zoukankan      html  css  js  c++  java
  • Scala

    Case classes are like regular classes with a few key differences which we will go over. Case classes are good for modeling immutable data. In the next step of the tour, we’ll see how they are useful in pattern matching.

    Defining a case class

    A minimal case class requires the keywords case class, an identifier, and a parameter list (which may be empty):

    case class Book(isbn: String)
    
    val frankenstein = Book("978-0486282114")
    

    Notice how the keyword new was not used to instantiate the Book case class. This is because case classes have an apply method by default which takes care of object construction.

    When you create a case class with parameters, the parameters are public vals.

    case class Message(sender: String, recipient: String, body: String)
    val message1 = Message("guillaume@quebec.ca", "jorge@catalonia.es", "Ça va ?")
    
    println(message1.sender)  // prints guillaume@quebec.ca
    message1.sender = "travis@washington.us"  // this line does not compile
    

    You can’t reassign message1.sender because it is a val (i.e. immutable). It is possible to use vars in case classes but this is discouraged.

    Comparison

    Case classes are compared by structure and not by reference:

    case class Message(sender: String, recipient: String, body: String)
    
    val message2 = Message("jorge@catalonia.es", "guillaume@quebec.ca", "Com va?")
    val message3 = Message("jorge@catalonia.es", "guillaume@quebec.ca", "Com va?")
    val messagesAreTheSame = message2 == message3  // true
    

    Even though message2 and message3 refer to different objects, the value of each object is equal.

    Copying

    You can create a (shallow) copy of an instance of a case class simply by using the copy method. You can optionally change the constructor arguments.

    case class Message(sender: String, recipient: String, body: String)
    val message4 = Message("julien@bretagne.fr", "travis@washington.us", "Me zo o komz gant ma amezeg")
    val message5 = message4.copy(sender = message4.recipient, recipient = "claire@bourgogne.fr")
    message5.sender  // travis@washington.us
    message5.recipient // claire@bourgogne.fr
    message5.body  // "Me zo o komz gant ma amezeg"
    

    The recipient of message4 is used as the sender of message5 but the bodyof message4 was copied directly.

  • 相关阅读:
    Hybrid APP基础篇(四)->JSBridge的原理
    剑指offer
    剑指offer
    如何在HTML中设置文本的大小写
    如何给HTML标签中的文本设置修饰线
    CSS设置文本的水平对齐方向
    如何在HTML中设置字体颜色,你知道这几种方式吗?
    CSS尺寸样式属性
    CSS基本选择器是什么?基本选择器是如何工作
    如何创建 CSS
  • 原文地址:https://www.cnblogs.com/yjyyjy/p/10694643.html
Copyright © 2011-2022 走看看