class Int is abstract; cannot be instantiated

Question

While going through Programming in Scala, i came across:

While you can define your own value classes (see Section 11.4), there are nine value classes built into Scala: 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 ofBoolean. 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.

Due to which new Int gives the error class Int is abstract; cannot be instantiated val a: Int = new Int in Scala. Java allows new Integer(23).

Question: What is the trick the author is taking about. Why Scala defines value classes to be abstract and final.

"Java allows new Integer(23)." So does Scala. scala.Int is not java.lang.Integer, and Java does not allow new int(23). — Alexey Romanov

Thilo Thilo · Accepted Answer · 2019-03-18T06:19:50

What is the trick the author is taking about ?

The "trick" is that

when a class is abstract, you cannot make instances of it (cannot call new).
when a class is final, you cannot make subclasses
when a class is abstract and you cannot make subclasses, then you also cannot make a concrete subclass that you could instantiate

So as a result, value classes cannot be instantiated by application code.

Why Scala defines value classes to be abstract and final.

The point of value classes is that they are defined by their (immutable) value/contents. The object identity is not relevant.

The Scala compiler also tries to optimize value classes by not creating any objects at all where possible (just using unboxed primitives directly). That only works if we can be sure that you can just box and unbox at will.

In Java new Integer(1) and another new Integer(1) are two different objects, but that is not useful for a pure value class (if you want to use these different instances as lock monitor objects or something else where you need object identity, you are just confusing yourself and others and should not have used Integer).

class Int is abstract; cannot be instantiated

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