chapter goals

1. Master the definition and use of variables and strings
2. Master the division of data types and the content of data type conversion
3. Master the keyboard entry function
4. Understand constants and identifiers in Scala

1. Output statement and semicolon

1.1 Output statement

Method 1: Newline output

Format:  println(Write the data you want to print to the console);

Method 2: output without wrapping

Format: print(Write the data you want to print to the console);

Note: Both println() and print() statements can print multiple values ​​at the same time. The format is: println(value1, value2, value3...)

1.2 Semicolons

In Scala statements, the semicolon at the end of a single line of code can be written or not. If multiple lines of code are written on one line, the semicolon in the middle cannot be omitted, and the semicolon of the last line of code can be omitted.

Example:

println("Hello, Scala!")			//The final semicolon can be written or not
//If multiple lines of code are written on one line, the semicolon of the previous statement must be written, and the semicolon of the last statement can be omitted.
println("Hello"); println("Scala")	

2. Constants in Scala

2.1 Overview

A constant refers to a quantity whose value cannot be changed during the running of the program.

2.2 Classification

• Literal constants (commonly used are the following)
  • integer constant
  • floating point constant
  • character constant
  • string constant
  • boolean constant
  • empty constant
• Custom constants (explained later)

2.3 Code Demonstration

//integer constant
println(10)
//floating point constant
println(10.3)
//Character constant, the value should be enclosed in single quotes
println('a')
//String constant, the value should be enclosed in double quotes
println("abc")
//Boolean constant, the only values ​​are true and false
println(true)
//empty constant
println(null)

3. Variables in Scala

3.1 Overview

We will define variables every day when we write scala programs in the future, so what is a variable and how is it defined?

Variable refers to the amount whose value can change during the execution of the program. The definition format is as follows:

3.2 Syntax format

Java variable definition

int a = 0;

In scala, you can use val or var to define variables, the syntax is as follows:

val/var variable name:variable type = initial value

in

• val defines a variable that cannot be reassigned, that is, a custom constant.
• var defines a variable that can be reassigned

Note: When defining variables in scala, the type is written after the variable name

3.3 Examples

**Requirement:** Define a variable to save a person's name "tom"

step

1. Open the scala interpreter
2. Define a variable of type string to hold the name

Reference Code

scala> val name:String = "tom"
name: String = tom

3.4 The difference between val and var variables

example

Reassign the name variable to Jim, and observe its running results

Reference Code

scala> name = "Jim"
<console>:12: error: reassignment to val
       name = "Jim"

example

Use var to redefine the variable to save the name "tom", and try to reassign it to Jim, and observe its running results

Reference Code

scala> var name:String = "tom"
name: String = tom

scala> name = "Jim"
name: String = Jim

Note: Use val first to define variables, and only use var if the variable needs to be reassigned

3.5 Using type inference to define variables

scala's syntax is more concise than Java, and we can use a more concise way to define variables.

example

Use a cleaner syntax to define a variable to hold a person's name "tom"

Reference Code

scala> val name = "tom"
name: String = tom

scala can automatically infer the type of the variable according to the value of the variable, so that the code is more concise.

4. string

scala provides a variety of ways to define strings. In the future, we can choose the most convenient way to define according to our needs.

• use double quotes
• Use interpolation expressions
• use triple quotes

4.1 Using double quotes

grammar

val/var variable name = "string"

example

There is a person whose name is "hadoop", please print his name and the length of his name.

Reference Code

scala> println(name + name.length)
hadoop6

4.2 Using interpolation expressions

In scala, you can use interpolation expressions to define strings, effectively avoiding the concatenation of a large number of strings.

grammar

val/var variable name = s"${variable/expression}string"

Notice:

• add s before the definition string
• In strings, you can use ${} to refer to variables or write expressions

example

Please define several variables and save them separately: "zhangsan", 23, "male", define a string and save these information.

Printout: name=zhangsan, age=23, sex=male

Reference Code

scala> val name = "zhangsan"
name: String = zhangsan

scala> val age = 23
age: Int = 23

scala> val sex = "male"
sex: String = male

scala> val result = s"name=${name}, age=${age}, sex=${sex}"
result: String = name=zhangsan, age=23, sex=male

scala> println(result)
name=zhangsan, age=23, sex=male

4.3 Using triple quotes

If there is a large section of text that needs to be saved, you can use triple quotes to define the string. For example: save a large section of SQL statement. Everything between the three quotes will be the value of the string.

grammar

val/var variable name = """String 1
 string 2"""

example

Define a string to save the following SQL statement

select
	*
from
    t_user
where
    name = "zhangsan"

print the SQL statement

Reference Code

val sql = """select
     | *
     | from
     |     t_user
     | where
     |     name = "zhangsan""""

println(sql)

4.4 Extension: lazy assignment

In the big data development of enterprises, sometimes very complex SQL statements are written, and these SQL statements may have hundreds or even thousands of lines. If these SQL statements are directly loaded into the JVM, there will be a large memory overhead. How to solve this problem?

