abstract-class

问题 I have a class that has to take a custom widget. This one can have two different implementations, so I would like to have an abstract class as interface and create two other classes those extend the abstract one. So, I have: abstract class ICustomWidget extends StatelessWidget{} class A extends ICustomWidget{ @override Widget build(BuildContext context) => //Implementation } class B extends ICustomWidget { @override Widget build(BuildContext context) => //Implementation } I want to ask if

问题 I have a class that has to take a custom widget. This one can have two different implementations, so I would like to have an abstract class as interface and create two other classes those extend the abstract one. So, I have: abstract class ICustomWidget extends StatelessWidget{} class A extends ICustomWidget{ @override Widget build(BuildContext context) => //Implementation } class B extends ICustomWidget { @override Widget build(BuildContext context) => //Implementation } I want to ask if

问题 I have a class that has to take a custom widget. This one can have two different implementations, so I would like to have an abstract class as interface and create two other classes those extend the abstract one. So, I have: abstract class ICustomWidget extends StatelessWidget{} class A extends ICustomWidget{ @override Widget build(BuildContext context) => //Implementation } class B extends ICustomWidget { @override Widget build(BuildContext context) => //Implementation } I want to ask if

问题 I have a class that has to take a custom widget. This one can have two different implementations, so I would like to have an abstract class as interface and create two other classes those extend the abstract one. So, I have: abstract class ICustomWidget extends StatelessWidget{} class A extends ICustomWidget{ @override Widget build(BuildContext context) => //Implementation } class B extends ICustomWidget { @override Widget build(BuildContext context) => //Implementation } I want to ask if

问题 There are two way for choosing between abstract class or interface. Microsoft solution and Oracle solution: Microsoft, design guideline: Do use abstract (MustInherit in Visual Basic) classes instead of interfaces to decouple the contract from implementations. http://msdn.microsoft.com/en-us/library/ms229013.aspx Oracle, The Java Tutorials: If an abstract class contains only abstract method declarations, it should be declared as an interface instead. http://docs.oracle.com/javase/tutorial/java

问题 I read source code of Java Graphics abstract class, I am curious how does this abstract void drawline method draws lines in JComponent's paint(Graphics g) and paintComponent(Graphics g). I know that abstract methods have no method body. I couldn't find any relevant examples with Google. If possible, can you provide me a link to source code of this method. 回答1: Mad Programmer is right, all of java's graphics and graphics2d methods is directed with native codes. If you're curious about these

问题 I have two classes that are supposed to implement the same test cases for two independent libraries (let's call them LibA and LibB ). So far I define the test methods to be implemented in an abstract base class which ensures that both test classes implement all desired tests: from abc import ABC, abstractmethod class MyTests(ABC): @abstractmethod def test_foo(self): pass class TestsA(MyTests): def test_foo(self): pass class TestsB(MyTests): def test_foo(self): pass This works as expected, but

问题 In many languages if you try to instantiate abstract class you get compile time error. In Dart however, you get warning while compiling and a runtime exception AbstractClassInstantiationError . Why is that? Can someone provide an example, where it's reasonable to compile such code? 回答1: Dart tries to allow you to run your program while you are developing it. That is why many things that are compile time errors in other languages are compile-time warnings and runtime errors in Dart. This

问题 I have created a custom abstract class, which in turn of course derived classes were also created. public abstract class AbstractBaseClass ... public class ChildClass1 : AbstractBaseClass ... Now, whenever I declare for example AbstractBaseClass baseClass = null , and wherever null checks follow after this initialization, it always fails. if (baseClass == null) { // this block is never reached - condition always evaluates to false // let's say AbstractBaseClass baseClass = null is at line 10

问题 I have this models: class BillHeader(models.Model): billno = models.IntegerField(primary_key=True, blank=True) class BillData(models.Model): price = models.DecimalField(_('Price'), max_digits=12, decimal_places=2) amount = models.DecimalField(_('Amount'), max_digits=6, decimal_places=2) [... rest of the model ...] class Meta: abstract = True class BillFooter(models.Model): total = models.DecimalField(_('Total'), max_digits=12, decimal_places=2) [... rest of the model ...] class Meta: abstract