I want to use String's subsequence method in blackberry which is given in java API but not present in Blackberry API. Here is the method :
String.subSequence(int beginIndex, int endIndex)
I want to use that method so how should it be possible if I cant access that API. Can u please post the code/url with code/api in Blackberry which will perform same function like subsequence method.
Java SE's subSequence is implemented in java.lang.String just because String was made to implement CharSequence interface. If you read the docs, it says:
An invocation of this method of the form
str.subSequence(begin, end)
behaves in exactly the same way as the invocation
str.substring(begin, end)
This method is defined so that the String class can implement the CharSequence interface.
So in BlackBerry, as there's no CharSequence interface because it was introduced in Java 1.4, you can use substring instead with identical result.
Related
I am trying to call a method of a class that I only know by name as a String. Now therefore I would need a ClassMirror of that class that allowes me to instantiate an instance. However, creating ClassMirrors seems to be only possible by entering a type using reflectClass(Type) or by passing an already existing instance of that class into reflect(dynamic). So these aren`t helping if I only have a String.
In Java you can do this pretty easily, by calling Class.forName(String). Then you would get a Constructor instance, make it accessibly and call it.
Does anyone know if this is even possible in dart? What seems weird is that once you have a ClassMirror you can access fields and methods by passing symbols, which can be created by Strings.
You can put a specific list of strings to map to a specific list of closures to create a new object with specific parameters.
But you can't get a reflection without using dart:mirrors, which is being deprecated, and also had a negative impact on tree shaking to get the payload size down.
In general, you're invited to look at the package:reflectable to achieve most of what you'd want out of dart:mirrors, using source-to-source builders.
I tried to use method overloading in some dart code and quickly learned that overloading is not offered in dart.
My questions are: why is it not offered, and what is the recommended alternative? Is there a standard naming convention since methods that do the same thing but with different inputs must have different names?
Is it standard to use named parameters and then check that the caller has supplied enough information to complete the calculation?
Say I have a method that returns how much money someone makes in a year, called yearlyIncome.
In Java, I would create a method like this
double yearlyIncome(double hourlyRate, double hoursWorkedPerYear)
And maybe another method like this
double yearlyIncome(double monthlyRate, int monthsWorkedPerYear)
and so on. They're all used to calculate the same thing, but with different inputs. What's the best, standardized way to do this in dart?
Thanks so much in advance.
Function overloading is not supported in Dart at all.
Function overloading requires static types. Dart at its core is a dynamically typed language.
You can either use different names for the methods or optional named or unnamed parameters
// optional unnamed
void foo(int a, [String b]);
foo(5);
foo(5, 'bar');
// optional named
void foo(int a, {String b});
foo(5);
foo(5, b :'bar');
Optional parameters can also have default values. Optional named and unnamed parameters can not be used together (only one or the other for a single function)
In the case of a constructor you can use named constructors as an alternative
Dart did not support overloading originally because it was a much more dynamic language where the declared types did not have any semantic effect. That made it impossible to use static type based overload resolution.
Dart has since changed to be more statically type, and there is nothing fundamentally preventing Dart from adding overloading today, except that it would be a huge work and a huge change to the language. Or so I'd assume, because there isn't any obvious design that isn't either highly complicated or hugely breaking.
What you do instead in Dart is to use optional parameters. A method like:
String toString([int radix]);
effectively have two signatures: String Function() and String Function(int). It can act at both signatures.
There are definite limits to how far you can go with just optional parameters, because they still need to have exactly one type each, but that is the alternative that Dart currently provides. (Or use different names, but that's not overloading, you can do that in languages with overloading too).
Optional parameters is also one of the complications if we wanted to add overloading to the Dart language - would existing functions with optional parameters would count as multiple overloadings? If you declare a class like:
abstract class WithOverloading {
String toString();
String toString(int radix);
}
is that then the same signature as:
abstract class WithoutOverloading {
String toString([int radix]);
}
Probably not because you can tear off the latter and get one function with an optional parameter, and you might not be able to tear off both functions from the former and combine them into one function. Or maybe you can, that's why it's not a trivial design question how to include overloading into the existing Dart language.
In F#, can I omit the class name when calling a static method?
Example:
In C#, I can do something like:
using static Bizmonger.Patterns.MessageBus;
...
Publish("SOME_MESSAGE");
instead of:
MessageBus.Publish("SOME_MESSAGE");
Can I do something like this in F#?
