81st: The structure and type constraint inversion, covariance, and the nether of the list in Scala

Learn about the structure and type constraints of list in Scala today.

Let's take a look at the code

Package Scala.learn

/**
* @author Zhang
*/
Abstract class Big_data

Class Hadoop extends Big_data
Class Spark extends Big_data

Object List_constructor {
def main (args:array[string]) {
Val Hadoop = new Hadoop:: Nil//Last list type for Hadoop
Val big_data = new Spark:: hadoop//The last list type is Big_data, promoted to parent type
}
}

As we said last, nil is the nothing type, it can be connected to all types of list, so Hadoop is finally a list of Hadoop types.

While new Spark is a spark type and is a subtype of Big_data, Hadoop is a type of Hadoop and a subtype of Big_data, then the final form of big_data, if it can hold two different subtypes, Then it becomes their common upper bound, that is, the parent type: Big_data.

This mechanism of list is very beneficial for the extension of the listing itself.

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81st: The structure and type constraint inversion, covariance, and the nether of the list in Scala