理解 Scala 中的类型参数 [英] Understanding type Parameters in Scala
问题描述
我正在尝试理解 Scala 中的类型参数.让我们看下面的一般示例:
def func1[T](a : T) : T = a
我知道 func1 接受任何类型的 1 个参数,并返回完全相同类型的参数.我不明白的是为什么:
def func1[T]
为什么 [T] 紧跟在 function1 之后??我们可以简单地在 func1 之后不加 [T] 来编写它,例如:
def func1(a : T) : T = a
1) func1 后面的 [T] 是什么意思,为什么我们把它放在那里?
2) 为什么我们对类做同样的事情?
class MyClass[T]{...}
我的意思是 MyClass 实例是 MyClass 类型.[T] 在那里是什么意思?你不是说我有一个 MyClass 类型的布尔类,你说我有一个 MyClass 类型的对象对吗?
提前致谢.
func1
后面的 [T] 是什么意思,为什么要把它放在那里?
func[T]
中的[T]
定义了一个类型参数T
.你的函数可以像 func[String]("Hello")
一样被调用,其中 String
被替换为 T
.您也可以像 func("Hello")
一样调用它,因为 Scala 编译器足够聪明,可以推断 T
必须是 String
.>
那为什么我们在定义的时候一定要写func[T]
呢?我们需要区分由类型参数给出的类型参数和由实际类型给出的参数.如果你这样写:def func1(a : T) : T = a
,那么 T
必须是一个实际类型.例如:
T 类def func1(a : T) : T = a//<-- 现在编译
- 为什么我们要对类做同样的事情?
您经常希望在类中包含某种类型的对象.如果您在类级别定义类型参数,则该类型将在整个类中保持不变.考虑这个例子:
class Container[T](val t: T) {def isValueEqual(obj: T): Boolean = t.equals(obj)}
这里,obj:T
中的T
与Container[T]
中定义的T
类型相同代码>.现在考虑这个例子:
class Container[T](val t: T) {def isValueEqual[T](obj: T): Boolean = t.equals(obj)}
请注意,我也在方法级别定义了一个新的类型参数 (isValueEqual[T]
).在这种情况下,方法中定义的 T
将隐藏在类级别定义的 T
.这意味着它们可能不是同一类型!你可以这样称呼它:
val c = new Container("Hello")println(c.isValueEqual(5))//5 不是 String 类型!
I am trying to understand the type parameters in Scala. Let's look the following general example:
def func1[T](a : T) : T = a
I understand that func1 takes 1 parameter of any type, and returns that parameter of the very same type. What i don't understand is why:
def func1[T]
Why [T] right after function1?? We could simply write it without [T] after func1, like:
def func1(a : T) : T = a
1) What does that [T] means after func1 and why we put it there?
2) Why we do the same with classes?
class MyClass[T]{...}
I mean MyClass instantiations are of type MyClass. What does [T] means there? You don't say i have a boolean Class of type MyClass, you say i have an object of type MyClass right?
Thanks in advance.
- What does [T] mean after
func1
, and why do we put it there?
The [T]
in func[T]
defines a type parameter T
. Your function can be called like func[String]("Hello")
, in which String
is replaced with T
. You can also call it like func("Hello")
because the Scala compiler is smart enough to infer that T
must be String
.
So why do we have to write func[T]
when we define it? We need the distinction between arguments of a type given by a type parameter, and arguments given by an actual type. If you write it this: def func1(a : T) : T = a
, then T
has to be an actual type. For example:
class T
def func1(a : T) : T = a // <-- This compiles now
- Why do we do the same with classes?
You often want to contain an object of some type inside a class. If you define the type parameter at the class level, the type will remain the same throughout your class. Consider this example:
class Container[T](val t: T) {
def isValueEqual(obj: T): Boolean = t.equals(obj)
}
Here, the T
in obj: T
is the same type as the T
defined in Container[T]
. Now consider this example:
class Container[T](val t: T) {
def isValueEqual[T](obj: T): Boolean = t.equals(obj)
}
Notice that I defined a new type parameter at the method level as well (isValueEqual[T]
). In this case, the T
defined in the method will shadow the T
defined on the class level. This means that they might not be the same type! You could call it like this:
val c = new Container("Hello")
println(c.isValueEqual(5)) // 5 is not of type String!
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