Kotlin操作符重載與中綴表示法筆記

1.操作符重載

操作符重載在java中并沒有這樣的概念，它可以讓我們?yōu)橹付ǖ念愋吞峁╊A(yù)定義的一組操作符實(shí)現(xiàn)。這些操作符具有固定的符號表示（如+、* 或 in）和固定的優(yōu)先級。怎樣實(shí)現(xiàn)操作符重載呢？其實(shí)只要你實(shí)現(xiàn)一個(gè)固定名稱的函數(shù)，然后用operator來修飾這個(gè)函數(shù)。查看Kotlin源碼可以發(fā)現(xiàn)有大量的操作符重載的實(shí)現(xiàn)，我們來看看Int類型做了那些操作符重載。

    /**
     * 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 the [bitCount] number of bits. */
    public infix fun shl(bitCount: Int): Int
    /** Shifts this value right by the [bitCount] number of bits, filling the leftmost bits with copies of the sign bit. */
    public infix fun shr(bitCount: Int): Int
    /** Shifts this value right by the [bitCount] number of 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

2. 一元操作

2.1 一元前綴操作符

表達(dá)式 翻譯為
+a a.unaryPlus()
-a a.unaryMinus()
!a a.not()

當(dāng)編譯器處理+a時(shí)，它執(zhí)行以下步驟：

  • 確定a實(shí)例的類型，如果類似為T。
  • 在T類型中查找一個(gè)帶有operator修飾符的無參函數(shù)unaryPlus()，可以是成員函數(shù)或者擴(kuò)展函數(shù)。
  • 如果沒有找到，則會(huì)發(fā)生編譯錯(cuò)誤。

下面代碼展示下如何實(shí)現(xiàn)一個(gè)一元減運(yùn)算符的示例：

data class Point(val x: Int, val y: Int)

operator fun Point.unaryMinus() : Point = Point(-x, -y)

fun main(args: Array<String>) {
    val p = Point(1, 2)
    println(p)  //打印Point(x=1, y=2)
    println(-p)  //打印Point(x=-1, y=-2)
}

2.2 遞增與遞減

表達(dá)式 翻譯為
a++ a.inc()
a-- a.dec()

下面示例下用法：

data class Point(val x: Int, val y: Int)

operator fun Point.unaryMinus() : Point = Point(-x, -y)

operator fun Point.inc(): Point = Point(x + 1, y + 1)

operator fun Point.dec(): Point = Point(x - 1, y - 1)

3. 二元操作符

3.1 算術(shù)運(yùn)算符

表達(dá)式 翻譯為
a + b a.plus(b)
a - b a.minus(b)
a * b a.times(b)
a / b a.div(b)
a % b a.rem(b)、 a.mod(b) （已棄用）
a..b a.rangeTo(b)

請注意，自 Kotlin 1.1 起支持 rem 運(yùn)算符。Kotlin 1.0 使用 mod 運(yùn)算符，它在 Kotlin 1.1 中被棄用。
以下為示例：

data class Point(val x: Int, val y: Int)

operator fun Point.unaryMinus() : Point = Point(-x, -y)

operator fun Point.inc(): Point = Point(x + 1, y + 1)

operator fun Point.dec(): Point = Point(x - 1, y - 1)

operator fun Point.plus(point: Point): Point = Point(x + point.x, y + point.y)

operator fun Point.minus(point: Point): Point = Point(x - point.x, y - point.y)

operator fun Point.times(point: Point): Point = Point(x * point.x, y * point.y)

3.2 “in”操作符

表達(dá)式 翻譯為
a in b b.contains(a)
a !in b !b.contains(a)

以下為示例代碼：

data class Point(val x: Int, val y: Int)

data class Rectangle(val x: Int, val y: Int, val width: Int, val height: Int)

operator fun Point.unaryMinus() : Point = Point(-x, -y)

operator fun Point.inc(): Point = Point(x + 1, y + 1)

operator fun Point.dec(): Point = Point(x - 1, y - 1)

operator fun Point.plus(point: Point): Point = Point(x + point.x, y + point.y)

operator fun Point.minus(point: Point): Point = Point(x - point.x, y - point.y)

operator fun Point.times(point: Point): Point = Point(x * point.x, y * point.y)

operator fun Rectangle.contains(point: Point): Boolean = point.x > x && point.x <x +width && point.y > y && point.y < y +height

fun main(args: Array<String>) {
    val p = Point(1, 2)
    val rectangle = Rectangle(0, 0, 2,3)
    println(p in rectangle)  //打印true
}

3.3 索引訪問操作符

表達(dá)式 翻譯為
a[i] a.get(i)
a[i, j] a.get(i, j)
a[i_1, ……, i_n] a.get(i_1, ……, i_n)
a[i] = b a.set(i, b)
a[i, j] = b a.set(i, j, b)
a[i_1, ……, i_n] = b a.set(i_1, ……, i_n, b)

3.4 廣義賦值

表達(dá)式 翻譯為
a += b a.plusAssign(b)
a -= b a.minusAssign(b)
a *= b a.timesAssign(b)
a /= b a.divAssign(b)
a %= b a.remAssign(b), a.modAssign(b)（已棄用）

