subclass

问题 class BaseDoc { static Collection; static find(){ // which is expected to return an instance of SubDoc when run SubDoc.find() return this.Collection.find(); } } class SubDoc extends BaseDoc {} What I hope is: When running SubDoc.find(), the app knows the type of the return value to be an instance of SubDoc rather than BaseDoc. How can I achieve that? 回答1: You can create a generic version of find that will have the this parameter inferred to the type of the class it's being called on, and use

问题 class BaseDoc { static Collection; static find(){ // which is expected to return an instance of SubDoc when run SubDoc.find() return this.Collection.find(); } } class SubDoc extends BaseDoc {} What I hope is: When running SubDoc.find(), the app knows the type of the return value to be an instance of SubDoc rather than BaseDoc. How can I achieve that? 回答1: You can create a generic version of find that will have the this parameter inferred to the type of the class it's being called on, and use

问题 In the following code- class Mammal { String name = "furry "; String makeNoise() { return "generic noise"; } } class Zebra extends Mammal { String name = "stripes "; String makeNoise() { return "bray"; } } public class ZooKeeper { public static void main(String[] args) { new ZooKeeper().go(); } void go() { Mammal m = new Zebra(); System.out.println(m.name + m.makeNoise()); Zebra z = new Zebra(); System.out.println(z.name + z.makeNoise()); } } Both objects ( m and z ), if I see in debug

问题 In the following code- class Mammal { String name = "furry "; String makeNoise() { return "generic noise"; } } class Zebra extends Mammal { String name = "stripes "; String makeNoise() { return "bray"; } } public class ZooKeeper { public static void main(String[] args) { new ZooKeeper().go(); } void go() { Mammal m = new Zebra(); System.out.println(m.name + m.makeNoise()); Zebra z = new Zebra(); System.out.println(z.name + z.makeNoise()); } } Both objects ( m and z ), if I see in debug

问题 I am simply trying to add two UIButton s and a UILabel to a subclass of UIView that I made. Here is that subclass: InvitedView.m #import "InvitedView.h" #import "AppDelegate.h" @class ViewController; @interface InvitedView() { UIButton *accept; UIButton *decline; UILabel *question; UIView *gray; ViewController *myViewController; NSString *holduser; } @end @implementation InvitedView - (id)initWithFrame:(CGRect)frame { self = [super initWithFrame:frame]; if (self) { gray = [[UIView alloc]

问题 I am simply trying to add two UIButton s and a UILabel to a subclass of UIView that I made. Here is that subclass: InvitedView.m #import "InvitedView.h" #import "AppDelegate.h" @class ViewController; @interface InvitedView() { UIButton *accept; UIButton *decline; UILabel *question; UIView *gray; ViewController *myViewController; NSString *holduser; } @end @implementation InvitedView - (id)initWithFrame:(CGRect)frame { self = [super initWithFrame:frame]; if (self) { gray = [[UIView alloc]

问题 I'm trying to make a new CLLocation subclass. This is the skeleton: #import <UIKit/UIKit.h> #import <CoreLocation/CoreLocation.h> @interface JFLocation : CLLocation { } @end #import "JFLocation.h" @implementation JFLocation @end When I build the class I'm getting this errors: Undefined symbols: ".objc_class_name_CLLocation", referenced from: .objc_class_name_JFLocation in JFLocation.o ld: symbol(s) not found Any Idea?? Thank you!! 回答1: You should add CoreLocation.framework to the frameworks

问题 I'm trying to make a new CLLocation subclass. This is the skeleton: #import <UIKit/UIKit.h> #import <CoreLocation/CoreLocation.h> @interface JFLocation : CLLocation { } @end #import "JFLocation.h" @implementation JFLocation @end When I build the class I'm getting this errors: Undefined symbols: ".objc_class_name_CLLocation", referenced from: .objc_class_name_JFLocation in JFLocation.o ld: symbol(s) not found Any Idea?? Thank you!! 回答1: You should add CoreLocation.framework to the frameworks

问题 I have subclassed NSException class to create CustomException class. Whenever I catch an exception in code (in @catch), i want to initialize an object of CustomException (subclass of NSException) with the object of NSException that is passed as a parameter to @catch. Something like this @catch (NSException * e) { CustomException * ex1=[[CustomException alloc]initWithException:e errorCode:@"-11011" severity:1]; } I tried doing it by passing the NSException object to the init method of

问题 Below is sample code that subclasses datetime. Since pass is the only subclass body, method '__new__' of datetime is expected to be preserved. The following code has been tested on Python 3.4.2 on Mac OS 10.12.3 and Python 3.6.0 on Arch Linux. In both cases, same result. The question is, why is 'a' an instance of MyDatetime when 'b' is not an instance of MyDatetime when they differ only by argument tz? Thank you for your feedback. Now on with the example... #!/usr/bin/env python3 from