When some variables hold large data, these data do not need to be loaded into the JVM memory immediately. You can use lazy assignment to improve efficiency.

Grammar format:

lazy val/var variable name = expression

example

The following complex SQL statement needs to be executed in the program, and we hope to load it only when this SQL statement is used.

"""insert overwrite table adm.itcast_adm_personas
    select
    a.user_id,
    a.user_name,
    a.user_sex,
    a.user_birthday,
    a.user_age,
    a.constellation,
    a.province,
    a.city,
    a.city_level,
    a.hex_mail,
    a.op_mail,
    a.hex_phone,
    a.fore_phone,
    a.figure_model,
    a.stature_model,
    b.first_order_time,
    b.last_order_time,
      ...
    d.month1_hour025_cnt,
    d.month1_hour627_cnt,
    d.month1_hour829_cnt,
    d.month1_hour10212_cnt,
    d.month1_hour13214_cnt,
    d.month1_hour15217_cnt,
    d.month1_hour18219_cnt,
    d.month1_hour20221_cnt,
    d.month1_hour22223_cnt
    from gdm.itcast_gdm_user_basic a
      left join gdm.itcast_gdm_user_consume_order b on a.user_id=b.user_id
    left join gdm.itcast_gdm_user_buy_category c on a.user_id=c.user_id
    left join gdm.itcast_gdm_user_visit d on a.user_id=d.user_id;"""

Reference Code

scala> lazy val sql = """insert overwrite table adm.itcast_adm_personas
     |     select
     |     a.user_id,
	....
     |     left join gdm.itcast_gdm_user_buy_category c on a.user_id=c.user_id
     |     left join gdm.itcast_gdm_user_visit d on a.user_id=d.user_id;"""
sql: String = <lazy>

5. Identifier

5.1 Overview

​ In actual development, we will write a lot of code, and there will definitely be variables, methods, classes, etc. in these codes. How should they be named? This requires the use of identifiers. Identifiers are used to give variables and methods , class names etc. Identifiers in Scala are very similar to identifiers in Java.

5.2 Naming rules

• Must be composed of uppercase and lowercase English letters, numbers, underscore _, dollar sign $, any combination of these four parts.
• Numbers cannot begin with.
• Cannot have the same name as a keyword in Scala.
• It's best to be known.

5.3 Naming convention

• Variables or methods: Starting from the second word, the first letter of each word is capitalized, and all other letters are lowercase (small camel case).

  zhangSanAge, student_Country, getSum
  

• Class or trait (Trait): the first letter of each word is capitalized, and all other letters are all lowercase (Upper CamelCase)

  Person, StudentDemo, OrderItems
  

• Package: All lowercase, usually the company's domain name is reversed, and the multi-level packages are separated by .

  com.itheima.add,  cn.itcast.update
  

6. Data Types

6.1 Brief introduction

Data types are used to constrain the value range of variables (constants). Scala is also a strongly typed language, and most of the data types in it are the same as Java. We mainly learn

• Some usages that are different from Java
• Inheritance system of data types in scala

6.2 Data Types

Note the difference between the scala type and Java

[!NOTE]

1. All types in scala start with an uppercase letter
2. Use Int instead of Integer for shaping
3. You can define variables in scala without writing the type, and let the scala compiler automatically infer
4. The default integer type in Scala is Int, and the default floating point type is: Double

6.3 The Scala Type Hierarchy

6.4 Thinking questions

Is there something wrong with the following code?

val b:Int = null

Scala will explain the error: Null type cannot be converted to Int type, indicating that Null type is not a subclass of Int type

7. Type conversion

7.1 Overview

When a Scala program is performing operations or assignments, the data type values ​​with a small range will be automatically converted to data type values ​​with a large range, and then calculated. For example: 1 + 1.1 The result of the operation is a Double type of 2.1. And some Sometimes, we will involve some actions similar to "rounding". We need to convert a decimal to an integer before calculating. These are the type conversions in Scala.

Type conversion in Scala is divided into type conversion of value type and type conversion of reference type. Here we will focus on: type conversion of value type.

Type conversion of value types is divided into:

• automatic type conversion
• cast

7.2 Automatic type conversion

1. Explanation

   A data type value with a small range will be automatically converted to a data type value with a large range. This action is called: automatic type conversion.

   Automatic type conversion from small to large: Byte, Short, Char -> Int -> Long -> Float -> Double

2. sample code

   val a:Int = 3
   val b:Double = 3 + 2.21	//Because the calculation is performed on the values ​​of int type and double type, the final result is: Double type
   val c:Byte = a + 1	//Writing in this way will cause an error, because the final calculation result is data of type Int, and it will definitely not work to assign it to type Byte.
   

