3.7 语法与标识符

我们知道，任何一门编程语言都会有一些自己专用的关键字、符号以及规定的语法规则等等。程序员们使用这些基础词汇和语法规则来表达算法步骤，也就是写代码的过程。

词法分析是编译器对源码进行编译的基础步骤之一。词法分析是将源程序读入的字符序列，按照一定的规则转换成词法单元（Token）序列的过程。词法单元是语言中具有独立意义的最小单元，包括修饰符、关键字、常数、运算符、边界符等等。

3.7.1 修饰符

在Kotlin源码工程中的kotlin/grammar/src/modifiers.grm文件中，描述了Kotlin语言的修饰符，我们在此作简要注释说明：

/**
## Modifiers
*/
modifiers
  : (modifier | annotations)*
  ;
typeModifiers
  : (suspendModifier | annotations)*
  ;
modifier
  : classModifier
  : accessModifier
  : varianceAnnotation
  : memberModifier
  : parameterModifier
  : typeParameterModifier
  : functionModifier
  : propertyModifier
  ;
classModifier 类修饰符
  : "abstract" 抽象类
  : "final" 不可被继承final类
  : "enum" 枚举类
  : "open" 可继承open类
  : "annotation" 注解类
  : "sealed" 密封类
  : "data" 数据类
  ;
memberModifier
  : "override" 重写函数
  : "open" 可被重写
  : "final" 不可被重写
  : "abstract" 抽象函数
  : "lateinit" 后期初始化
  ;
accessModifier 访问权限控制, 默认是public
  : "private"
  : "protected"
  : "public"
  : "internal"  整个模块内（模块(module)是指一起编译的一组 Kotlin 源代码文件: 例如，一个 IntelliJ IDEA 模块，一个 Maven 工程, 或 Gradle 工程，通过 Ant 任务的一次调用编译的一组文件等）可访问
  ;
varianceAnnotation 泛型可变性
  : "in" 
  : "out"
  ;
parameterModifier
  : "noinline" 
  : "crossinline" 
  : "vararg" 变长参数
  ;
typeParameterModifier
  : "reified"
  ;
functionModifier
  : "tailrec" 尾递归
  : "operator"
  : "infix"
  : "inline"
  : "external"
  : suspendModifier
  ;
propertyModifier
  : "const" 
  ;
suspendModifier
  : "suspend"
  ;

这些修饰符的完整定义，在kotlin/compiler/frontend/src/org/jetbrains/kotlin/lexer/KtTokens.java源码中：

KtModifierKeywordToken[] MODIFIER_KEYWORDS_ARRAY =
            new KtModifierKeywordToken[] {
                    ABSTRACT_KEYWORD, ENUM_KEYWORD, OPEN_KEYWORD, INNER_KEYWORD, OVERRIDE_KEYWORD, PRIVATE_KEYWORD,
                    PUBLIC_KEYWORD, INTERNAL_KEYWORD, PROTECTED_KEYWORD, OUT_KEYWORD, IN_KEYWORD, FINAL_KEYWORD, VARARG_KEYWORD,
                    REIFIED_KEYWORD, COMPANION_KEYWORD, SEALED_KEYWORD, LATEINIT_KEYWORD,
                    DATA_KEYWORD, INLINE_KEYWORD, NOINLINE_KEYWORD, TAILREC_KEYWORD, EXTERNAL_KEYWORD, ANNOTATION_KEYWORD, CROSSINLINE_KEYWORD,
                    CONST_KEYWORD, OPERATOR_KEYWORD, INFIX_KEYWORD, SUSPEND_KEYWORD, HEADER_KEYWORD, IMPL_KEYWORD
            };
    TokenSet MODIFIER_KEYWORDS = TokenSet.create(MODIFIER_KEYWORDS_ARRAY);
    TokenSet TYPE_MODIFIER_KEYWORDS = TokenSet.create(SUSPEND_KEYWORD);
    TokenSet TYPE_ARGUMENT_MODIFIER_KEYWORDS = TokenSet.create(IN_KEYWORD, OUT_KEYWORD);
    TokenSet RESERVED_VALUE_PARAMETER_MODIFIER_KEYWORDS = TokenSet.create(OUT_KEYWORD, VARARG_KEYWORD);
    TokenSet VISIBILITY_MODIFIERS = TokenSet.create(PRIVATE_KEYWORD, PUBLIC_KEYWORD, INTERNAL_KEYWORD, PROTECTED_KEYWORD);

3.7.2 关键字(保留字)

