Scala编程语言指南：特性与优势解析

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"Scala教程，来自Tutorialspoint的Scala快速指南" Scala是一种混合型函数式编程语言，由Martin Odersky创建，并于2003年首次发布。它旨在结合面向对象和函数式编程的优点，同时编译运行在Java虚拟机（JVM）上。由于许多公司依赖Java来构建关键业务应用，他们转向Scala以提高开发效率、应用程序可扩展性和整体可靠性。 1. Scala是纯面向对象的：在Scala中，每个值都是一个对象，类型和对象的行为由类和特质来描述。类可以通过子类化进行扩展，而灵活的混合组合机制则作为多重继承的清洁替代方案。这种设计使得Scala的面向对象编程更加灵活且易于维护。 2. Scala是函数式的：在Scala中，函数也是值，这意味着它可以被赋给变量、作为参数传递给其他函数，甚至可以作为返回值。这使得函数式编程风格在Scala中变得自然，有助于编写无副作用的代码，从而提高并发性并简化调试。 3. 类型系统： Scala有一个强大的静态类型系统，可以在编译时捕获许多潜在的错误。它支持泛型，允许编写可复用的、类型安全的代码。此外，Scala还有模式匹配功能，使得处理复杂数据结构变得更加简单。 4. 特质（Traits）：特质是一种轻量级的代码重用机制，可以看作是接口与部分实现的结合。它们可以用于实现多重继承，避免了传统多继承可能导致的菱形问题。 5. 高级特性： Scala支持高阶函数、闭包、匿名函数和函数对象，以及类型推断，这些特性使得代码更简洁、更易于理解和维护。另外，Scala还提供了Actor模型，用于并发和分布式计算，这是受Akka框架启发的，非常适合大规模并行处理。 6. 集成与Java的兼容性：由于Scala运行在JVM上，它可以无缝地与现有的Java库和框架集成。Java代码可以调用Scala代码，反之亦然，这使得迁移现有Java项目到Scala成为可能。 7. 互操作性： Scala提供了一些工具，如`javabridge`，使得与Java代码的交互变得更加方便。这包括对Java集合的内置支持，以及能够直接调用Java方法和访问Java字段。 8. 函数式数据结构： Scala提供了许多内置的函数式数据结构，如列表、集合、映射和队列，这些数据结构在并发环境中表现出色，因为它们是不可变的。 9. 类型类和隐式转换：类型类允许以一种类型安全的方式添加行为到任意类型上，而隐式转换则可以优雅地解决类型之间的转换问题，使得代码更加简洁。 10. 元编程： Scala支持元编程，允许在编译时动态生成代码，这为创建高度灵活的库和框架提供了可能。 Scala的综合特性使其成为开发高性能、可扩展和易于维护的应用程序的理想选择，尤其适合大数据处理和云计算环境。通过学习Scala，开发者可以利用其丰富的特性和工具来提升开发效率和软件质量。

A string literal is a sequence of characters in double quotes. The characters are either printable unicode

character or are described by escape sequences. Here are some string literals:

"Hello,\nWorld!"

"This string contains a \" character."

Multi-Line String s

A multi-line string literal is a sequence of characters enclosed in triple quotes """ ... """. The sequence of

characters is arbitrary, except that it may contain three or more consuctive quote characters only at the very end.

Characters must not necessarily be printable; newlines or other control characters are also permitted. Here is a

multi-line string literal:

"""the present string

spans three

lines."""

The Null Value

The null value is of type scala.Null and is thus compatible with every reference type. It denotes a reference

value which refers to a special "null" object.

Escape Sequences:

The following escape sequences are recog nized in character and string literals.

Escape

Sequenc es

Unic ode Description

\b \u0008 backspace BS

\t \u0009 horizontal tab HT

\n \u000c formfeed FF

\f \u000c formfeed FF

\r \u000d carriag e return CR

\" \u0022 double quote "

\' \u0027 sing le quote .

\\ \u005c backslash \

A character with Unicode between 0 and 255 may also be represented by an octal escape, i.e., a backslash '\'

followed by a sequence of up to three octal characters. Following is the example to show few escape sequence

characters:

object Test {

def main(args: Array[String]) {

println("Hello\tWorld\n\n" );

}

When the above code is compiled and executed, it produces the following result:

Hello World

SCALA - VARIABLES

Variables are nothing but reserved memory locations to store values. This means that when you create a variable

you reserve some space in memory.

Based on the data type of a variable, the compiler allocates memory and decides what can be stored in the

reserved memory. Therefore, by assig ning different data types to variables, you can store integ ers, decimals,

or characters in these variables.

