Simple Scala getter/setter override

Question

Let's say we have a class with a 'name' property:

class SuperFoo(var name: String)

If I wish to override this, to eg add some locking around the calls:

class SubFoo(n: String) extends SuperFoo(n) {
  val lock = new ReentrantLock
  override def name(): String =
    {
      lock.lock
      try {
        super.name
      } finally {
        lock.unlock
      }
    }
  override def name_=(arg: String): Unit = {
    lock.lock
    try {
      super.name = arg
    } finally {
      lock.unlock
    }
  }
}

The above produces a compilation error:

super may be not be used on variable name

Any ideas how to correctly implement this? (i.e. override the getter & setter to add locking around them). Thanks!

The compiler seems to think I'm overriding a variable: if I remove the super.name statement from the getter, I get this error message: "overriding variable name in class SuperFoo of type String; method name cannot override a mutable" — Nikolaos, Dec 28 '12 at 12:23

paradigmatic · Accepted Answer · 2012-12-30T18:26:35.020

Here you need to refer directly to the super class setter/getter. Normally you should write something like:

class SubFoo(n: String) extends SuperFoo(n) {
  val lock = new ReentrantLock
  override def name(): String =
    {
      lock.lock
      try {
        super.name()
      } finally {
        lock.unlock
      }
    }
  override def name_=(arg: String): Unit = {
    lock.lock
    try {
      super.name_=(arg)
    } finally {
      lock.unlock
    }
  }
}

However, if the setter will compile without any problem, the getter won't because the compiler will view it as super.name.apply() (Strings can get this method through implicit conversions).

I see several options:

Favor composition over inheritance (a classic).
Change the variable name, make it private and write accessor in the super-class (see below).
Resort to reflection/manual name umangling voodoo.

I'll go for option #1, but here is code for option #2:

class SuperFoo( private var nameVar: String) {
  def name: String = nameVar
  def name_=(arg: String): Unit = nameVar = arg
}

class SubFoo(n: String) extends SuperFoo(n) {
  val lock = new ReentrantLock
  override def name(): String =
    {
      lock.lock
      try {
    super.name
      } finally {
        lock.unlock
      }
    }
  override def name_=(arg: String): Unit = {
    lock.lock
    try {
      super.name = arg
    } finally {
      lock.unlock
    }
  }
}

EDIT: Here is a workable implementation of option #1:

trait Foo {
  def name: String
  def name_=(arg: String): Unit
}

class SimpleFoo( var name: String) extends Foo

class LockedFoo(foo: Foo) extends Foo {
  val lock = new ReentrantLock
  def name(): String =
    {
      lock.lock
      try {
        foo.name
      } finally {
        lock.unlock
      }
    }
  def name_=(arg: String): Unit = {
    lock.lock
    try {
      foo.name = arg
    } finally {
      lock.unlock
    }
  }
}

Thanks- I was trying to avoid option 2 and use your option 1 somehow (option 3 is out of the question due to reflection). However, option 1 does not compile even if the property is not a string, try with Int and you'll see what I mean. Scalac thinks that you're trying to override the 'name' field for some reason... Any ideas how to get option 1 working with eg an Int? — Nikolaos, Dec 30 '12 at 16:57
I added option 1 solution. Following GoF guidelines, the decorator design pattern allows to add a behavior to an existing implementation. — paradigmatic, Dec 30 '12 at 18:29

Simple Scala getter/setter override

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