7.3 Mandatory type conversion

1. Explanation

   A data type value with a large range can be converted into a data type value with a small range through a certain format (coercive conversion function). This action is called: forced type conversion.

   Note: When using mandatory type conversion, it may cause loss of precision!

2. Format

val/var variable name:type of data = specific value.toXxx		//Xxx represents the data type you want to convert to

1. Reference Code

val a:Double = 5.21
val b:Int = a.toInt

7.4 Mutual conversion between value type and String type

1. Data of value type is converted to String type

Format one:

val/var variable name:String = value type data + ""

Format two:

val/var variable name:String = value type data.toString

example

Convert data of type Int, Double, Boolean to its corresponding string form.

Reference Code:

val a1:Int = 10
val b1:Double = 2.1
val c1:Boolean = true

//Method 1: Realized by concatenating with an empty string
val a2:String = a1 + ""
val b2:String = b1 + ""
val c2:String = c1 + ""

//Method 2: Realized by the toString function
val a3:String = a1.toString
val b3:String = b1.toString
val c3:String = c1.toString

2. String type data is converted into its corresponding value type

Format:

val/var variable name:value type = string value.toXxx	//Xxx represents the data type you want to convert to

Notice:

• The data of String type is converted into data of Char type, the method is a bit special, instead of calling toChar, but toCharArray
• This point can be understood for now, and we will explain it in detail later

need:

​ Convert string-type integers, floating-point numbers, and Boolean data into their corresponding value type data.

Reference Code:

val s1:String = "100"
val s2:String = "2.3"
val s3:String = "false"

//Convert string type data to its corresponding: Int type
val a:Int = s1.toInt
//Convert string type data to its corresponding: Double type
val b:Double = s2.toDouble
//Convert string type data to its corresponding: Boolean type
val c:Boolean = s3.toBoolean

8. Keyboard entry

8.1 Overview

The data we mentioned above are all "dead" and fixed data written by us, so the user experience is not particularly good. If these data are entered by users and then we receive them through code, it will be very fun. This is the "keyboard entry" function in Scala that we will learn next.

8.2 Steps to use

1. Guide package

   Format: import scala.io.StdIn

2. Receive the data entered by the user's keyboard through StdIn.readXxx()

   Receive string data: StdIn.readLine()

   Receive integer data: StdIn.readInt()

8.3 Examples

• Prompt the user to enter a string, and receive the printout.

  println("Please enter a string: ")
  val str = StdIn.readLine()
  println("The content of the string you entered is: " + str)
  

• Prompt the user to enter an integer, and receive the printout.

  println("Please enter an integer: ")
  val num = StdIn.readInt()
  println("The number you entered is: " + num)
  

9. Case: Greeting

9.1 Overview

After chatting for so long, hurry up and say hello to your friends.

Requirements: Prompt user to enter his/her name and age, receive and print.

9.2 Specific steps

1. Prompt the user to enter a name.
2. Receive the name entered by the user.
3. Prompt the user to enter their age.
4. Receive the age entered by the user.
5. Print the data entered by the user (name and age) to the console.

9.3 Reference code

//1. Prompt the user to enter their name.
println("Please enter your name: ")
//2. Receive the name entered by the user.
val name = StdIn.readLine()
//3. Prompt the user to enter their age.
println("Please enter your age: ")
//4. Receive the age entered by the user.
val age = StdIn.readInt()
//5. Print the data entered by the user (name and age) to the console.
println(s"Hello everyone, My name is ${name}, this year, I ${age}Years old, It's a pleasure to study with you Scala!")
for: " + str)

• Prompt the user to enter an integer, and receive the printout.

  println("Please enter an integer: ")
  val num = StdIn.readInt()
  println("The number you entered is: " + num)
  

9. Case: Greeting

9.1 Overview

After chatting for so long, hurry up and say hello to your friends.

Requirements: Prompt user to enter his/her name and age, receive and print.

9.2 Specific steps

1. Prompt the user to enter a name.
2. Receive the name entered by the user.
3. Prompt the user to enter their age.
4. Receive the age entered by the user.
5. Print the data entered by the user (name and age) to the console.

9.3 Reference code

//1. Prompt the user to enter their name.
println("Please enter your name: ")
//2. Receive the name entered by the user.
val name = StdIn.readLine()
//3. Prompt the user to enter their age.
println("Please enter your age: ")
//4. Receive the age entered by the user.
val age = StdIn.readInt()
//5. Print the data entered by the user (name and age) to the console.
println(s"Hello everyone, My name is ${name}, this year, I ${age}Years old, It's a pleasure to study with you Scala!")