Kotlin Data Types

Kotlin 数据类型

In Kotlin, the type of a variable is decided by its value

val myNum = 5             // Int
val myDoubleNum = 5.99    // Double
val myLetter = 'D'        // Char
val myBoolean = true      // Boolean
val myText = "Hello"      // String

However, you learned from the previous chapter that it is possible to specify the type if you want

val myNum: Int = 5                // Int
val myDoubleNum: Double = 5.99    // Double
val myLetter: Char = 'D'          // Char
val myBoolean: Boolean = true     // Boolean
val myText: String = "Hello"      // String

Sometimes you have to specify the type, and often you don't. Anyhow, it is good to know what the different types represent.

You will learn more about when you need to specify the type later.

Data types are divided into different groups

• 数字
• 字符
• 布尔值
• 字符串
• 数组

数字

Number types are divided into two groups

Integer types store whole numbers, positive or negative (such as 123 or -456), without decimals. Valid types are Byte, Short, Int and Long.

Floating point types represent numbers with a fractional part, containing one or more decimals. There are two types: Float and Double.

整数类型

Byte

The Byte data type can store whole numbers from -128 to 127. This can be used instead of Int or other integer types to save memory when you are certain that the value will be within -128 and 127

val myNum: Byte = 100
println(myNum)

Short

The Short data type can store whole numbers from -32768 to 32767

val myNum: Short = 5000
println(myNum)

Int

The Int data type can store whole numbers from -2147483648 to 2147483647

val myNum: Int = 100000
println(myNum)

Long

The Long data type can store whole numbers from -9223372036854775808 to 9223372036854775807. This is used when Int is not large enough to store the value. Optionally, you can end the value with an "L"

val myNum: Long = 15000000000L
println(myNum)

Difference Between Int and Long

A whole number is an Int as long as it is up to 2147483647. If it goes beyond that, it is defined as Long

val myNum1 = 2147483647  // Int
val myNum2 = 2147483648  // Long

浮点数类型

Floating point types represent numbers with a decimal, such as 9.99 or 3.14515.

The Float and Double data types can store fractional numbers

val myNum: Float = 5.75F
println(myNum)

val myNum: Double = 19.99
println(myNum)

科学计数法

A floating point number can also be a scientific number with an "e" or "E" to indicate the power of 10

val myNum1: Float = 35E3F
val myNum2: Double = 12E4
println(myNum1)
println(myNum2)

布尔值

The Boolean data type and can only take the values true or false

val isKotlinFun: Boolean = true
val isFishTasty: Boolean = false
println(isKotlinFun)   // Outputs true
println(isFishTasty)   // Outputs false 

布尔值主要用于条件测试，你将在后面的章节中了解更多。

字符

The Char data type is used to store a single character. A char value must be surrounded by single quotes, like 'A' or 'c'

val myGrade: Char = 'B'
println(myGrade)

Unlike Java, you cannot use ASCII values to display certain characters. The value 66 would output a "B" in Java, but will generate an error in Kotlin

val myLetter: Char = 66
println(myLetter) // Error

字符串

The String data type is used to store a sequence of characters (text). String values must be surrounded by double quotes

val myText: String = "Hello World"
println(myText)

You will learn more about strings in the Strings chapter.

数组

数组用于在单个变量中存储多个值，而不是为每个值声明单独的变量。

You will learn more about arrays in the Arrays chapter.

类型转换

Type conversion is when you convert the value of one data type to another type.

In Kotlin, numeric type conversion is different from Java. For example, it is not possible to convert an Int type to a Long type with the following code

val x: Int = 5
val y: Long = x
println(y) // Error: Type mismatch 

To convert a numeric data type to another type, you must use one of the following functions: toByte(), toShort(), toInt(), toLong(), toFloat(), toDouble() or toChar()

val x: Int = 5
val y: Long = x.toLong()
println(y)