TokenSet KEYWORDS = TokenSet.create(PACKAGE_KEYWORD, AS_KEYWORD, TYPE_ALIAS_KEYWORD, CLASS_KEYWORD, INTERFACE_KEYWORD,
                                        THIS_KEYWORD, SUPER_KEYWORD, VAL_KEYWORD, VAR_KEYWORD, FUN_KEYWORD, FOR_KEYWORD,
                                        NULL_KEYWORD,
                                        TRUE_KEYWORD, FALSE_KEYWORD, IS_KEYWORD,
                                        IN_KEYWORD, THROW_KEYWORD, RETURN_KEYWORD, BREAK_KEYWORD, CONTINUE_KEYWORD, OBJECT_KEYWORD, IF_KEYWORD,
                                        ELSE_KEYWORD, WHILE_KEYWORD, DO_KEYWORD, TRY_KEYWORD, WHEN_KEYWORD,
                                        NOT_IN, NOT_IS, AS_SAFE,
                                        TYPEOF_KEYWORD
    );
    TokenSet SOFT_KEYWORDS = TokenSet.create(FILE_KEYWORD, IMPORT_KEYWORD, WHERE_KEYWORD, BY_KEYWORD, GET_KEYWORD,
                                             SET_KEYWORD, ABSTRACT_KEYWORD, ENUM_KEYWORD, OPEN_KEYWORD, INNER_KEYWORD,
                                             OVERRIDE_KEYWORD, PRIVATE_KEYWORD, PUBLIC_KEYWORD, INTERNAL_KEYWORD, PROTECTED_KEYWORD,
                                             CATCH_KEYWORD, FINALLY_KEYWORD, OUT_KEYWORD, FINAL_KEYWORD, VARARG_KEYWORD, REIFIED_KEYWORD,
                                             DYNAMIC_KEYWORD, COMPANION_KEYWORD, CONSTRUCTOR_KEYWORD, INIT_KEYWORD, SEALED_KEYWORD,
                                             FIELD_KEYWORD, PROPERTY_KEYWORD, RECEIVER_KEYWORD, PARAM_KEYWORD, SETPARAM_KEYWORD,
                                             DELEGATE_KEYWORD,
                                             LATEINIT_KEYWORD,
                                             DATA_KEYWORD, INLINE_KEYWORD, NOINLINE_KEYWORD, TAILREC_KEYWORD, EXTERNAL_KEYWORD,
                                             ANNOTATION_KEYWORD, CROSSINLINE_KEYWORD, CONST_KEYWORD, OPERATOR_KEYWORD, INFIX_KEYWORD,
                                             SUSPEND_KEYWORD, HEADER_KEYWORD, IMPL_KEYWORD
    );

