Questions tagged [generics]
Generics are a form of parametric polymorphism found in a range of languages, including .NET languages, Java, Swift, Rust and Go (since 1.18).
51,124
questions
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answers
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How to Sort a List<T> by a property in the object
I have a class called Order which has properties such as OrderId, OrderDate, Quantity, and Total. I have a list of this Order class:
List<Order> objListOrder = new List<Order>();
...
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1438
votes
22
answers
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views
Create Generic method constraining T to an Enum
I'm building a function to extend the Enum.Parse concept that
Allows a default value to be parsed in case that an Enum value is not found
Is case insensitive
So I wrote the following:
public static ...
- 39.9k
1295
votes
9
answers
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views
How do I call a generic method using a Type variable?
What's the best way to call a generic method when the type parameter isn't known at compile time, but instead is obtained dynamically at runtime?
Consider the following sample code - inside the ...
- 44k
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votes
32
answers
1.0m
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How can I create a generic array in Java?
Due to the implementation of Java generics, you can't have code like this:
public class GenSet<E> {
private E a[];
public GenSet() {
a = new E[INITIAL_ARRAY_LENGTH]; // Error: ...
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1052
votes
13
answers
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Difference between <? super T> and <? extends T> in Java [duplicate]
What is the difference between List<? super T> and List<? extends T> ?
I used to use List<? extends T>, but it does not allow me to add elements to it list.add(e), whereas the List&...
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908
votes
17
answers
169k
views
What is PECS (Producer Extends Consumer Super)?
I came across PECS (short for Producer extends and Consumer super) while reading up on generics.
Can someone explain to me how to use PECS to resolve confusion between extends and super?
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908
votes
19
answers
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views
Is List<Dog> a subclass of List<Animal>? Why are Java generics not implicitly polymorphic?
I'm a bit confused about how Java generics handle inheritance / polymorphism.
Assume the following hierarchy -
Animal (Parent)
Dog - Cat (Children)
So suppose I have a method doSomething(List<...
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905
votes
25
answers
999k
views
How do I get a class instance of generic type T?
I have a generics class, Foo<T>. In a method of Foo, I want to get the class instance of type T, but I just can't call T.class.
What is the preferred way to get around it using T.class?
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850
votes
18
answers
879k
views
How to get the type of T from a member of a generic class or method
Let's say I have a generic member in a class or method, like so:
public class Foo<T>
{
public List<T> Bar { get; set; }
public void Baz()
{
// get type of T
} ...
- 139k
820
votes
15
answers
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views
What is a raw type and why shouldn't we use it?
Questions:
What are raw types in Java, and why do I often hear that they shouldn't be used in new code?
What is the alternative if we can't use raw types, and how is it better?
- 380k
758
votes
30
answers
905k
views
How do I clone a generic list in C#?
I have a generic list of objects in C#, and wish to clone the list. The items within the list are cloneable, but there doesn't seem to be an option to do list.Clone().
Is there an easy way around ...
- 7,887
704
votes
19
answers
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views
How do I make the method return type generic?
Consider this example (typical in OOP books):
I have an Animal class, where each Animal can have many friends.
And subclasses like Dog, Duck, Mouse etc which add specific behavior like bark(), ...
- 20.9k
704
votes
22
answers
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views
How do I address unchecked cast warnings?
Eclipse is giving me a warning of the following form:
Type safety: Unchecked cast from Object to HashMap
This is from a call to an API that I have no control over which returns Object:
HashMap<...
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669
votes
30
answers
862k
views
Get generic type of class at runtime
How can I achieve this?
public class GenericClass<T>
{
public Type getMyType()
{
//How do I return the type of T?
}
}
Everything I have tried so far always returns type ...
- 6,715
665
votes
13
answers
249k
views
How can I return NULL from a generic method in C#?
I have a generic method with this (dummy) code (yes I'm aware IList has predicates, but my code is not using IList but some other collection, anyway this is irrelevant for the question...)
static ...
656
votes
29
answers
612k
views
Create instance of generic type in Java?
Is it possible to create an instance of a generic type in Java? I'm thinking based on what I've seen that the answer is no (due to type erasure), but I'd be interested if anyone can see something I'm ...