In F#, you can use open on namespaces (just like using in C#) or on modules (which is useful when the API you are calling has been written in F#), but not on static classes (which is what you'd need when calling C# libraries).
One thing that you can do though to make the code a bit shorter is to define a type alias:
type M = Bizmonger.Patterns.MessageBus;
// Now you can write just
M.Publish("SOME_MESSAGE")
// Rather than writing the full
MessageBus.Publish("SOME_MESSAGE");
There is a feature request on the F# UserVoice to allow using open on static classes (just like in C#) and so if you'd like this to happen, please upvote and comment there.
I also learned that I could implement a function to serve as a wrapper for clients to call instead.
Create a wrapper function
module Messages
open Bizmonger.Patterns
let Publish (message:string, payload:_) =
MessageBus.Publish(message, payload)
Client
Then a client can now invoke a function without specifying a class name.
open Messages
...
Publish("SOME_MESSAGE", null);
What does external mean in Dart? For example: external DateTime._now();
I'm new to Dart, I can't find documentation for external, so can you give an example to help explain？
9.4 External Functions
An external function is a function whose body is provided separately from its
declaration. An external function may be a top-level function (17), a method
The body of the function is defined somewhere else.
As far as I know this is used to fix different implementations for Dart VM in the browser and Dart VM on the Server.
When we make an external function inside a class like toString()
external String toString();
means this method is abstract and the child of the parent class will add the function body, that's because in Dart we only can make an abstract class.
Summary:
external function = abstract function in not abstract classes
I don't think external keyword is meant to be used to mark methods as abstract, even if that's possible
It's enough to leave a method with no implementation to set it abstract, inside an abstract class
It's the equivalent of declare in TypeScript, and extern in C#, those are used for interoperability with other runtimes, which means you're telling the compiler "Don't worry about this method's implementation, I promise it will exist at runtime", the runtime may be in C or Javascript or whatever
In case, if you are wondering why or where should I even use external keyword, here is a one more example for Flutter.
class MyStruct extends Struct {
#Int32()
external int a;
#Float()
external double b;
external Pointer<Void> c;
}
Sometimes, but not often when you play with native libraries, in this case with Struct to access native struct's field in memory. Under Struct We must declare all fields as external because it will be external fields from dart:ffi (C / C++).
So, external is more than just way to declare "abstract method".
9.4 External Functions
An external function is a function whose
body is provided separately from its
declaration.
What this does mean is that you define the function, but without implementation. It's exactly how you define the abstract method, but the only difference is that with external you don't implement the method in dart but in C or something else.
Something like String class it can be considered as external functions except that the String class it marked with #pragma('vm:entry-point') which make the entire class use native code.
See the following example to understand:
This dart's side.
https://github.com/dart-lang/sdk/blob/main/sdk/lib/core/string.dart#L711
This the implementation in C++.
https://github.com/dart-lang/sdk/blob/main/runtime/lib/string.cc#L467-#L472
In my opinion it is an equivalent of Java native keyword. For example, since current time milliseconds is implemented differently on Android, iOS, Linux etc, DateTime.now().millisecondsSinceEpoch will be linked to different implementations at runtime. So it is not initially known how this method will look like. For this reason it is marked as external meaning it is platform dependent.
Is there any integrated solution in Delphi 2007 to check whether a TStringList contains a part of a certain value?
e.g.:
List.AddObject('This is a string', customStringObject1);
List.AddObject('This is a mushroom', customStringObject2);
List.AddObject('Random stuff', customStringObject3);
Searching for "This is a" is supposed to deliver me "true", since the first two elements contain this partwise.
The only method i'm aware of so far is TStringList.find(string,integer), but this performs a complete string comparision, i.e. only searching for This is a string will return true.
Any suggestions?
Not integrated, but you can use the Pos function on the Text property:
Pos('This is a', List.Text)
And if you want it to be integrated, you can create a class helper for TStrings.
Not directly, no. You would have to either:
1) call Pos() on the Text property, which is not efficient if you have a lot of strings.
2) loop through the list manually, calling Pos() on each String. More efficient, but also more coding.
3) derive a new class from TStringList and override its virtual CompareStrings() method to compare strings however you want (the default implementation simple calls AnsiCompareStr() or AnsiCompareText(), depending on the CaseSensitive property). Return 0 if you find a match. You can then use the TStringList.Find() method, which calls CompareStrings() internally (be careful, so does TStringList.Sort(), but you can avoid that if you call TStringList.CustomSort() instead).