generics

问题 regarding the following code: public class Test <T extends Comparable>{ public static void main(String[] args){ List<String> lst = Array.asList("abc","def"); System.out.println(func(lst)); } public static boolean func(List<**here**> lst){ return lst.get(0).compareTo(lst.get(1)) == 0; } } why writing " ? extends Comparable" here would compile , and writing "Comparable" would not compile? thanks in advance. 回答1: This happens because generics are invariant . Even if String is a Comparable ,

问题 regarding the following code: public class Test <T extends Comparable>{ public static void main(String[] args){ List<String> lst = Array.asList("abc","def"); System.out.println(func(lst)); } public static boolean func(List<**here**> lst){ return lst.get(0).compareTo(lst.get(1)) == 0; } } why writing " ? extends Comparable" here would compile , and writing "Comparable" would not compile? thanks in advance. 回答1: This happens because generics are invariant . Even if String is a Comparable ,

问题 I have a helper class calling a REST web service. This helper class receives the information from the web service within a class defined as a generic "R". I can easily use GSON to read my JSON in a POJO, but I can't figure out how to give it a List and read it properly. So far I have tried to extract the type of R in a variable and pass it to the List without success. I have tried using Jackson with its TypeFactory, also without success, it won't take R as its parameter. What I want to do is

问题 I have a helper class calling a REST web service. This helper class receives the information from the web service within a class defined as a generic "R". I can easily use GSON to read my JSON in a POJO, but I can't figure out how to give it a List and read it properly. So far I have tried to extract the type of R in a variable and pass it to the List without success. I have tried using Jackson with its TypeFactory, also without success, it won't take R as its parameter. What I want to do is

问题 This doesn't compile because Initializer 'init(_:)' requires that 'Number' conform to 'BinaryInteger' struct Percentage<Number: Numeric> { let value: Number let total: Number var percentage: Double { Double(value) / Double(total) } } Does anyone have a nice solution? To give some context to the problem from real life: I'm coding a SwiftUI app, that has a CircularProgress-view. I would like to use the same CircularProgress-view with different number types and to be able to show the current

问题 Everything is in the title, but let me put some code. Let's say we have a class with 2 methods with the same name: class MyClass { /// <summary> /// The first version /// </summary> public TItem Foo(long param1) { ... } /// <summary> /// Overload with a generic parameters /// </summary> public bool Foo<TItem>(out TItem item, long param1) { ... } } An we need to get the MethodInfo for the second 'Foo' method which has a generic out parameter: public class AnotherClass { public MethodInfo

问题 I have simple generic class: class MyClass<T> { let closure: (T -> Void) init(closure: T -> Void) { self.closure = closure } } I would like to write an extension for UIView which would apply closure to any subclass of UIView : extension UIView { func apply(c: MyClass<Self>) -> Self { c.closure(self) return self } } But it gives me an error: 'Self' is only available in a protocol or as the result of a method in a class . Is there any solution to fix this code? 回答1: You can achieve this by

问题 When comes to bridge method, i do know that java compiler will add them if there's a need so that overriding can be done properly by the subclass (after reading SCJP by mughal and angelikalanger website). But this is a bit confusing as per below: Before erasure: class x <T> { void set(T t){} } class y <E> extends x { void set(E e) {} // name clash here } class z<E> extends x { void set(Object y) {} // no name clash here } class z1<E> extends x<T> { void set(Object y) {} // name clash here }

问题 From Effective Java by Joshua Bloch, Arrays differ from generic type in two important ways. First arrays are covariant. Generics are invariant. Covariant simply means if X is subtype of Y then X[] will also be sub type of Y[]. Arrays are covariant As string is subtype of Object So String[] is subtype of Object[] Invariant simply means irrespective of X being subtype of Y or not , List<X> will not be subType of List<Y>. My question is why the decision to make arrays covariant in Java? There

问题 From Effective Java by Joshua Bloch, Arrays differ from generic type in two important ways. First arrays are covariant. Generics are invariant. Covariant simply means if X is subtype of Y then X[] will also be sub type of Y[]. Arrays are covariant As string is subtype of Object So String[] is subtype of Object[] Invariant simply means irrespective of X being subtype of Y or not , List<X> will not be subType of List<Y>. My question is why the decision to make arrays covariant in Java? There