转:scala override的学习总结

1) 可以用override val  覆盖父类或trait中不带参数的def , 但反过来不行, 既不可以用override def 覆盖父类的val,比如

class Father{
    def name = "Archer"
    def score() = 1.0
    val id = 12
}

class Children extends Father{
    override val name = "Jack"
    override val score  = 2.0
    override def id = 10  // error: error overriding value id in class Father of type Int; method id is not stable
}

个人认为这条规则可以这么记忆,因为def表示一个函数,函数返回值都有一个确定的范围,比如上面的name函数,它的返回值范围是全体的字符串,由于val 表示一个常量,值的范围已经却确定,因此如果子类要覆盖父类的同名成员,其值的范围不能更宽泛, 否则就破坏了里氏代换原则。显然, def 所蕴含的范围要比 val 广(注: 这段话是我瞎说,只是方便记忆)

2.  子类的def 可以 override 父类的def, 比如

class Father{
    def name = "Archer"
    def say() = "go to bed"
    def moveTo(x: Int, y: Int) = println("move to ("+x+","+y);
}

class Children extends Father{
   override def name() = "Jack"
   override def say() = "I'm not tired"
   override moveTo(x: Int, y: Int) = super.moveTo(x,y)
}

3. 子类的var要override 父类的def 成员 , 不过def 必须成对出现;但反过来不行

class Father{
    def credit = "abc"
    def credit_=(s: String) = (println("s"))
}

class Children extends Father{
    override var credit = "fff"   //  var can override  a dual def
}

你可能感兴趣的:(scala,override,var,val)