Kotlin Variables

Kotlin 变量

Variables are containers for storing data values.

To create a variable, use var 或者val, and assign a value to it with the equal sign (=):

var variableName = value
val variableName = value

var name = "John"
val birthyear = 1975

println(name)          // Print the value of name
println(birthyear)     // Print the value of birthyear

The difference between var and val is that variables declared with the var keyword can be changed/modified, while val variables cannot.

Variable Type

Unlike many other programming languages, variables in Kotlin do not need to be declared with a specified type (like "String" for text or "Int" for numbers, if you are familiar with those).

To create a variable in Kotlin that should store text and another that should store a number, look at the following example:

var name = "John"      // String (text)
val birthyear = 1975   // Int (number)

println(name)          // Print the value of name
println(birthyear)     // Print the value of birthyear

Kotlin is smart enough to understand that "John" is a String (text), and that 1975 is an Int (number) variable.

However, it is possible to specify the type if you insist:

var name: String = "John" // String
val birthyear: Int = 1975 // Int

println(name)
println(birthyear)

You can also declare a variable without assigning the value, and assign the value later. However, this is only possible when you specify the type:

var name: String
name = "John"
println(name)

var name
name = "John"
println(name)

Notes on val

When you create a variable with the val keyword, the value cannot be changed/reassigned.

The following example will generate an error:

val name = "John"
name = "Robert"  // Error (Val cannot be reassigned)
println(name)

When using var, you can change the value whenever you want:

var name = "John"
name = "Robert"
println(name)

val pi = 3.14159265359
println(pi)

Display Variables

Like you have seen with the examples above, the println() method is often used to display variables.

To combine both text and a variable, use the + character:

val name = "John"
println("Hello " + name)

You can also use the + character to add a variable to another variable:

val firstName = "John "
val lastName = "Doe"
val fullName = firstName + lastName
println(fullName)

For numeric values, the + character works as a mathematical operator:

val x = 5
val y = 6
println(x + y) // Print the value of x + y 

From the example above, you can expect:

• x stores value 5
• y stores value 6
• Then we use the println() method to display the value of x + y, which is 11

Variable Names

A variable can have a short name (like x and y) or more descriptive names (age, sum, totalVolume).

The general rule for Kotlin variables are:

• Names can contain letters, digits, underscores, and dollar signs
• Names should start with a letter
• Names can also begin with $ and _ (but we will not use it in this tutorial)
• Names are case sensitive ("myVar" and "myvar" are different variables)
• Names should start with a lowercase letter and it cannot contain whitespace
• Reserved words (like Kotlin keywords, such as var 或者String) cannot be used as names