其中，对应的关键字如下：

KtKeywordToken PACKAGE_KEYWORD          = KtKeywordToken.keyword("package");
KtKeywordToken AS_KEYWORD               = KtKeywordToken.keyword("as");
KtKeywordToken TYPE_ALIAS_KEYWORD       = KtKeywordToken.keyword("typealias");
KtKeywordToken CLASS_KEYWORD            = KtKeywordToken.keyword("class");
KtKeywordToken THIS_KEYWORD             = KtKeywordToken.keyword("this");
KtKeywordToken SUPER_KEYWORD            = KtKeywordToken.keyword("super");
KtKeywordToken VAL_KEYWORD              = KtKeywordToken.keyword("val");
KtKeywordToken VAR_KEYWORD              = KtKeywordToken.keyword("var");
KtKeywordToken FUN_KEYWORD              = KtKeywordToken.keyword("fun");
KtKeywordToken FOR_KEYWORD              = KtKeywordToken.keyword("for");
KtKeywordToken NULL_KEYWORD             = KtKeywordToken.keyword("null");
KtKeywordToken TRUE_KEYWORD             = KtKeywordToken.keyword("true");
KtKeywordToken FALSE_KEYWORD            = KtKeywordToken.keyword("false");
KtKeywordToken IS_KEYWORD               = KtKeywordToken.keyword("is");
KtModifierKeywordToken IN_KEYWORD       = KtModifierKeywordToken.keywordModifier("in");
KtKeywordToken THROW_KEYWORD            = KtKeywordToken.keyword("throw");
KtKeywordToken RETURN_KEYWORD           = KtKeywordToken.keyword("return");
KtKeywordToken BREAK_KEYWORD            = KtKeywordToken.keyword("break");
KtKeywordToken CONTINUE_KEYWORD         = KtKeywordToken.keyword("continue");
KtKeywordToken OBJECT_KEYWORD           = KtKeywordToken.keyword("object");
KtKeywordToken IF_KEYWORD               = KtKeywordToken.keyword("if");
KtKeywordToken TRY_KEYWORD              = KtKeywordToken.keyword("try");
KtKeywordToken ELSE_KEYWORD             = KtKeywordToken.keyword("else");
KtKeywordToken WHILE_KEYWORD            = KtKeywordToken.keyword("while");
KtKeywordToken DO_KEYWORD               = KtKeywordToken.keyword("do");
KtKeywordToken WHEN_KEYWORD             = KtKeywordToken.keyword("when");
KtKeywordToken INTERFACE_KEYWORD        = KtKeywordToken.keyword("interface");
// Reserved for future use:
KtKeywordToken TYPEOF_KEYWORD           = KtKeywordToken.keyword("typeof");
...
KtKeywordToken FILE_KEYWORD    = KtKeywordToken.softKeyword("file");
KtKeywordToken FIELD_KEYWORD     = KtKeywordToken.softKeyword("field");
KtKeywordToken PROPERTY_KEYWORD     = KtKeywordToken.softKeyword("property");
KtKeywordToken RECEIVER_KEYWORD     = KtKeywordToken.softKeyword("receiver");
KtKeywordToken PARAM_KEYWORD     = KtKeywordToken.softKeyword("param");
KtKeywordToken SETPARAM_KEYWORD  = KtKeywordToken.softKeyword("setparam");
KtKeywordToken DELEGATE_KEYWORD  = KtKeywordToken.softKeyword("delegate");
KtKeywordToken IMPORT_KEYWORD    = KtKeywordToken.softKeyword("import");
KtKeywordToken WHERE_KEYWORD     = KtKeywordToken.softKeyword("where");
KtKeywordToken BY_KEYWORD        = KtKeywordToken.softKeyword("by");
KtKeywordToken GET_KEYWORD       = KtKeywordToken.softKeyword("get");
KtKeywordToken SET_KEYWORD       = KtKeywordToken.softKeyword("set");
KtKeywordToken CONSTRUCTOR_KEYWORD = KtKeywordToken.softKeyword("constructor");
KtKeywordToken INIT_KEYWORD        = KtKeywordToken.softKeyword("init");
KtModifierKeywordToken ABSTRACT_KEYWORD  = KtModifierKeywordToken.softKeywordModifier("abstract");
KtModifierKeywordToken ENUM_KEYWORD      = KtModifierKeywordToken.softKeywordModifier("enum");
KtModifierKeywordToken OPEN_KEYWORD      = KtModifierKeywordToken.softKeywordModifier("open");
KtModifierKeywordToken INNER_KEYWORD     = KtModifierKeywordToken.softKeywordModifier("inner");
KtModifierKeywordToken OVERRIDE_KEYWORD  = KtModifierKeywordToken.softKeywordModifier("override");
KtModifierKeywordToken PRIVATE_KEYWORD   = KtModifierKeywordToken.softKeywordModifier("private");
KtModifierKeywordToken PUBLIC_KEYWORD    = KtModifierKeywordToken.softKeywordModifier("public");
KtModifierKeywordToken INTERNAL_KEYWORD  = KtModifierKeywordToken.softKeywordModifier("internal");
KtModifierKeywordToken PROTECTED_KEYWORD = KtModifierKeywordToken.softKeywordModifier("protected");
KtKeywordToken CATCH_KEYWORD     = KtKeywordToken.softKeyword("catch");
KtModifierKeywordToken OUT_KEYWORD       = KtModifierKeywordToken.softKeywordModifier("out");
KtModifierKeywordToken VARARG_KEYWORD    = KtModifierKeywordToken.softKeywordModifier("vararg");
KtModifierKeywordToken REIFIED_KEYWORD   = KtModifierKeywordToken.softKeywordModifier("reified");
KtKeywordToken DYNAMIC_KEYWORD   = KtKeywordToken.softKeyword("dynamic");
KtModifierKeywordToken COMPANION_KEYWORD = KtModifierKeywordToken.softKeywordModifier("companion");
KtModifierKeywordToken SEALED_KEYWORD    = KtModifierKeywordToken.softKeywordModifier("sealed");
KtModifierKeywordToken DEFAULT_VISIBILITY_KEYWORD = PUBLIC_KEYWORD;
KtKeywordToken FINALLY_KEYWORD   = KtKeywordToken.softKeyword("finally");
KtModifierKeywordToken FINAL_KEYWORD     = KtModifierKeywordToken.softKeywordModifier("final");
KtModifierKeywordToken LATEINIT_KEYWORD = KtModifierKeywordToken.softKeywordModifier("lateinit");
KtModifierKeywordToken DATA_KEYWORD    = KtModifierKeywordToken.softKeywordModifier("data");
KtModifierKeywordToken INLINE_KEYWORD    = KtModifierKeywordToken.softKeywordModifier("inline");
KtModifierKeywordToken NOINLINE_KEYWORD    = KtModifierKeywordToken.softKeywordModifier("noinline");
KtModifierKeywordToken TAILREC_KEYWORD    = KtModifierKeywordToken.softKeywordModifier("tailrec");
KtModifierKeywordToken EXTERNAL_KEYWORD    = KtModifierKeywordToken.softKeywordModifier("external");
KtModifierKeywordToken ANNOTATION_KEYWORD    = KtModifierKeywordToken.softKeywordModifier("annotation");
KtModifierKeywordToken CROSSINLINE_KEYWORD    = KtModifierKeywordToken.softKeywordModifier("crossinline");
KtModifierKeywordToken OPERATOR_KEYWORD = KtModifierKeywordToken.softKeywordModifier("operator");
KtModifierKeywordToken INFIX_KEYWORD = KtModifierKeywordToken.softKeywordModifier("infix");
KtModifierKeywordToken CONST_KEYWORD = KtModifierKeywordToken.softKeywordModifier("const");
KtModifierKeywordToken SUSPEND_KEYWORD = KtModifierKeywordToken.softKeywordModifier("suspend");
KtModifierKeywordToken HEADER_KEYWORD = KtModifierKeywordToken.softKeywordModifier("header");
KtModifierKeywordToken IMPL_KEYWORD = KtModifierKeywordToken.softKeywordModifier("impl");

