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Chapter 11 of Programming in Scala, First Edition
Scala's Hierarchy
by Martin Odersky, Lex Spoon, and Bill Venners
December 10, 2008

Now that you've seen the details of class inheritance in the previous chapter, it is a good time to take a step back and look at Scala's class hierarchy as a whole. In Scala, every class inherits from a common superclass named Any. Because every class is a subclass of Any, the methods defined in Any are "universal" methods: they may be invoked on any object. Scala also defines some interesting classes at the bottom of the hierarchy, Null and Nothing, which essentially act as common subclasses. For example, just as Any is a superclass of every other class, Nothing is a subclass of every other class. In this chapter, we'll give you a tour of Scala's class hierarchy.

11.1 Scala's class hierarchy [link]

Figure 11.1 shows an outline of Scala's class hierarchy. At the top of the hierarchy is class Any, which defines methods that include the following:

  final def ==(that: Any): Boolean
  final def !=(that: Any): Boolean
  def equals(that: Any): Boolean
  def hashCode: Int
  def toString: String
Because every class inherits from Any, every object in a Scala program can be compared using ==, !=, or equals; hashed using hashCode; and formatted using toString. The equality and inequality methods, == and !=, are declared final in class Any, so they cannot be overridden in subclasses. In fact, == is always the same as equals and != is always the negation of equals. So individual classes can tailor what == or != means by overriding the equals method. We'll show an example later in this chapter.

image images/scalaHierarchy.jpg
Class hierarchy of Scala.

The root class Any has two subclasses: AnyVal and AnyRef. AnyVal is the parent class of every built-in value class in Scala. There are nine such value classes: Byte, Short, Char, Int, Long, Float, Double, Boolean, and Unit. The first eight of these correspond to Java's primitive types, and their values are represented at run time as Java's primitive values. The instances of these classes are all written as literals in Scala. For example, 42 is an instance of Int, 'x' is an instance of Char, and false an instance of Boolean. You cannot create instances of these classes using new. This is enforced by the "trick" that value classes are all defined to be both abstract and final. So if you were to write:

  scala> new Int
you would get:

  <console>:5: error: class Int is abstract; cannot be 
  instantiated
         new Int
         ^

The other value class, Unit, corresponds roughly to Java's void type; it is used as the result type of a method that does not otherwise return an interesting result. Unit has a single instance value, which is written (), as discussed in Section 7.2.

As explained in Chapter 5, the value classes support the usual arithmetic and boolean operators as methods. For instance, Int has methods named + and *, and Boolean has methods named || and &&. Value classes also inherit all methods from class Any. You can test this in the interpreter:

  scala> 42.toString
  res1: java.lang.String = 42
  
scala> 42.hashCode res2: Int = 42
scala> 42 equals 42 res3: Boolean = true
Note that the value class space is flat; all value classes are subtypes of scala.AnyVal, but they do not subclass each other. Instead there are implicit conversions between different value class types. For example, an instance of class scala.Int is automatically widened (by an implicit conversion) to an instance of class scala.Long when required.

As mentioned in Section 5.9, implicit conversions are also used to add more functionality to value types. For instance, the type Int supports all of the operations below:

  scala> 42 max 43
  res4: Int = 43
  
scala> 42 min 43 res5: Int = 42
scala> 1 until 5 res6: Range = Range(1, 2, 3, 4)
scala> 1 to 5 res7: Range.Inclusive = Range(1, 2, 3, 4, 5)
scala> 3.abs res8: Int = 3
scala> (-3).abs res9: Int = 3
Here's how this works: The methods min, max, until, to, and abs are all defined in a class scala.runtime.RichInt, and there is an implicit conversion from class Int to RichInt. The conversion is applied whenever a method is invoked on an Int that is undefined in Int but defined in RichInt. Similar "booster classes" and implicit conversions exist for the other value classes. Implicit conversions will be discussed in detail in Chapter 21.

The other subclass of the root class Any is class AnyRef. This is the base class of all reference classes in Scala. As mentioned previously, on the Java platform AnyRef is in fact just an alias for class java.lang.Object. So classes written in Java as well as classes written in Scala all inherit from AnyRef.[1] One way to think of java.lang.Object, therefore, is as the way AnyRef is implemented on the Java platform. Thus, although you can use Object and AnyRef interchangeably in Scala programs on the Java platform, the recommended style is to use AnyRef everywhere.

Scala classes are different from Java classes in that they also inherit from a special marker trait called ScalaObject. The idea is that the ScalaObject contains methods that the Scala compiler defines and implements in order to make execution of Scala programs more efficient. Right now, Scala object contains a single method, named $tag, which is used internally to speed up pattern matching.

11.2 How primitives are implemented [link]

How is all this implemented? In fact, Scala stores integers in the same way as Java: as 32-bit words. This is important for efficiency on the JVM and also for interoperability with Java libraries. Standard operations like addition or multiplication are implemented as primitive operations. However, Scala uses the "backup" class java.lang.Integer whenever an integer needs to be seen as a (Java) object. This happens for instance when invoking the toString method on an integer number or when assigning an integer to a variable of type Any. Integers of type Int are converted transparently to "boxed integers" of type java.lang.Integer whenever necessary.

