Scala learning day 1: Variables

Learning objectives

Grammatical format
 Define a variable in the interpreter
 val and var variables
 Use type inference to define variables
 Lazy assignment

Grammatical format

Java variable definition

int a = 0;

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

val/var variable ID: variable type = initial value

among

	val defines variables that cannot be reassigned
	var defines a variable that can be reassigned

Note:

The variable type defined in scala is written after the variable name
 scala statements don't need semicolons at the end

Define a variable in the interpreter

Example: define a variable to hold a person's name "tom"
Steps:

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

Reference code

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

val and var variables

Example

Reassign the name variable to Jim and observe its operation results

Reference code

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

It can be seen from the result that name cannot be reassigned to "Jim" because val variable cannot be reassigned

Example

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

Reference code

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

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

Note:

Use 'val' first to define variables. If the variables need to be reassigned, use 'var`

Use type inference to define variables

scala's syntax is simpler than Java's. We can define variables in a more concise way

Example

Define a variable in a more concise way to save a person's name "tom"

Reference code

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

Note:

scala can automatically infer the type of variable according to the value of variable, which makes the code more concise

Lazy assignment

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

When some variables hold large data, but do not need to be loaded into the JVM memory immediately, lazy assignment can be used to improve efficiency

Grammatical format

lazy val/var variable name = expression

Example

We need to execute the following complex SQL statement in the program. We hope to load it only when we use this SQL statement.

"""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>