this 关键字

this关键字持有当前对象的引用。我们可以使用this来引用变量或者成员函数，也可以使用return this，来返回某个类的引用。

代码示例

class ThisDemo {
    val thisis = "THIS IS"
    fun whatIsThis(): ThisDemo {
        println(this.thisis) //引用变量
        this.howIsThis()// 引用成员函数
        return this // 返回此类的引用
    }
    fun howIsThis(){
        println("HOW IS THIS ?")
    }
}

测试代码

@Test
fun testThisDemo(){
    val demo = ThisDemo()
    println(demo.whatIsThis())
}

输出

THIS IS
HOW IS THIS ?
com.easy.kotlin.ThisDemo@475232fc

在类的成员中，this 指向的是该类的当前对象。

在扩展函数或者带接收者的函数字面值中， this 表示在点左侧传递的 接收者参数。

代码示例：

>>> val sum = fun Int.(x:Int):Int = this + x
>>> sum
kotlin.Int.(kotlin.Int) -> kotlin.Int
>>> 1.sum(1)
2
>>> val concat = fun String.(x:Any) = this + x
>>> "abc".concat(123)
abc123
>>> "abc".concat(true)
abctrue

如果 this 没有限定符，它指的是最内层的包含它的作用域。如果我们想要引用其他作用域中的 this，可以使用 this@label 标签。

代码示例：

class Outer {
    val oh = "Oh!"
    inner class Inner {
        fun m() {
            val outer = this@Outer
            val inner = this@Inner
            val pthis = this
            println("outer=" + outer)
            println("inner=" + inner)
            println("pthis=" + pthis)
            println(this@Outer.oh)
            val fun1 = hello@ fun String.() {
                val d1 = this // fun1 的接收者
                println("d1" + d1)
            }
            val fun2 = { s: String ->
                val d2 = this
                println("d2=" + d2)
            }
            "abc".fun1()
            fun2
        }
    }
}

测试代码：

@Test
fun testThisKeyWord() {
    val outer = Outer()
    outer.Inner().m()
}

输出

outer=com.easy.kotlin.Outer@5114e183
inner=com.easy.kotlin.Outer$Inner@5aa8ac7f
pthis=com.easy.kotlin.Outer$Inner@5aa8ac7f
Oh!
d1abc

super 关键字

super关键字持有指向其父类的引用。

代码示例：

open class Father {
    open val firstName = "Chen"
    open val lastName = "Jason"
    fun ff() {
        println("FFF")
    }
}
class Son : Father {
    override var firstName = super.firstName
    override var lastName = "Jack"
    constructor(lastName: String) {
        this.lastName = lastName
    }
    fun love() {
        super.ff() // 调用父类方法
        println(super.firstName + " " + super.lastName + " Love " + this.firstName + " " + this.lastName)
    }
}