All this sounds a lot like auto-boxing in Java 5 and it is indeed quite similar. There's one crucial difference, though, in that boxing in Scala is much less visible than boxing in Java. Try the following in Java:

  // This is Java
  boolean isEqual(int x, int y) {
    return x == y;
  }
  System.out.println(isEqual(421421));
You will surely get true. Now, change the argument types of isEqual to java.lang.Integer (or Object, the result will be the same):
  // This is Java
  boolean isEqual(Integer x, Integer y) {
    return x == y;
  }
  System.out.println(isEqual(421421));
You will find that you get false! What happens is that the number 421 gets boxed twice, so that the arguments for x and y are two different objects. Because == means reference equality on reference types, and Integer is a reference type, the result is false. This is one aspect where it shows that Java is not a pure object-oriented language. There is a difference between primitive types and reference types that can be clearly observed.

Now try the same experiment in Scala:

  scala> def isEqual(x: Int, y: Int) = x == y
  isEqual: (Int,Int)Boolean
  
scala> isEqual(421421) res10: Boolean = true
scala> def isEqual(x: Any, y: Any) = x == y isEqual: (Any,Any)Boolean
scala> isEqual(421421) res11: Boolean = true
In fact, the equality operation == in Scala is designed to be transparent with respect to the type's representation. For value types, it is the natural (numeric or boolean) equality. For reference types, == is treated as an alias of the equals method inherited from Object. That method is originally defined as reference equality, but is overridden by many subclasses to implement their natural notion of equality. This also means that in Scala you never fall into Java's well-known trap concerning string comparisons. In Scala, string comparison works as it should:
  scala> val x = "abcd".substring(2)
  x: java.lang.String = cd
  
scala> val y = "abcd".substring(2) y: java.lang.String = cd
scala> x == y res12: Boolean = true
In Java, the result of comparing x with y would be false. The programmer should have used equals in this case, but it is easy to forget.

However, there are situations where you need reference equality instead of user-defined equality. For example, in some situations where efficiency is paramount, you would like to hash cons with some classes and compare their instances with reference equality.[2] For these cases, class AnyRef defines an additional eq method, which cannot be overridden and is implemented as reference equality (i.e., it behaves like == in Java for reference types). There's also the negation of eq, which is called ne. For example:

  scala> val x = new String("abc")
  x: java.lang.String = abc
  
scala> val y = new String("abc") y: java.lang.String = abc
scala> x == y res13: Boolean = true
scala> x eq y res14: Boolean = false
scala> x ne y res15: Boolean = true
Equality in Scala is discussed further in Chapter 28.

11.3 Bottom types [link]

At the bottom of the type hierarchy in Figure 11.1 you see the two classes scala.Null and scala.Nothing. These are special types that handle some "corner cases" of Scala's object-oriented type system in a uniform way.

Class Null is the type of the null reference; it is a subclass of every reference class (i.e., every class that itself inherits from AnyRef). Null is not compatible with value types. You cannot, for example, assign a null value to an integer variable:

  scala> val i: Int = null
  <console>:4: error: type mismatch;
   found   : Null(null)
   required: Int

Type Nothing is at the very bottom of Scala's class hierarchy; it is a subtype of every other type. However, there exist no values of this type whatsoever. Why does it make sense to have a type without values? As discussed in Section 7.4, one use of Nothing is that it signals abnormal termination. For instance there's the error method in the Predef object of Scala's standard library, which is defined like this:

  def error(message: String): Nothing =
    throw new RuntimeException(message)
The return type of error is Nothing, which tells users that the method will not return normally (it throws an exception instead). Because Nothing is a subtype of every other type, you can use methods like error in very flexible ways. For instance:
  def divide(x: Int, y: Int): Int = 
    if (y != 0) x / y 
    else error("can't divide by zero")

The "then" branch of the conditional, x / y, has type Int, whereas the else branch, the call to error, has type Nothing. Because Nothing is a subtype of Int, the type of the whole conditional is Int, as required.

11.4 Conclusion [link]

In this chapter we showed you the classes at the top and bottom of Scala's class hierarchy. Now that you've gotten a good foundation on class inheritance in Scala, you're ready to understand mixin composition. In the next chapter, you'll learn about traits.


Footnotes for Chapter 11:

[1] The reason the AnyRef alias exists, instead of just using the name java.lang.Object, is because Scala was designed to work on both the Java and .NET platforms. On .NET, AnyRef is an alias for System.Object.

[2] You hash cons instances of a class by caching all instances you have created in a weak collection. Then, any time you want a new instance of the class, you first check the cache. If the cache already has an element equal to the one you are about to create, you can reuse the existing instance. As a result of this arrangement, any two instances that are equal with equals() are also equal with reference equality.


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