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1. Variable declaration
(1) Brief description
Variables are declared in Scala using the keywords val and var. val is similar to a final variable in Java, that is, a constant, which cannot be modified once initialized; var is similar to a non-final variable in Java, which can be assigned and modified multiple times.
val - value - value
var - variable - variable
(2) Use val to declare variables
1. Declaration method
val <variable name>[: data type] = variable value It
is more flexible to define variables, you can specify the data type, or you can not specify the data type, and Scala can judge the data type by itself according to the assigned value. In declaring variables,
Scala has both Java and Features of Python. Java is a static language, declaring variables must specify the data type, Python is a dynamic language, declaring variables does not need to specify the data type.
2. Case demonstration
(3) Use var to declare variables
1. Declaration method
var <variable name>[:datatype] = variable value
2. Case demonstration
(4) Newline input statement (continued line)
If you want to enter a multi-line string, it is impossible to use double quotes, you have to use triple quotes
(5) Declare multiple variables at the same time
Scala cannot assign different values to multiple variables at once
(6) Summary variable declaration
1. Variables must be initialized
2. Define variables without specifying the data type
3. Encourage the priority use of val (constant)
Scala encourages the use of val (constant) in preference to var unless it is really necessary to modify it.
4. The statement does not need to write the terminator
2. Data type
(1) Scala type hierarchy
1. Any type
Any is the root of the Scala class hierarchy, also known as a superclass or top-level class. Every class in the Scala execution environment inherits from this class, directly or indirectly. Some common methods are defined in this class, such as equals(), hashCode() and toString(). Any has two direct subclasses: AnyVal and AnyRef
2. AnyVal type
AnyVal represents a value type, and there are 9 predefined value types, which are non-empty Double, Float, Long, Int, Short, Byte, Char, Unit, and Boolean. Unit is a value type that does not contain any information, equivalent to void in the Java language, and is used as the result type of a method that does not return any results. Unit has only one instance value, written as ().
3. AnyRef type
AnyRef represents a reference type. All non-value types are defined as reference types. Every user-defined type in Scala is a subtype of AnyRef. AnyRef corresponds to java.lang.Object in Java.
4. Case demonstration
Define a variable list whose type is List[Any]. The list includes strings, integers, characters, Boolean values and functions. Since these elements belong to the instance of the object Any, they can be added to the list.
Through the enhanced for loop to traverse the list elements and display their data types
scala> list.foreach(x => println(x + ": " x.getClass.getSimpleName))
love:String
100:Integer
66.6:Double
c:Charater
true: Bootlean
<function0>: anonfun$1
Get the sublist of the first three elements of the list
scala> list.reverse
res6:List[Any] = List(<function0>, true, c, 66.6, 100, love)
scala>List
res7: List[Any] = List(love, 100, 66.6, c, true, <function0>)
(2) Basic data types
1. Basic type table
2. Brief description
(1) String is under the java.lang package, and other types are under the scala package. Since Scala will automatically import java.lang and scala packages, these types can be used directly in the program.
(2) In fact, there is no real basic type in scala (such as the basic type of Java), and the above ten basic types are all classes.
(3) In Scala, the difference between the basic data type and other classes is that its objects are embodied in the form of direct quantity, which is basically the same as that in Java.
(4) Usage of """ of the String type: the syntax of """...""" can be used in the String literal to directly include a piece of content, so that the content can contain any character without escaping. At this time You can use the pipe character | to control the indentation format before each line, and call stripMargin on the string to align the string at |.Use
.stripMargin and the pipe character "|" to adjust the string output format
3. Basic types of rich wrappers
The methods provided by the nine basic types are relatively limited. In order to expand its functions, Scala provides corresponding nine rich wrappers, which are actually nine classes. The literals of these nine classes can be automatically converted to their corresponding rich wrappers when needed. Wrapper classes to call extra methods provided by rich wrapper classes.
(3) Data type conversion
1. Conversion of value types
Value types in Scala can be converted as shown in the figure below, and the conversion is one-way.
Convert Long to Float. Note that some precision will be lost in this case.
If you convert Float to Long, an error will be reported.
It cannot be automatically converted from Float type to Long type, but it can be implemented by calling a method
2. Conversion of reference types
Additionally, Scala can convert reference types to their subtypes.
Nothing is a subclass of all types, similar to the identifying interface in Java (such as serializable, which is used to indicate that the class can be serialized). For example, if a method throws an exception, the return value type of the exception is Nothing (although will not return).
Null is a subclass of all reference types (AnyRef), so Null can be assigned to all reference types, but not to value types. This is the same semantics as Java. Null has a unique single column value of null.