测试代码

@Test
fun testSuperKeyWord() {
    val son = Son("Harry")
    son.love()
}

输出

FFF
Chen Jason Love Chen Harry

3.7.3 操作符和操作符的重载

Kotlin 允许我们为自己的类型提供预定义的一组操作符的实现。这些操作符具有固定的符号表示（如 + 或 *）和固定的优先级。这些操作符的符号定义如下：

KtSingleValueToken LBRACKET    = new KtSingleValueToken("LBRACKET", "[");
KtSingleValueToken RBRACKET    = new KtSingleValueToken("RBRACKET", "]");
KtSingleValueToken LBRACE      = new KtSingleValueToken("LBRACE", "{");
KtSingleValueToken RBRACE      = new KtSingleValueToken("RBRACE", "}");
KtSingleValueToken LPAR        = new KtSingleValueToken("LPAR", "(");
KtSingleValueToken RPAR        = new KtSingleValueToken("RPAR", ")");
KtSingleValueToken DOT         = new KtSingleValueToken("DOT", ".");
KtSingleValueToken PLUSPLUS    = new KtSingleValueToken("PLUSPLUS", "++");
KtSingleValueToken MINUSMINUS  = new KtSingleValueToken("MINUSMINUS", "--");
KtSingleValueToken MUL         = new KtSingleValueToken("MUL", "*");
KtSingleValueToken PLUS        = new KtSingleValueToken("PLUS", "+");
KtSingleValueToken MINUS       = new KtSingleValueToken("MINUS", "-");
KtSingleValueToken EXCL        = new KtSingleValueToken("EXCL", "!");
KtSingleValueToken DIV         = new KtSingleValueToken("DIV", "/");
KtSingleValueToken PERC        = new KtSingleValueToken("PERC", "%");
KtSingleValueToken LT          = new KtSingleValueToken("LT", "<");
KtSingleValueToken GT          = new KtSingleValueToken("GT", ">");
KtSingleValueToken LTEQ        = new KtSingleValueToken("LTEQ", "<=");
KtSingleValueToken GTEQ        = new KtSingleValueToken("GTEQ", ">=");
KtSingleValueToken EQEQEQ      = new KtSingleValueToken("EQEQEQ", "===");
KtSingleValueToken ARROW       = new KtSingleValueToken("ARROW", "->");
KtSingleValueToken DOUBLE_ARROW       = new KtSingleValueToken("DOUBLE_ARROW", "=>");
KtSingleValueToken EXCLEQEQEQ  = new KtSingleValueToken("EXCLEQEQEQ", "!==");
KtSingleValueToken EQEQ        = new KtSingleValueToken("EQEQ", "==");
KtSingleValueToken EXCLEQ      = new KtSingleValueToken("EXCLEQ", "!=");
KtSingleValueToken EXCLEXCL    = new KtSingleValueToken("EXCLEXCL", "!!");
KtSingleValueToken ANDAND      = new KtSingleValueToken("ANDAND", "&&");
KtSingleValueToken OROR        = new KtSingleValueToken("OROR", "||");
KtSingleValueToken SAFE_ACCESS = new KtSingleValueToken("SAFE_ACCESS", "?.");
KtSingleValueToken ELVIS       = new KtSingleValueToken("ELVIS", "?:");
KtSingleValueToken QUEST       = new KtSingleValueToken("QUEST", "?");
KtSingleValueToken COLONCOLON  = new KtSingleValueToken("COLONCOLON", "::");
KtSingleValueToken COLON       = new KtSingleValueToken("COLON", ":");
KtSingleValueToken SEMICOLON   = new KtSingleValueToken("SEMICOLON", ";");
KtSingleValueToken DOUBLE_SEMICOLON   = new KtSingleValueToken("DOUBLE_SEMICOLON", ";;");
KtSingleValueToken RANGE       = new KtSingleValueToken("RANGE", "..");
KtSingleValueToken EQ          = new KtSingleValueToken("EQ", "=");
KtSingleValueToken MULTEQ      = new KtSingleValueToken("MULTEQ", "*=");
KtSingleValueToken DIVEQ       = new KtSingleValueToken("DIVEQ", "/=");
KtSingleValueToken PERCEQ      = new KtSingleValueToken("PERCEQ", "%=");
KtSingleValueToken PLUSEQ      = new KtSingleValueToken("PLUSEQ", "+=");
KtSingleValueToken MINUSEQ     = new KtSingleValueToken("MINUSEQ", "-=");
KtKeywordToken NOT_IN      = KtKeywordToken.keyword("NOT_IN", "!in");
KtKeywordToken NOT_IS      = KtKeywordToken.keyword("NOT_IS", "!is");
KtSingleValueToken HASH        = new KtSingleValueToken("HASH", "#");
KtSingleValueToken AT          = new KtSingleValueToken("AT", "@");
KtSingleValueToken COMMA       = new KtSingleValueToken("COMMA", ",");

3.7.4 操作符优先级（Precedence)

