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Copy file name to clipboardExpand all lines: overviews/collections/arrays.md
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@@ -58,7 +58,7 @@ The `ArrayOps` object gets inserted automatically by the implicit conversion. So
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scala> intArrayOps(a1).reverse
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res5: Array[Int] = Array(3, 2, 1)
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where `intArrayOps` is the implicit conversion that was inserted previously. This raises the question how the compiler picked `intArrayOps` over the other implicit conversion to `WrappedArray` in the line above. After all, both conversions map an array to a type that supports a reverse method, which is what the input specified. The answer to that question is that the two implicit conversions are prioritized. The `ArrayOps` conversion has a higher priority than the `WrappedArray` conversion. The first is defined in the `Predef` object whereas the second is defined in a class `scala.LowPritoryImplicits`, which is inherited from `Predef`. Implicits in subclasses and subobjects take precedence over implicits in base classes. So if both conversions are applicable, the one in `Predef` is chosen. A very similar scheme works for strings.
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where `intArrayOps` is the implicit conversion that was inserted previously. This raises the question how the compiler picked `intArrayOps` over the other implicit conversion to `WrappedArray` in the line above. After all, both conversions map an array to a type that supports a reverse method, which is what the input specified. The answer to that question is that the two implicit conversions are prioritized. The `ArrayOps` conversion has a higher priority than the `WrappedArray` conversion. The first is defined in the `Predef` object whereas the second is defined in a class `scala.LowPriorityImplicits`, which is inherited from `Predef`. Implicits in subclasses and subobjects take precedence over implicits in base classes. So if both conversions are applicable, the one in `Predef` is chosen. A very similar scheme works for strings.
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So now you know how arrays can be compatible with sequences and how they can support all sequence operations. What about genericity? In Java you cannot write a `T[]` where `T` is a type parameter. How then is Scala's `Array[T]` represented? In fact a generic array like `Array[T]` could be at run-time any of Java's eight primitive array types `byte[]`, `short[]`, `char[]`, `int[]`, `long[]`, `float[]`, `double[]`, `boolean[]`, or it could be an array of objects. The only common run-time type encompassing all of these types is `AnyRef` (or, equivalently `java.lang.Object`), so that's the type to which the Scala compiler maps `Array[T]`. At run-time, when an element of an array of type `Array[T]` is accessed or updated there is a sequence of type tests that determine the actual array type, followed by the correct array operation on the Java array. These type tests slow down array operations somewhat. You can expect accesses to generic arrays to be three to four times slower than accesses to primitive or object arrays. This means that if you need maximal performance, you should prefer concrete over generic arrays. Representing the generic array type is not enough, however, There must also be a way to create generic arrays. This is an even harder problem, which requires a little bit of help from you. To illustrate the problem, consider the following attempt to write a generic method that creates an array.
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