Kotlin 中的 Any、Unit 、Nothing

1.官方文档

　　Any　　　　https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-any

　　Unit　　　　https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-unit

　　Nothing　　 https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-nothing.html

2.Any

2.1 源码

/*
 * Copyright 2010-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
 * that can be found in the LICENSE file.
 */
 
package kotlin
 
import kotlin.native.identityHashCode
import kotlin.native.internal.fullName
import kotlin.native.internal.ExportTypeInfo
import kotlin.native.internal.GCUnsafeCall
 
/**
 * The root of the Kotlin class hierarchy. Every Kotlin class has [Any] as a superclass.
 */
@ExportTypeInfo("theAnyTypeInfo")
public open class Any {
    /**
     * Indicates whether some other object is "equal to" this one. Implementations must fulfil the following
     * requirements:
     *
     * * Reflexive: for any non-null value `x`, `x.equals(x)` should return true.
     * * Symmetric: for any non-null values `x` and `y`, `x.equals(y)` should return true if and only if `y.equals(x)` returns true.
     * * Transitive:  for any non-null values `x`, `y`, and `z`, if `x.equals(y)` returns true and `y.equals(z)` returns true, then `x.equals(z)` should return true.
     * * Consistent:  for any non-null values `x` and `y`, multiple invocations of `x.equals(y)` consistently return true or consistently return false, provided no information used in `equals` comparisons on the objects is modified.
     * * Never equal to null: for any non-null value `x`, `x.equals(null)` should return false.
     *
     * Read more about [equality](https://kotlinlang.org/docs/reference/equality.html) in Kotlin.
     */
    @GCUnsafeCall("Kotlin_Any_equals")
    external public open operator fun equals(other: Any?): Boolean
 
    /**
     * Returns a hash code value for the object.  The general contract of `hashCode` is:
     *
     * * Whenever it is invoked on the same object more than once, the `hashCode` method must consistently return the same integer, provided no information used in `equals` comparisons on the object is modified.
     * * If two objects are equal according to the `equals()` method, then calling the `hashCode` method on each of the two objects must produce the same integer result.
     */
    public open fun hashCode(): Int = this.identityHashCode()
 
    /**
     * Returns a string representation of the object.
     */
    public open fun toString(): String {
        val className = this::class.fullName ?: "<object>"
        // TODO: consider using [identityHashCode].
        val unsignedHashCode = this.hashCode().toLong() and 0xffffffffL
        val hashCodeStr = unsignedHashCode.toString(16)
        return "$className@$hashCodeStr"
    }
}

2.2 使用

　　kotlin中，所有类型默认继承自它，相当于java 中的 Object。

2.3 示例

 @Test @SmallTest
 fun kotlin_any(){
     val list = ArrayList<Any>()             
     list.add(1)                             //ok
     list.add("two")                         //ok
 
     fun faa() : Any { return 1     }        //ok
     fun fbb() : Any { throw Exception()}    //ok
     fun fcc() : Any?{ throw Exception()}    //ok
     fun fdd() : Any?{ return null      }    //ok
 
     fun <T> fee(t : T  ){}
     fun     fff(t : Any){}
 
     fee(1);fee("t")
     fff(1);fff("t")
     
     fun <T,Any>fgg(t : T,any: Any){}
     fgg(1,1     )
     fgg("e","e" )
     fgg(0b1,1f  )
 }

3.Unit

3.1 源码

 /*
  * Copyright 2010-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
  * that can be found in the LICENSE file.
  */
 
 package kotlin
 
 import kotlin.native.internal.ExportTypeInfo
 
 /**
  * The type with only one value: the `Unit` object.
  */
 @ExportTypeInfo("theUnitTypeInfo")
 public object Unit {
     override fun toString() = "kotlin.Unit"
 }

3.2 作用

　　一个单例类，只有一个对象（Unit）。在jvm环境中，它相当于void。

3.3 示例

 @Test @SmallTest
 fun kotlin_unit(){
     fun faa() = Unit                //ok
     //fun fbb() = Unit()            //error
     fun fcc():Unit {   }            //ok
 }

4.Nothing

4.1 源码

 /*
  * Copyright 2010-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
  * that can be found in the LICENSE file.
  */
 
 package kotlin
 
 /**
  * Nothing has no instances. You can use Nothing to represent "a value that never exists": for example,
  * if a function has the return type of Nothing, it means that it never returns (always throws an exception).
  */
 public final class Nothing private constructor() {}

4.2 作用

　　一个不可实例化的类型，final 且私有构造。

• 它表示一个值
• 该值是不存在的值
• 通常用在的函数的返回值上，表示该函数返回了一个不可能存在的值，return 什么值都错。但不影响抛异常

4.3 示例

示例1.

 @kotlin.internal.InlineOnly
 public inline fun TODO(): Nothing = throw NotImplementedError()

示例2.

 @Test @SmallTest
 fun kotlin_nothing(){
     val list = ArrayList<Nothing>()         //编译通过，但是无意义
     //list.add(1)                           //编译失败,1是整型
     //list.add(Nothing())                   //Nothing 不可实例化
     fun foo() : Nothing{ while(true){ } }   //ok
     //fun fee() : Nothing{ return 1     }   //failed,1不是Nothing类型
     fun fuu() : Nothing{ throw Exception()} //ok,Nothing不影响抛异常
 }