Variable Declaration

Scala has the different syntax for the declaration of variables and they can be defined as value, i.e., constant or a

variable. Following is the syntax to define a variable using var keyword:

var myVar : String = "Foo"

Here, myVar is declared using the keyword var. This means that it is a variable that can chang e value and this is

called mutable variable. Following is the syntax to define a variable using val keyword:

val myVal : String = "Foo"

Here, myVal is declared using the keyword val. This means that it is a variable that can not be chang ed and this is

called immutable variable.

Variable Data Types:

The type of a variable is specified after the variable name and before equals sig n. You can define any type of

Scala variable by mentioning its data type as follows:

val or val VariableName : DataType [= Initial Value]

If you do not assig n any initial value to a variable, then it is valid as follows:

var myVar :Int;

val myVal :String;

Variable Type Inference:

When you assig n an initial value to a variable, the Scala compiler can fig ure out the type of the variable based on

the value assig ned to it. This is called variable type inference. Therefore, you could write these variable

declarations like this:

var myVar = 10;

val myVal = "Hello, Scala!";

Here by default myVar will be Int type and myVal will become String type variable.

Multiple assig nments:

Scala supports multiple assig nments. If a code block or method returns a Tuple, the Tuple can be assig ned to a

val variable. [Note: We will study Tuple in subsequent chapters.]

val (myVar1: Int, myVar2: String) = Pair(40, "Foo")

And the type inference gets it rig ht:

val (myVar1, myVar2) = Pair(40, "Foo")

Variable Types:

Variables in Scala can have three different scopes depending on the place where they are being used. They can

exist as fields, as method parameters and as loc al variables. Below are the details about each type of

scope:

Fields:

Fields are variables that belong to an object. The fields are accessible from inside every method in the object.

Fields can also be accessible outside the object depending on what access modifiers the field is declared with.

Object fields can be both mutable or immutable types and can be defined using either var or val.

Method Parameters:

Method parameters are variables which are used to pass the value inside a method when the method is called.

Method parameters are only accessible from inside the method but the objects passed in may be accessible

from the outside, if you have a reference to the object from outside the method. Method parameters are always

mutable and defined by val keyword.

Local Variables:

Local variables are variables declared inside a method. Local variables are only accessible from inside the

method, but the objects you create may escape the method if you return them from the method. Local variables

can be both mutable or immutable types and can be defined using either var or val.

SCALA - ACCESS MODIFIERS

Members of packag es, classes or objects can be labeled with the access modifiers private and protected,

and if we are not using either of these two keywords, then access will be assumed as public . These modifiers

restrict accesses to the members to certain reg ions of code. To use an access modifier, you include its keyword

in the definition of members of packag e, class or object as we will see in the following section.

Private members:

A private member is visible only inside the class or object that contains the member definition. Following is the

example:

class Outer {

class Inner {

private def f() { println("f") }

class InnerMost {

f() // OK

}

(new Inner).f() // Error: f is not accessible

}

In Scala, the access (new Inner).f() is illeg al because f is declared private in Inner and the access is not from

within class Inner. By contrast, the first access to f in class InnerMost is OK, because that access is contained in

the body of class Inner. Java would permit both accesses because it lets an outer class access private members

of its inner classes.

Protected members:

A protected member is only accessible from subclasses of the class in which the member is defined. Following

is the example:

package p {

class Super {

protected def f() { println("f") }

}

class Sub extends Super {

f()

}

class Other {

(new Super).f() // Error: f is not accessible

}

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Scala编程语言指南：特性与优势解析

scala-parser-combinators_2.12-1.1.0-API文档-中英对照版.zip

scala-parser-combinators_2.11-1.0.4-API文档-中英对照版.zip

scala-java8-compat_2.11-0.7.0-API文档-中英对照版.zip

flink-1.13.2-bin-scala_2.11-el7.parcel包下载

flink-1.14.3-bin-scala_2.12

flink-1.15.0-bin-scala_2.12.tgz

scalac: Multiple 'scala-library*.jar' files (scala-library.jar, scala-library-2.12.10.jar) in Scala compiler classpath in Scala SDK scala-sdk-2.11.8

以下代码的输出结果为（ ）。 val m1 = Map("scala1" -> 100, "scala2" -> 200) val m2 = m1 + ("scala3" -> 300) val v1 = m1("scala2") println(s"v1=$v1") val v2 = m2.getOrElse("scala5", "default") println(s"v2=$v2")

scala和maven-scala-plugin对应的版本

dockerfile 自己打包flink-1.15.3-bin-scala_2.12.tgz 成镜像

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以下代码的输出结果为（）。 val m1 = Map("scala1" -> 100, "scala2" -> 200) val m2 = m1 + ("scala3" -> 300) val v1 = m1("scala2") println(s"v1=$v1") val v2 = m2.getOrElse("scala5", "default") println(s"v2=$v2")