- 43.4k
638
votes
32
answers
852k
views
Remove duplicates from a List<T> in C#
Anyone have a quick method for de-duplicating a generic List in C#?
- 39.8k
612
votes
28
answers
564k
views
How to remove elements from a generic list while iterating over it?
I am looking for a better pattern for working with a list of elements which each need processed and then depending on the outcome are removed from the list.
You can't use .Remove(element) inside a ...
- 10.8k
561
votes
18
answers
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List<T> or IList<T> [closed]
Can anyone explain to me why I would want to use IList over List in C#?
Related question: Why is it considered bad to expose List<T>
- 19.2k
553
votes
9
answers
87k
views
Why does C# forbid generic attribute types?
This causes a compile-time exception:
public sealed class ValidatesAttribute<T> : Attribute
{
}
[Validates<string>]
public static class StringValidation
{
}
I realize C# does not ...
- 45.2k
544
votes
6
answers
155k
views
Possible heap pollution via varargs parameter
I understand this occurs with Java 7 when using varargs with a generic type;
But my question is..
What exactly does Eclipse mean when it says "its use could potentially pollute the heap?"
And
How ...
- 9,714
536
votes
15
answers
437k
views
Deserialize a List<T> object with Gson?
I want to transfer a list object via Google Gson, but I don't know how to deserialize generic types.
What I tried after looking at this (BalusC's answer):
MyClass mc = new Gson().fromJson(result, new ...
- 7,948
502
votes
11
answers
502k
views
What is SuppressWarnings ("unchecked") in Java?
Sometime when looking through code, I see many methods specify an annotation:
@SuppressWarnings("unchecked")
What does this mean?
- 13.7k
501
votes
12
answers
479k
views
ArrayList vs List<> in C#
What is the difference between ArrayList and List<> in C#?
Is it only that List<> has a type while ArrayList doesn't?
- 14.3k
500
votes
7
answers
176k
views
What is the point of the diamond operator (<>) in Java?
The diamond operator in java 7 allows code like the following:
List<String> list = new LinkedList<>();
However in Java 5/6, I can simply write:
List<String> list = new LinkedList()...
- 7,752
499
votes
1
answer
344k
views
C# generics syntax for multiple type parameter constraints [duplicate]
Possible Duplicate:
Generic methods and multiple constraints
I need a generic function that has two type constraints, each inheriting from a different base class. I know how to do this with one ...
- 51.5k
485
votes
10
answers
797k
views
List<T> OrderBy Alphabetical Order
I'm using C# on Framework 3.5. I'm looking to quickly sort a Generic List<T>. For the sake of this example, let's say I have a List of a Person type with a property of lastname. How would I ...
- 9,845
484
votes
17
answers
264k
views
Passing arguments to C# generic new() of templated type
I'm trying to create a new object of type T via its constructor when adding to the list.
I'm getting a compile error: The error message is:
'T': cannot provide arguments when creating an instance ...
- 13.9k
484
votes
22
answers
462k
views
A generic list of anonymous class
In C# 3.0 you can create anonymous class with the following syntax
var o = new { Id = 1, Name = "Foo" };
Is there a way to add these anonymous class to a generic list?
Example:
var o = new { Id = ...
- 6,470
466
votes
24
answers
157k
views
Is there a constraint that restricts my generic method to numeric types?
Can anyone tell me if there is a way with generics to limit a generic type argument T to only:
Int16
Int32
Int64
UInt16
UInt32
UInt64
I'm aware of the where keyword, but can't find an interface for ...
- 17.9k
464
votes
20
answers
138k
views
Passing a single item as IEnumerable<T>
Is there a common way to pass a single item of type T to a method which expects an IEnumerable<T> parameter? Language is C#, framework version 2.0.
Currently I am using a helper method (it's ....
- 50.6k
454
votes
11
answers
63k
views
What are the reasons why Map.get(Object key) is not (fully) generic
What are the reasons behind the decision to not have a fully generic get method
in the interface of java.util.Map<K, V>.
To clarify the question, the signature of the method is
V get(Object ...
- 12.9k
437
votes
2
answers
193k
views
Java Generics With a Class & an Interface - Together
I want to have a Class object, but I want to force whatever class it represents to extend class A and implement interface B.
I can do:
Class<? extends ClassA>
Or:
Class<? extends ...
