If we try to add an element, then we will get a UnsupportedOperationException. This makes the initialized list fixed in size, and we cannot add more elements to it. The returned instance from this method is a List backed by the input array. List listOfStrings = Arrays.asList("Justin", "Jessica", "Simon", "Harry") We can directly pass the elements to this method. This method can take varargs, so we don't even need an array to initialize the list. We can create a List from an array by using the asList() method. In this tutorial, we will learn some one-liners to initialize a List in Java. The List interface implements the Collection interface. To illustrate the issue, consider the following attempt to write a generic method that creates an array.The List is an interface in Java and is implemented by classes like the ArrayList and the LinkedList. This is an even harder problem, which requires a little of help from you. Representing the generic array type is not enough, however, there must also be a way to create generic arrays. This means that if you need maximal performance, you should prefer concrete to generic arrays. You can expect accesses to generic arrays to be three to four times slower than accesses to primitive or object arrays. These type tests slow down array operations somewhat. At run-time, when an element of an array of type Array 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. The only common run-time type encompassing all of these types is AnyRef (or, equivalently ), so that’s the type to which the Scala compiler maps Array. How then is Scala’s Array represented? In fact a generic array like Array 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. What about genericity? In Java, you cannot write a T where T is a type parameter. So now you know how arrays can be compatible with sequences and how they can support all sequence operations. So if both conversions are applicable, the one in Predef is chosen. Implicits in subclasses and subobjects take precedence over implicits in base classes. The first is defined in the Predef object whereas the second is defined in a class scala.LowPriorityImplicits, which is inherited by Predef. The ArrayOps conversion has a higher priority than the WrappedArray conversion. The answer to that question is that the two implicit conversions are prioritized. After all, both conversions map an array to a type that supports a reverse method, which is what the input specified. This raises the question of how the compiler picked intArrayOps over the other implicit conversion to WrappedArray in the line above. Where intArrayOps is the implicit conversion that was inserted previously. The difference between the two implicit conversions on arrays is shown in the next REPL dialogue: Modern VMs often avoid creating this object entirely. Typically, this ArrayOps object is short-lived it will usually be inaccessible after the call to the sequence method and its storage can be recycled. “Adding” means that the array is wrapped in another object of type ArrayOps which supports all sequence methods. This conversion simply “adds” all sequence methods to arrays but does not turn the array itself into a sequence. There is yet another implicit conversion that gets applied to arrays. The last REPL line above shows that wrapping and then unwrapping with toArray gives the same array you started with. To go the other way, from a WrappedArray to an Array, you can use the toArray method defined in Traversable. The interaction above demonstrates that arrays are compatible with sequences, because there’s an implicit conversion from arrays to WrappedArrays. Instead, there is an implicit “wrapping” conversion between arrays and instances of class, which is a subclass of Seq. It can’t really be that because the data type representation of a native array is not a subtype of Seq. In Scala 2.8 an array does not pretend to be a sequence. Instead, the Scala 2.8 array implementation makes systematic use of implicit conversions. There were some puzzling corner cases and the performance of array operations was not all that predictable. The details of this were quite complicated, in particular when one created a new array of generic type Array. Previously, the Scala compiler somewhat “magically” wrapped and unwrapped arrays to and from Seq objects when required in a process called boxing and unboxing. Given that Scala arrays are represented just like Java arrays, how can these additional features be supported in Scala? In fact, the answer to this question differs between Scala 2.8 and earlier versions.
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