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When I try to convert using Java 8 streams, I get a compile error:
Incompatible types
Required: List
Found: java.lang.Object
ArrayList list = new ArrayList();
list.add("Test");
list.add("Java");
list.add("Other");
//This won't compile
List<String> strings = list.stream()
.map(object -> Objects.toString(object, null))
.collect(Collectors.toList());
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While brand name drug refers to the name giving by the producing company, generic drug refers to a drug produced after the active ingredient of the brand name drug. Generic drugs will, however, be sold under different brand names, but will contain the same active ingredients as the brand-name drug.
The Java Generics programming is introduced in J2SE 5 to deal with type-safe objects. It makes the code stable by detecting the bugs at compile time. Before generics, we can store any type of objects in the collection, i.e., non-generic. Now generics force the java programmer to store a specific type of objects.
That is, ArrayList<String> is a subtype of List<String> , since ArrayList is a subtype of List and both have the same parametric type String .
Non-generic, in other words, concrete, classes can inherit from closed constructed base classes, but not from open constructed classes or from type parameters because there is no way at run time for client code to supply the type argument required to instantiate the base class.
List comprehension in python generates a list of elements from an existing list. It then employs the for loop to traverse the iterable objects in an element-wise pattern. To convert a list to a string, use Python List Comprehension and the join () function.
While refactoring old code, you might have already encountered that the classes are inundated with lots of non-generic List. It is important for a codebase to evolve and that’s why I would like to discuss about how to refactor your old non-generic List to generic List.
T is the type of each item in the resulting list. L is the type of the list that I wish to create (which extends List<T> ). untypedList is the "untyped" input list, effectively the same as List<Object>. Show activity on this post.
The point to be noted here is that a list is an ordered sequence of object types and a string is an ordered sequence of characters. This is the main difference between the two. A sequence is a data type composed of multiple elements of the same data type, such as integer, float, character, etc.
From comment:
I'm actually calling a 3rd party method that returns
List
Then cast the value to a List<Object> or List<?> (thanks, Sean Van Gorder) so it is not a raw generic anymore, and the stream code will work:
List<?> list = thirdPartyMethod();
List<String> strings = list.stream()
.map(object -> Objects.toString(object, null))
.collect(Collectors.toList());
Logically, there is no difference between a List and a List<Object>, but there is a big difference to the compiler, because a raw generic forces backwards-compatibility mode in the compiler, where all generics are ignored, causing the entire stream-chain to behave badly.
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