HashSet的迭代器出现问题 [英] problem with a HashSet's Iterator

问题描述

我试图查看HashSet是否将是我下一个项目的解决方案，所以我正在做一些非常简单的测试来检查功能.我有一个简单的类 Klant :

I'm trying to see if HashSet would be the solution for my next project so i'm doing some very easy test to check functionalities. I have a simple class Klant:

public class Klant {
    private int klantNummer;

    public Klant(int nummer) {
        this.klantNummer = nummer;
    }

    public int getKlantNummer() {
        return this.klantNummer;
    }
}

和具有直通组成的类使用 HashSet

and a class with through composition uses a HashSet

public class MySet<Klant> { 
    private Collection<Klant> mySet = null;

    public MySet() {
        mySet=new HashSet<Klant>();
    }

    public void add(Klant elem) {
        mySet.add(elem);
    }

    public void toon() {
        Iterator<Klant> i = mySet.iterator();   
        while(i.hasNext()) {
            Klant k = i.next();
            System.out.println(k.);
        }
    }
}

问题出在方法 toon()基本上，即使我指定迭代器将包含Klant对象< Klant> 本地 k 对象不向我提供 Klant 中定义的 getKlantNummer()方法 k 对象仍然是一个 Object 实例，甚至通过以下方式对其进行强制转换:

The problem is in the method toon() Basically even though i specify that the Iterator will contain Klant objects <Klant> The local k object does not provide me with the getKlantNummer() mthod defined in Klant The k object its still an Object instance, and even by casting it with:

Object k = (Klant)i.next();

它不起作用.下投是危险的，但据我所知，这是不被禁止的.

it won't work. Down-casting is dangerous, but as far as i remember it is not prohibited.

有什么建议吗?

推荐答案

在类定义中，您拥有

public class MySet<Klant> {

该 Klant 被解释为您的类的类型参数(就像 E 用于 Collection 或 K 和 V 用于 Map .当您随后在 MySet 中使用它时，它会覆盖您的实际类 Klant ，并且由于它的擦除是 Object (如您未指定上限)您的 MySet 类中类型为 Klant 的变量将仅查看 Object 的方法.删除type参数并使用

That Klant is being interpreted as a type parameter for your class (just like E is for Collection or K and V are for Map). It is overriding your actual class Klant when you subsequently use it within MySet, and since its erasure is Object (as you specified no upper bound) a variable of type Klant within your MySet class will only see Object's methods. Remove the type parameter and use

public class MySet {

你应该很好.

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