abstract

Protected abstract or public abstract method in abstract class

由 回眸只為那壹抹淺笑 提交于 2019-12-23 10:13:31
问题 Hi I have an abstract class in which I have some public methods and some abstract ones. I have the public so that they implement the common methods for the derived classes. What is confusing me is why I will want to define a public abstract method instead of protected abstract. That makes no sense to me to define a public abstract method in abstract class.... because if is an abstract will be overridden, in the derived classes, but the same is if is defined as public but somehow it makes more

AbstractMethodError on calling Exception.printStackTrace

由 依然范特西╮ 提交于 2019-12-23 09:26:50
问题 Inside a catch clause I want to print the strack trace of the exception: try { ... } catch (Exception exc) { exc.printStackTrace(); ... } But in some cases I don't get a stack trace and instead see something like this: Exception in thread "pool-1-thread-2" java.lang.AbstractMethodError: java.lang.Exception.printStackTrace()V ... Usually, this exception should occur if a library has not the same version at runtime as at compile time, but in this case I work with a class from the Java library.

Understanding how drawLine works

由 喜欢而已 提交于 2019-12-23 08:33:07
问题 Given the following code: import javax.swing.*; import java.awt.*; public class NewClass extends JPanel { public void paintComponent(Graphics g) { g.drawLine(0, 0, 90, 90); } public static void main(String[] args) { JFrame jf = new JFrame(); jf.add(new NewClass()); jf.setSize(500, 500); jf.setVisible(true); } } Why does it draw a line if the method drawLine is abstract and, as I managed to understand, an abstract method has no definition? Thank you in advance! 回答1: paintComponent() gets a non

Abstract class with default value

由 我与影子孤独终老i 提交于 2019-12-23 06:51:12
问题 I am trying to define a abstract Range class which will serve as the base implementation of a number of range classes. The intended use is irrelevant for this question, but so far I have: /** * Abstract generic utility class for handling ranges */ public abstract class Range<T extends Number> { // Variables to hold the range configuration private T start; private T stop; private T step; /** * Constructs a range by defining it's limits and step size. * * @param start The beginning of the range

Can an overriding method have a different access specifier from that in the base class?

由 烈酒焚心 提交于 2019-12-23 06:48:38
问题 Which access modifier, in an abstract class, do I have to use for a method, so the subclasses can decide whether it should be public or not? Is it possible to "override" a modifier in Java or not? public abstract class A { ??? void method(); } public class B extends A { @Override public void method(){ // TODO } } public class C extends B { @Override private void method(){ // TODO } } I know that there will be a problem with static binding, if someone calls: // Will work A foo = new B() foo

PYOMO: Operation on sets of abstract model

由 为君一笑 提交于 2019-12-23 04:57:19
问题 I want to operate on abstract sets. But it dosen't work. from pyomo.environ import * m = AbstractModel() m.A = Set(initialize=[0,1,2]) m.B = Set(initialize=[0]) m.C = m.A-m.B instance = m.create_instance() for c in instance.C.value: print(c) TypeError: 'NoneType' object is not iterable 回答1: Based on what you told to Qi Chen, here is a working example of your code if you used the AbstractModel formulation. The thing, with abstract models, is that it doesn't do much more than delay the

Why the forEach method in Iterable interface is not abstract? [duplicate]

由 孤街醉人 提交于 2019-12-23 04:43:39
问题 This question already has answers here : Interface with default methods vs Abstract class in Java 8 (15 answers) Closed 2 years ago . When I see the Iterable interface source, it looks like the foreach method and Spliterator methods are not abstract. How an interface can have non abstract methods? Or is there anything I am missing in this? See the Iterbale interface source below. package java.lang; import java.util.Iterator; import java.util.Objects; import java.util.Spliterator; import java

java abstarct与Interface的区别

由 会有一股神秘感。 提交于 2019-12-22 19:20:28
【推荐】2019 Java 开发者跳槽指南.pdf(吐血整理) >>> abstract class和interface是Java语言中对于抽象类定义进行支持的两种机制，正是由于这两种机制的存在，才赋予了Java强大的面向对象能力。 abstract class和interface之间在对于抽象类定义的支持方面具有很大的相似性，甚至可以相互替换，因此很多开发者在进行抽象类定义时对于 abstract class和interface的选择显得比较随意。其实，两者之间还是有很大的区别的，对于它们的选择甚至反映出对于问题领域本质的理解、对于设计意图的理解是否正确、合理。本文将对它们之间的区别进行一番剖析，试图给开发者提供一个在二者之间进行选择的依据。 一、理解抽象类 abstract class和interface在Java语言中都是用来进行抽象类（本文中的抽象类并非从abstract class翻译而来，它表示的是一个抽象体，而abstract class为Java语言中用于定义抽象类的一种方法，请读者注意区分）定义的，那么什么是抽象类，使用抽象类能为我们带来什么好处呢？在面向对 象的概念中，我们知道所有的对象都是通过类来描绘的，但是反过来却不是这样。并不是所有的类都是用来描绘对象的，如果一个类中没有包含足够的信息来描绘一个具体的对象，这样的类就是抽象类

Can a class still be pure abstract if it has a non-pure destructor?

由 旧巷老猫 提交于 2019-12-22 18:05:11
问题 I am working on an exercise which asks me to take a base class Rodent and make it a pure abstract class. My understanding of a pure abstract class is that it acts as an interface and only contains pure virtual functions. Although this is an easy exercise I have a problem with the solution provided by the book: class Rodent { public: virtual ~Rodent() {cout << "Destroy rodent" << endl;} virtual void run() = 0; virtual void squeak() = 0; }; As you can see the author has added a dummy definition

How to force an implementation of a protected static function

由 ↘锁芯ラ 提交于 2019-12-22 10:36:24
问题 I'm trying to write an abstract class (or interface) which forces the extending class to implement a protected static function. But this is neither possible with an abstract class nor an interface. Errors: static functions should not be abstract access type for interface members must be omitted Any ideas how to accomplish that? UPDATE The purpose is basically to call the public function statically. This way the class does not need to be instanciated. It is also not necessary to make
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