scala编程第18章学习笔记——有状态的对象

银行账号的简化实现：

scala> class BankAccount{
     | private var bal: Int = 0
     | def balance: Int = bal
     | def deposit(amount: Int) {
     | require(amount > 0)
     | bal += amount
     | }
     |
     | def withdraw(amount: Int): Boolean =
     | if (amount > bal) false
     | else{
     | bal -= amount
     | true
     | }
     | }
defined class BankAccount

BankAccount类定义了私有变量bal，以及三个公开的方法：balance返回当前余额；deposit向bal添加指定amount的金额；withdraw尝试从bal减少指定amount的金额并须要确保操作之后的余额不能变为负数。withdraw的返回值为Boolean类型，说明请求的资金是否被成功提取。

scala> val account = new BankAccount
account: BankAccount = BankAccount@18532dc

scala> account deposit 100

scala> account withdraw 80
res1: Boolean = true

scala> account withdraw 80
res2: Boolean = false

scala> account.balance
res3: Int = 20

只定义getter和setter方法而不带有关联字段，这种做法不但可行，有时甚至很有必要。

scala> class Thermometer {
     | var celsius: Float = _
     | def fahrenheit = celsius * 9 / 5 + 32
     | def fahrenheit_= (f: Float) {
     | celsius = (f - 32) * 5 / 9
     | }
     | override def toString = fahrenheit + "F/" + celsius + "C"
     | }
defined class Thermometer

celsius变量初始化设置为缺省值‘—'，这个符号指定了变量的”初始化值“。精确的说，字段的初始化器”=_”把零值赋给该字段。这里的“零”的取值取决于字段的类型。对于数值类型来说是0，布尔类型是false，应用类型则是null。

scala> val t = new Thermometer
t: Thermometer = 32.0F/0.0C

scala> t.celsius = 100
t.celsius: Float = 100.0

scala> t
res0: Thermometer = 212.0F/100.0C

scala> t.fahrenheit = -40
t.fahrenheit: Float = -40.0

scala> t
res1: Thermometer = -40.0F/-40.0C