1.操作符重载
操作符重载在java中并没有这样的概念，它可以让我们为指定的类型提供预定义的一组操作符实现。这些操作符具有固定的符号表示（如+、* 或 in）和固定的优先级。怎样实现操作符重载呢？其实只要你实现一个固定名称的函数，然后用operator来修饰这个函数。查看Kotlin源码可以发现有大量的操作符重载的实现，我们来看看Int类型做了那些操作符重载。
/**
* 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 一元前缀操作符
| 表达式 | 翻译为 |
|---|---|
| +a | a.unaryPlus() |
| -a | a.unaryMinus() |
| !a | a.not() |
当编译器处理+a时，它执行以下步骤：
- 确定a实例的类型，如果类似为T。
- 在T类型中查找一个带有
operator修饰符的无参函数unaryPlus()，可以是成员函数或者扩展函数。 - 如果没有找到，则会发生编译错误。
下面代码展示下如何实现一个一元减运算符的示例：
data class Point(val x: Int, val y: Int)
operator fun Point.unaryMinus() : Point = Point(-x, -y)
fun main(args: Array) {
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 递增与递减
| 表达式 | 翻译为 |
|---|---|
| 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 算术运算符
| 表达式 | 翻译为 |
|---|---|
| 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) （已弃用） |
| a..b | a.rangeTo(b) |
请注意，自 Kotlin 1.1 起支持 rem 运算符。Kotlin 1.0 使用 mod 运算符，它在 Kotlin 1.1 中被弃用。
以下为示例：
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
| 表达式 | 翻译为 |
|---|---|
| a in b | b.contains(a) |
| a !in b | !b.contains(a) |
以下为示例代码：
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 y && point.y < y +height
fun main(args: Array) {
val p = Point(1, 2)
val rectangle = Rectangle(0, 0, 2,3)
println(p in rectangle) //打印true
}
3.3 索引访问操作符
| 表达式 | 翻译为 |
|---|---|
| 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 广义赋值
| 表达式 | 翻译为 |
|---|---|
| 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)（已弃用） |
通常来说，当你已经定义一个plus操作符时，Kotlin不仅仅支持+操作符，还同时支持+=操作了。重载+=操作符，除了实现plusAssign函数外，还需要函数返回值为Unit，而重载+操作符，是需要返回值的。当同时重载了+=和+操作符时，调用的时候会有编译错误，所以你只能保留一种操作符。如下：
operator
所以我们什么时候该用+操作符，什么时候该用+=呢？原则就是，如果你的类是一个mutable（可变化的）类，那么就使用+=操作符，反之就使用+操作符。查看源码可知MutableCollection就定义了plusAssign方法，而不是plus方法。
plusAssign
3.5 相等与不等操作符
| 表达式 | 翻译为 |
|---|---|
| a == b | a?.equals(b) ?: (b === null) |
| a != b | !(a?.equals(b) ?: (b === null)) |
Kotlin判断相等跟java是有区别的。Kotlin中判断值相等是用==，而java是调用equals方法。Kotlin中判断引用相等是用===，而java使用==。在Kotlin中null==null总是true，对于非空的x，x==null总是返回false而不会调用x.equals(null)。
当与null显示比较时，a==null会自动转换为a===null，注意：===和!==不可重载。
3.6 Elvis操作符 ?:
在Kotlin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。主要用于null的安全检查，Elvis操作符是一个二元运算符，如果第一个操作数不为null，则返回自己，否则返回第二个操作数。
在Kotlin中没有java中的三元运算符true?1:0，只有类似的`if (true) 1 else 0。而Evlis操作符是精简版的三元操作符。在java中使用三元运算符通常要重复变量两次，如下：
String name = ...;
String displayName = name !=null ? name : "Unknown";
而在Kotlin中：
val name = ...
val displayName = name?:"Unkown"
3.7 比较操作符
| 表达式 | 翻译为 |
|---|---|
| a > b | a.compareTo(b) > 0 |
| a < b | a.compareTo(b) < 0 |
| a >= b | a.compareTo(b) >= 0 |
| a <= b | a.compareTo(b) <= 0 |
所有的比较都转换为对 compareTo 的调用，这个函数需要返回 Int 值。
3.8 操作符优先级
| 优先级 | 标题 | 符号 |
|---|---|---|
| 最高 | 后缀（Postfix） | ++, --, ., ?., ? |
| 前缀（Prefix） | -, +, ++, --, !, labelDefinition@ |
|
| 右手类型运算（Type RHS，right-hand side class type） | :, as, as? | |
| 乘除去余（Multiplicative） | *, /, % | |
| 加减（Additive） | *, /, % | |
| 区间范围（Range） | .. | |
| infix函数 | 1 shl 2 | |
| Elvis操作符 | ?: | |
| 命名检查符（Named checks） | in, !in, is, !is | |
| 比较大小（Comparison） | <, >, <=, >= | |
| 相等行判断（Equality） | ==, !== | |
| 与（Conjunction） | && | |
| 或（Disjunction） | ll | |
| 最低 | 赋值（Assignment） | =, +=, -=, *=, /=, %= |
4. 中缀表示法
函数还可以用中缀表示法调用，满足下面三个条件时：
- 他们是成员函数或扩展函数
- 他们只有一个参数
- 他们使用
infix关键字标注
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 y && point.y < y +height
infix fun Point.mut(point: Point): Point = Point(x * point.x, y * point.y)
fun main(args: Array) {
var x = Point(1, 2)
var y = Point(3, 6)
println(x mut y) //打印Point(x=3, y=12)
}