- 21.1k
435
votes
8
answers
234k
views
Method has the same erasure as another method in type
Why is it not legal to have the following two methods in the same class?
class Test{
void add(Set<Integer> ii){}
void add(Set<String> ss){}
}
I get the compilation error
Method ...
- 32.5k
416
votes
3
answers
131k
views
How does the reified keyword in Kotlin work?
I'm trying to understand the purpose of the reified keyword. Apparently, it's allowing us to do reflection on generics.
However, when I leave it out, it works just as fine. When does this make an ...
- 5,689
407
votes
14
answers
144k
views
Can't operator == be applied to generic types in C#?
According to the documentation of the == operator in MSDN,
For predefined value types, the
equality operator (==) returns true if
the values of its operands are equal,
false otherwise. For ...
- 42.1k
401
votes
11
answers
174k
views
What does "where T : class, new()" mean?
Can you please explain to me what where T : class, new() means in the following line of code?
void Add<T>(T item) where T : class, new();
- 6,363
393
votes
12
answers
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views
Using Mockito to mock classes with generic parameters
Is there a clean method of mocking a class with generic parameters? Say I have to mock a class Foo<T> which I need to pass into a method that expects a Foo<Bar>. I can do the following ...
- 6,341
377
votes
4
answers
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views
Case insensitive access for generic dictionary
I have an application that use managed dlls. One of those dlls return a generic dictionary:
Dictionary<string, int> MyDictionary;
The dictionary contains keys with upper and lower case.
...
- 4,889
370
votes
9
answers
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views
Why is "extends T" allowed but not "implements T"?
Is there a special reason in Java for using always "extends" rather than "implements" for defining bounds of type parameters?
For example:
public interface C {}
public class A<B ...
- 3,701
368
votes
13
answers
326k
views
Nullable type as a generic parameter possible?
I want to do something like this :
myYear = record.GetValueOrNull<int?>("myYear"),
Notice the nullable type as the generic parameter.
Since the GetValueOrNull function could return null my ...
- 3,723
356
votes
14
answers
123k
views
Null or default comparison of generic argument in C#
I have a generic method defined like this:
public void MyMethod<T>(T myArgument)
The first thing I want to do is check if the value of myArgument is the default value for that type, something ...
- 6,813
356
votes
16
answers
191k
views
Check if a class is derived from a generic class
I have a generic class in my project with derived classes.
public class GenericClass<T> : GenericInterface<T>
{
}
public class Test : GenericClass<SomeType>
{
}
Is there any way ...
- 11.8k
353
votes
12
answers
536k
views
What causes javac to issue the "uses unchecked or unsafe operations" warning
For example:
javac Foo.java
Note: Foo.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
user23987
344
votes
8
answers
92k
views
IEnumerable and Recursion using yield return
I have an IEnumerable<T> method that I'm using to find controls in a WebForms page.
The method is recursive and I'm having some problems returning the type I want when the yield return is ...
- 53.7k
336
votes
4
answers
136k
views
Generic method with multiple constraints
I have a generic method which has two generic parameters. I tried to compile the code below but it doesn't work. Is it a .NET limitation? Is it possible to have multiple constraints for different ...
- 40k
328
votes
12
answers
354k
views
Create instance of generic type whose constructor requires a parameter?
If BaseFruit has a constructor that accepts an int weight, can I instantiate a piece of fruit in a generic method like this?
public void AddFruit<T>()where T: BaseFruit{
BaseFruit fruit = ...
- 92.1k
325
votes
6
answers
207k
views
What is the difference between 'E', 'T', and '?' for Java generics?
I come across Java code like this:
public interface Foo<E> {}
public interface Bar<T> {}
public interface Zar<?> {}
What is the difference among all three of the above and what ...
- 11.8k
319
votes
16
answers
233k
views
What's the reason I can't create generic array types in Java?
What's the reason why Java doesn't allow us to do
private T[] elements = new T[initialCapacity];
I could understand .NET didn't allow us to do that, as in .NET you have value types that at run-time ...
- 103k
316
votes
15
answers
405k
views
Get generic type of java.util.List
I have;
List<String> stringList = new ArrayList<String>();
List<Integer> integerList = new ArrayList<Integer>();
Is there a (easy) way to retrieve the generic type of the ...
- 16.4k