最高的父类型为Any,最低类型为Nothing
Any is the supertype of all types, also called the top type. It defines certain universal methods such as equals, hashCode, and toString. Any has two direct subclasses: AnyVal and AnyRef
注意：hashCode,equals,toString方法在Any类型中
AnyVal represents value types.
There are nine predefined value types and they are non-nullable: Double, Float, Long, Int, Short, Byte, Char, Unit, and Boolean. Unit is a value type which carries no meaningful information. There is exactly one instance of Unit which can be declared literally like so: (). All functions must return something so sometimes Unit is a useful return type
值类型是不能为null,Unit代表不携带任何有意义的信息
//test AnyVal is not null-able //compile error //error msg an expression of type Null is ineligible for implicit conversion val xi :Int = null
AnyRef represents reference types. All non-value types are defined as reference types. Every user-defined type in Scala is a subtype of AnyRef. If Scala is used in the context of a Java runtime environment, AnyRef corresponds to java.lang.Object
//AnyRef在Java运行环境中相当于java.lang.Object
类型转换
val x: Long = 987654321 val y: Float = x // 9.8765434E8 (note that some precision is lost in this case) val face: Char = '☺' val number: Int = face // 9786
Casting is unidirectional. This will not compile
类型转换未按照图中的方向,不编译.
意思就是scala类型转换一定是安全的.不能由长类型转成短类型（这句话或许有错,因为Long可以转换成Float）
Char不能到Long应该是scala的编译不能识别这种转换,以后在爬这个事的原因
val x: Long = 987654321 val y: Float = x // 9.8765434E8 val z: Long = y // Does not conform
Nothing is a subtype of all types, also called the bottom type. There is no value that has type Nothing. A common use is to signal non-termination such as a thrown exception, program exit, or an infinite loop (i.e., it is the type of an expression which does not evaluate to a value, or a method that does not return normally).
Null is a subtype of all reference types (i.e. any subtype of AnyRef). It has a single value identified by the keyword literal null. Null is provided mostly for interoperability with other JVM languages and should almost never be used in Scala code. We’ll cover alternatives to null later in the tour.
Null在scala中用null代替,为了保证scala和其他Java虚拟机语言的互操作性