注：Markdown表格语法：ll是||。

为实现这些的操作符，Kotlin为二元操作符左侧的类型和一元操作符的参数类型，提供了相应的函数或扩展函数。

例如在kotlin/core/builtins/native/kotlin/Primitives.kt代码中，对基本类型Int的操作符的实现代码如下

public class Int private constructor() : Number(), Comparable<Int> {
    ...
    /**
     * Compares this value with the specified value for order.
     * Returns zero if this value is equal to the specified other value, a negative number if it's less than other,
     * or a positive number if it's greater than other.
     */
    public operator fun compareTo(other: Byte): Int
    /**
     * Compares this value with the specified value for order.
     * Returns zero if this value is equal to the specified other value, a negative number if it's less than other,
     * or a positive number if it's greater than other.
     */
    public operator fun compareTo(other: Short): Int
    /**
     * Compares this value with the specified value for order.
     * Returns zero if this value is equal to the specified other value, a negative number if it's less than other,
     * or a positive number if it's greater than other.
     */
    public override operator fun compareTo(other: Int): Int
    /**
     * Compares this value with the specified value for order.
     * Returns zero if this value is equal to the specified other value, a negative number if it's less than other,
     * or a positive number if it's greater than other.
     */
    public operator fun compareTo(other: Long): Int
    /**
     * Compares this value with the specified value for order.
     * Returns zero if this value is equal to the specified other value, a negative number if it's less than other,
     * or a positive number if it's greater than other.
     */
    public operator fun compareTo(other: Float): Int
    /**
     * Compares this value with the specified value for order.
     * Returns zero if this value is equal to the specified other value, a negative number if it's less than other,
     * or a positive number if it's greater than other.
     */
    public operator fun compareTo(other: Double): Int
    /** Adds the other value to this value. */
    public operator fun plus(other: Byte): Int
    /** Adds the other value to this value. */
    public operator fun plus(other: Short): Int
    /** Adds the other value to this value. */
    public operator fun plus(other: Int): Int
    /** Adds the other value to this value. */
    public operator fun plus(other: Long): Long
    /** Adds the other value to this value. */
    public operator fun plus(other: Float): Float
    /** Adds the other value to this value. */
    public operator fun plus(other: Double): Double
    /** Subtracts the other value from this value. */
    public operator fun minus(other: Byte): Int
    /** Subtracts the other value from this value. */
    public operator fun minus(other: Short): Int
    /** Subtracts the other value from this value. */
    public operator fun minus(other: Int): Int
    /** Subtracts the other value from this value. */
    public operator fun minus(other: Long): Long
    /** Subtracts the other value from this value. */
    public operator fun minus(other: Float): Float
    /** Subtracts the other value from this value. */
    public operator fun minus(other: Double): Double
    /** Multiplies this value by the other value. */
    public operator fun times(other: Byte): Int
    /** Multiplies this value by the other value. */
    public operator fun times(other: Short): Int
    /** Multiplies this value by the other value. */
    public operator fun times(other: Int): Int
    /** Multiplies this value by the other value. */
    public operator fun times(other: Long): Long
    /** Multiplies this value by the other value. */
    public operator fun times(other: Float): Float
    /** Multiplies this value by the other value. */
    public operator fun times(other: Double): Double
    /** Divides this value by the other value. */
    public operator fun div(other: Byte): Int
    /** Divides this value by the other value. */
    public operator fun div(other: Short): Int
    /** Divides this value by the other value. */
    public operator fun div(other: Int): Int
    /** Divides this value by the other value. */
    public operator fun div(other: Long): Long
    /** Divides this value by the other value. */
    public operator fun div(other: Float): Float
    /** Divides this value by the other value. */
    public operator fun div(other: Double): Double
    /** Calculates the remainder of dividing this value by the other value. */
    @Deprecated("Use rem(other) instead", ReplaceWith("rem(other)"), DeprecationLevel.WARNING)
    public operator fun mod(other: Byte): Int
    /** Calculates the remainder of dividing this value by the other value. */
    @Deprecated("Use rem(other) instead", ReplaceWith("rem(other)"), DeprecationLevel.WARNING)
    public operator fun mod(other: Short): Int
    /** Calculates the remainder of dividing this value by the other value. */
    @Deprecated("Use rem(other) instead", ReplaceWith("rem(other)"), DeprecationLevel.WARNING)
    public operator fun mod(other: Int): Int
    /** Calculates the remainder of dividing this value by the other value. */
    @Deprecated("Use rem(other) instead", ReplaceWith("rem(other)"), DeprecationLevel.WARNING)
    public operator fun mod(other: Long): Long
    /** Calculates the remainder of dividing this value by the other value. */
    @Deprecated("Use rem(other) instead", ReplaceWith("rem(other)"), DeprecationLevel.WARNING)
    public operator fun mod(other: Float): Float
    /** Calculates the remainder of dividing this value by the other value. */
    @Deprecated("Use rem(other) instead", ReplaceWith("rem(other)"), DeprecationLevel.WARNING)
    public operator fun mod(other: Double): Double
    /** Calculates the remainder of dividing this value by the other value. */
    @SinceKotlin("1.1")
    public operator fun rem(other: Byte): Int
    /** Calculates the remainder of dividing this value by the other value. */
    @SinceKotlin("1.1")
    public operator fun rem(other: Short): Int
    /** Calculates the remainder of dividing this value by the other value. */
    @SinceKotlin("1.1")
    public operator fun rem(other: Int): Int
    /** Calculates the remainder of dividing this value by the other value. */
    @SinceKotlin("1.1")
    public operator fun rem(other: Long): Long
    /** Calculates the remainder of dividing this value by the other value. */
    @SinceKotlin("1.1")
    public operator fun rem(other: Float): Float
    /** Calculates the remainder of dividing this value by the other value. */
    @SinceKotlin("1.1")
    public operator fun rem(other: Double): Double
    /** Increments this value. */
    public operator fun inc(): Int
    /** Decrements this value. */
    public operator fun dec(): Int
    /** Returns this value. */
    public operator fun unaryPlus(): Int
    /** Returns the negative of this value. */
    public operator fun unaryMinus(): Int
     /** Creates a range from this value to the specified [other] value. */
    public operator fun rangeTo(other: Byte): IntRange
     /** Creates a range from this value to the specified [other] value. */
    public operator fun rangeTo(other: Short): IntRange
     /** Creates a range from this value to the specified [other] value. */
    public operator fun rangeTo(other: Int): IntRange
     /** Creates a range from this value to the specified [other] value. */
    public operator fun rangeTo(other: Long): LongRange
    /** Shifts this value left by [bits]. */
    public infix fun shl(bitCount: Int): Int
    /** Shifts this value right by [bits], filling the leftmost bits with copies of the sign bit. */
    public infix fun shr(bitCount: Int): Int
    /** Shifts this value right by [bits], filling the leftmost bits with zeros. */
    public infix fun ushr(bitCount: Int): Int
    /** Performs a bitwise AND operation between the two values. */
    public infix fun and(other: Int): Int
    /** Performs a bitwise OR operation between the two values. */
    public infix fun or(other: Int): Int
    /** Performs a bitwise XOR operation between the two values. */
    public infix fun xor(other: Int): Int
    /** Inverts the bits in this value. */
    public fun inv(): Int
    public override fun toByte(): Byte
    public override fun toChar(): Char
    public override fun toShort(): Short
    public override fun toInt(): Int
    public override fun toLong(): Long
    public override fun toFloat(): Float
    public override fun toDouble(): Double
}

