c# barcode generator library open source Show Me Some Bad Java Code in Font

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Show Me Some Bad Java Code
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So, I ve talked about Java code not being as type-safe as Scala code. You re probably thinking, But Java is a statically typed language, doesn t it give me all the safety that Scala does The answer to that is no. Take a look at the following code and spot the problem:
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public class Bad { public static void main(String[] argv) { Object[] a = argv; a[0] = new Object(); } }
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This is legal Java code, and here s what happens when we run the code:
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> java Bad Hello
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Exception in thread "main" java.lang.ArrayStoreException: java.lang.Object at Bad.main(Bad.java:4)
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1. Among enthusiasts of other statically typed languages with rich type systems (Standard ML, Haskell, OCaml) the architect style is often referred to as typeful programming, referring exactly to this distinction between going with the type inference flow and using the type system deliberately to encode important invariants.
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CHAPTER 7 TRAITS AND TYPES AND GNARLY STUFF FOR ARCHITECTS
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Java allows us to assign a String[] to Object[]. This is because a String is a subclass of Object, so if the array was read-only, the assignment would make sense. However, the array can be modified. The modification that we ve demonstrated shows one of Java s type-unsafety features. We ll discuss why this happened and the very complex topic of invariant, covariant, and contravariant types later in this chapter. Let s start looking at how Scala makes the architect s job easier and makes the coder s job easier.
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Library Pimping, Implicit Conversions, and Vampires
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We ve seen a little bit of stuff so far that looks like magic. The String class seems to have grown methods:
scala> "Hello".toList
res0: List[Char] = List(H, e, l, l, o)
You may be wondering how a Java class that is final could have additional methods on it. Well, Scala has a feature called implicit conversion. If you have an instance of a particular type, and you need another type, and there s an implicit conversion in scope, Scala will call the implicit method to perform the conversion. For example, some date-related methods take Long, and some take java.util.Date. It s useful to have conversions between the two. We create a method that calculates the number of days based on a Long containing a millisecond count:
scala> def millisToDays(in: Long): Int = (in / (1000L * 3600L * 24L)).toInt
We can calculate the number of days by passing a Long to the method:
scala> millisToDays(5949440999L)
res3: Int = 68
However, if we try to pass a Date into the method, we correctly get an error:
scala> import java.util.Date import java.util.Date scala> millisToDays(new Date)
CHAPTER 7 TRAITS AND TYPES AND GNARLY STUFF FOR ARCHITECTS
<console>:7: error: type mismatch; found : java.util.Date required: Long millisToDays(new Date) ^
But sometimes it s valuable to convert between one type and another. We are used to the conversion in some contexts: Int Long, Int Double, and so on. We can define a method that will automatically be called when we need the conversion:
scala> implicit def dateToLong(d: Date) = d.getTime dateToLong: (java.util.Date)Long
And this allows us to call millisToDays with a Date instance:
scala> millisToDays(new Date)
res5: Int = 14286
You may think that implicit conversions are dangerous and reduce type safety. In some cases that s true. You should be very careful with them, and their use should be an explicit design choice. However, we see that sometimes implicit conversions (e.g., Int Long) are very valuable, for example, when we have a method that takes a parameter that must be a Long:
scala> def m2[T <: Long](in: T): Int = (in / (1000L * 3600L * 24L)).toInt m2: [T <: Long](T)Int scala> m2(33)
<console>:8: error: inferred type arguments [Int] do not conform to method m2's type parameter bounds [T <: Long] m2(33) ^
CHAPTER 7 TRAITS AND TYPES AND GNARLY STUFF FOR ARCHITECTS
So having to type the following could get very old:
scala> m2(33.toLong)
res8: Int = 0
This is why implicit conversion is built into the Java compiler and why it s part of the standard Scala Predef.2
Library Pimping3
The implicit conversion gets us halfway to adding methods to a final class. The second half of the journey is that the Scala compiler will look to a possible implicit conversion from the type you have to a type with the method that you re invoking. The Scala compiler will insert code to call the implicit conversion and then call the method on the resulting instance. For example:
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