通常來說，當(dāng)你已經(jīng)定義一個(gè)plus操作符時(shí)，Kotlin不僅僅支持+操作符，還同時(shí)支持+=操作了。重載+=操作符，除了實(shí)現(xiàn)plusAssign函數(shù)外，還需要函數(shù)返回值為Unit，而重載+操作符，是需要返回值的。當(dāng)同時(shí)重載了+=和+操作符時(shí)，調(diào)用的時(shí)候會(huì)有編譯錯(cuò)誤，所以你只能保留一種操作符。如下：


operator

所以我們什么時(shí)候該用+操作符，什么時(shí)候該用+=呢？原則就是，如果你的類是一個(gè)mutable（可變化的）類，那么就使用+=操作符，反之就使用+操作符。查看源碼可知MutableCollection就定義了plusAssign方法，而不是plus方法。


plusAssign

3.5 相等與不等操作符

表達(dá)式 翻譯為
a == b a?.equals(b) ?: (b === null)
a != b !(a?.equals(b) ?: (b === null))

Kotlin判斷相等跟java是有區(qū)別的。Kotlin中判斷值相等是用==，而java是調(diào)用equals方法。Kotlin中判斷引用相等是用===，而java使用==。在Kotlin中null==null總是true，對于非空的x，x==null總是返回false而不會(huì)調(diào)用x.equals(null)。
當(dāng)與null顯示比較時(shí)，a==null會(huì)自動(dòng)轉(zhuǎn)換為a===null，注意：===和!==不可重載。

3.6 Elvis操作符 ?:

在Kotlin中，Elvis操作符特定是跟null比較。也就是說y = x?:0等價(jià)于val y = if (x!==null) x else 0。主要用于null的安全檢查，Elvis操作符是一個(gè)二元運(yùn)算符，如果第一個(gè)操作數(shù)不為null，則返回自己，否則返回第二個(gè)操作數(shù)。

在Kotlin中沒有java中的三元運(yùn)算符true?1:0，只有類似的`if (true) 1 else 0。而Evlis操作符是精簡版的三元操作符。在java中使用三元運(yùn)算符通常要重復(fù)變量兩次，如下：

String name = ...;
String displayName = name !=null ? name : "Unknown";

而在Kotlin中：

val name = ...
val displayName = name?:"Unkown"

3.7 比較操作符

表達(dá)式 翻譯為
a > b a.compareTo(b) > 0
a < b a.compareTo(b) < 0
a >= b a.compareTo(b) >= 0
a <= b a.compareTo(b) <= 0

所有的比較都轉(zhuǎn)換為對 compareTo 的調(diào)用，這個(gè)函數(shù)需要返回 Int 值。

3.8 操作符優(yōu)先級

優(yōu)先級 標(biāo)題 符號
最高 后綴（Postfix） ++, --, ., ?., ?
前綴（Prefix） -, +, ++, --, !, labelDefinition@
右手類型運(yùn)算（Type RHS，right-hand side class type） :, as, as?
乘除去余（Multiplicative） *, /, %
加減（Additive） *, /, %
區(qū)間范圍（Range） ..
infix函數(shù) 1 shl 2
Elvis操作符 ?:
命名檢查符（Named checks） in, !in, is, !is
比較大?。–omparison） <, >, <=, >=
相等行判斷（Equality） ==, !==
與（Conjunction） &&
或（Disjunction） ll
最低 賦值（Assignment） =, +=, -=, *=, /=, %=

4. 中綴表示法

函數(shù)還可以用中綴表示法調(diào)用，滿足下面三個(gè)條件時(shí)：

  • 他們是成員函數(shù)或擴(kuò)展函數(shù)
  • 他們只有一個(gè)參數(shù)
  • 他們使用infix關(guān)鍵字標(biāo)注
    示例：
data class Point(var x: Int, var y: Int)

data class Rectangle(val x: Int, val y: Int, val width: Int, val height: Int)

operator fun Point.unaryMinus() : Point = Point(-x, -y)

operator fun Point.inc(): Point = Point(x + 1, y + 1)

operator fun Point.dec(): Point = Point(x - 1, y - 1)

operator fun Point.plusAssign(point: Point) {
    x += point.x
    y += point.y

}

operator fun Point.minus(point: Point): Point = Point(x - point.x, y - point.y)

operator fun Point.times(point: Point): Point = Point(x * point.x, y * point.y)

operator fun Rectangle.contains(point: Point): Boolean = point.x > x && point.x <x +width && point.y > y && point.y < y +height

infix fun Point.mut(point: Point): Point = Point(x * point.x, y * point.y)

fun main(args: Array<String>) {
    var x = Point(1, 2)
    var y = Point(3, 6)

    println(x mut y)  //打印Point(x=3, y=12)

}

Kotlin語言基礎(chǔ)筆記

Kotlin流程控制語句筆記

Kotlin操作符重載與中綴表示法筆記

Kotlin擴(kuò)展函數(shù)和擴(kuò)展屬性筆記

Kotlin空指針安全（null-safety）筆記

Kotlin類型系統(tǒng)筆記

Kotlin面向?qū)ο缶幊坦P記

Kotlin委托（Delegation）筆記

Kotlin泛型型筆記

Kotlin函數(shù)式編程筆記

Kotlin與Java互操作筆記

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