从源代码我们可以看出，重载操作符的函数需要用 operator 修饰符标记。中缀操作符的函数使用infix修饰符标记。

3.7.5 一元操作符（unary operation）

前缀操作符

例如，当编译器处理表达式 +a 时，它将执行以下步骤：

• 确定 a 的类型，令其为 T。
• 为接收者 T 查找一个带有 operator 修饰符的无参函数 unaryPlus（），即成员函数或扩展函数。
• 如果函数不存在或不明确，则导致编译错误。
• 如果函数存在且其返回类型为 R，那就表达式 +a 具有类型 R。

编译器对这些操作以及所有其他操作都针对基本类型做了优化，不会引入函数调用的开销。

以下是如何重载一元减运算符的示例：

package com.easy.kotlin
class OperatorDemo {
}
data class Point(val x: Int, val y: Int)
operator fun Point.unaryMinus() = Point(-x, -y)

测试代码：

package com.easy.kotlin
import org.junit.Test
import org.junit.runner.RunWith
import org.junit.runners.JUnit4
@RunWith(JUnit4::class)
class OperatorDemoTest {
    @Test
    fun testPointUnaryMinus() {
        val p = Point(1, 1)
        val np = -p
        println(np) //Point(x=-1, y=-1)
    }
}

递增和递减

inc() 和 dec() 函数必须返回一个值，它用于赋值给使用 ++ 或 -- 操作的变量。

编译器执行以下步骤来解析后缀形式的操作符，例如 a++：

• 确定 a 的类型，令其为 T。
• 查找一个适用于类型为 T 的接收者的、带有 operator 修饰符的无参数函数 inc()。
• 检查函数的返回类型是 T 的子类型。

计算表达式的步骤是：

• 把 a 的初始值存储到临时存储 a_ 中
• 把 a.inc() 结果赋值给 a
• 把 a_ 作为表达式的结果返回

( a-- 同理分析）。

对于前缀形式 ++a 和 --a 解析步骤类似，但是返回值是取的新值来返回：

• 把 a.inc() 结果赋值给 a
• 把 a 的新值a+1作为表达式结果返回

( --a 同理分析）。

3.7.6 二元操作符

算术运算符

代码示例

>>> val a=10
>>> val b=3
>>> a+b
13
>>> a-b
7
>>> a/b
3
>>> a%b
1
>>> a..b
10..3
>>> b..a
3..10

