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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.2 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.

11.2 How primitives are implemented

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(421, 421));

You will surely get true. Now, change the argument types of isEqual to java.lang.Integer (or Object, the result will be the same):

2The 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.

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Section 11.2

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// This is Java

boolean isEqual(Integer x, Integer y) { return x == y;

}

System.out.println(isEqual(421, 421));

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(421, 421) res10: Boolean = true

scala> def isEqual(x: Any, y: Any) = x == y isEqual: (Any,Any)Boolean

scala> isEqual(421, 421) 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 other than Java’s boxed numeric 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

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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.3 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 30.

11.3 Bottom types

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:

3You 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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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.

11.4Conclusion

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.

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