements on the

same line.

You can declare multiple values or variables together:

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val xmax, ymax = 100 // Sets xmax and ymax to 100

var greeting, message: String = null

// greeting and message are both strings, initialized with null

1.3 Commonly Used Types

You have already seen some of the data types of the Scala language, such as Int and Double. Like Java,

Scala has seven numeric types: Byte, Char, Short, Int, Long, Float, and Double, and a Boolean type. However,

unlike Java, these types are classes. There is no distinction between primitive types and class types

in Scala. You can invoke methods on numbers, for example:

1.toString() // Yields the string "1"

or, more excitingly,

Click here to view code image

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