字符串的+运算符重载

先用代码举个例子：

>>> ""+1
1
>>> 1+""
error: none of the following functions can be called with the arguments supplied: 
public final operator fun plus(other: Byte): Int defined in kotlin.Int
public final operator fun plus(other: Double): Double defined in kotlin.Int
public final operator fun plus(other: Float): Float defined in kotlin.Int
public final operator fun plus(other: Int): Int defined in kotlin.Int
public final operator fun plus(other: Long): Long defined in kotlin.Int
public final operator fun plus(other: Short): Int defined in kotlin.Int
1+""
 ^

从上面的示例，我们可以看出，在Kotlin中1+""是不允许的(这地方，相比Scala，写这样的Kotlin代码就显得不大友好)，只能显式调用toString来相加：

>>> 1.toString()+""
1

自定义重载的 + 运算符

下面我们使用一个计数类 Counter 重载的 + 运算符来增加index的计数值。

代码示例

data class Counter(var index: Int)
operator fun Counter.plus(increment: Int): Counter {
    return Counter(index + increment)
}

测试类

package com.easy.kotlin
import org.junit.Test
import org.junit.runner.RunWith
import org.junit.runners.JUnit4
@RunWith(JUnit4::class)
class OperatorDemoTest 
    @Test
    fun testCounterIndexPlus() {
        val c = Counter(1)
        val cplus = c + 10
        println(cplus) //Counter(index=11)
    }
}

in 操作符

索引访问操作符

方括号转换为调用带有适当数量参数的 get 和 set。

调用操作符

圆括号转换为调用带有适当数量参数的 invoke。

计算并赋值

对于赋值操作，例如 a += b，编译器会试着生成 a = a + b 的代码（这里包含类型检查：a + b 的类型必须是 a 的子类型）。

相等与不等操作符

Kotlin 中有两种类型的相等性：

• 引用相等 === !==（两个引用指向同一对象）
• 结构相等 == !=（ 使用equals() 判断）

这个 == 操作符有些特殊：它被翻译成一个复杂的表达式，用于筛选 null 值。

意思是：如果 a 不是 null 则调用 equals(Any?) 函数并返回其值；否则（即 a === null）就计算 b === null 的值并返回。

当与 null 显式比较时，a == null 会被自动转换为 a=== null

注意：=== 和 !==不可重载。

Elvis 操作符 ?:

在Kotin中，Elvis操作符特定是跟null比较。也就是说

y = x?:0

等价于

val y = if(x!==null) x else 0

主要用来作null安全性检查。

Elvis操作符 ?: 是一个二元运算符，如果第一个操作数为真，则返回第一个操作数，否则将计算并返回其第二个操作数。它是三元条件运算符的变体。命名灵感来自猫王的发型风格。

Kotlin中没有这样的三元运算符 true?1:0，取而代之的是if(true) 1 else 0。而Elvis操作符算是精简版的三元运算符。

我们在Java中使用的三元运算符的语法，你通常要重复变量两次， 示例：

String name = "Elvis Presley";
String displayName = (name != null) ? name : "Unknown";

取而代之，你可以使用Elvis操作符。

String name = "Elvis Presley";
String displayName = name?:"Unknown"

我们可以看出，用Elvis操作符（?:）可以把带有默认值的if/else结构写的及其短小。用Elvis操作符不用检查null（避免了NullPointerException），也不用重复变量。

这个Elvis操作符功能在Spring 表达式语言 (SpEL)中提供。

在Kotlin中当然就没有理由不支持这个特性。

代码示例：

>>> val x = null
>>> val y = x?:0
>>> y
0
>>> val x = false
>>> val y = x?:0
>>> y
false
>>> val x = ""
>>> val y = x?:0
>>> y
>>> val x = "abc"
>>> val y = x?:0
>>> y
abc

比较操作符

所有的比较都转换为对 compareTo 的调用，这个函数需要返回 Int 值

用infix函数自定义中缀操作符

我们可以通过自定义infix函数来实现中缀操作符。

代码示例

data class Person(val name: String, val age: Int)
infix fun Person.grow(years: Int): Person {
    return Person(name, age + years)
}

测试代码

package com.easy.kotlin
import org.junit.Test
import org.junit.runner.RunWith
import org.junit.runners.JUnit4
@RunWith(JUnit4::class)
class InfixFunctionDemoTest {
    @Test fun testInfixFuntion() {
        val person = Person("Jack", 20)
        println(person.grow(2))
        println(person grow 2)
    }
}

输出

Person(name=Jack, age=22)
Person(name=Jack, age=22)