when does it need explicit type when declare varia

问题:

Here is a simple sample code for my question.

var a:Int=1; //line 1

var a=1; //line 2

Is Int in line 1 needed? or must? if not ,can I delete it like in line 2?

回答1:

Since 1 is of type Int, compiler knows that a is of type Int too. This is called type inference.

You should specify a type explicitly when this is better for code readability.

You must specify a type when compiler can't infer the type or when this helps to infer other types.

In Scala type inference can go in both directions, from right to left and vice versa. For example in val a = 1 type of a is inferred from type of 1, so type inference went from right to left. In

def myMethod[T](): T = ???
val n: Int = myMethod()

since n is expected to be an Int, compiler infers that T in myMethod() should be Int too, so type inference went from left to right.

https://twitter.github.io/scala_school/type-basics.html#inference

http://www.scala-lang.org/old/node/127

http://allaboutscala.com/tutorials/chapter-2-learning-basics-scala-programming/scala-tutorial-overview-scala-type-inference/

How does scala infers the type of a variable?

回答2:

You don't need to specify the type Int in this case, as it is inferred by the compiler.

There is plenty of documentation about type inference in scala. Check this out: http://docs.scala-lang.org/tour/local-type-inference.html

回答3:

In most cases the Scala compiler can deduce types automatically. In these cases you do not have to explicitly define the type of your variable declaration.

var a = 1

is perfectly valid Scala code.

It is often recommended to declare explicitly the return types of public methods.