问题与HashSet的迭代器 [英] problem with a HashSet's Iterator

问题描述

我试图看看HashSet是否会成为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 ：

  public class Klant {
 private int klantNummer; 
 
 public Klant（int nummer）{
 this.klantNummer = nummer; 
} 
 
 public int getKlantNummer（）{
 return this.klantNummer; 
 
 
 
 
 
 
 
以及具有贯穿组合的类使用 HashSet  
  public class MySet< Klant> {
私人收藏< 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。）; 
} 
} 
} 
  
问题在于方法 toon（） 
基本上，即使我指定Iterator将包含Klant对象< Klant>  
本地 k 对象不能为我提供 getKlantNummer（）方法> Klant  
  k 对象仍然是 Object 实例，甚至通过投射它：
 对象k =（Klant）i.next（）; 
  
不起作用。 
向下投射是危险的，但据我记忆它是不被禁止的。
 
 
 
有什么建议？
 
 
 
  public class MySet< Klant> =h2_lin>解决方案
 ; {
  

Klant 被解释为类的参数（就像 E 是用于集合或 K 和 V 用于 Map ）。当您随后在 MySet 中使用它时，它重写实际的类 Klant ，并且由于其擦除操作是<$ c $在你的 MySet 中有一个类型 Klant 的变量c> Object c> class只会看到 Object 的方法。删除类型参数并使用

  public class MySet {
  

，你应该很好。

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;
    }
}

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.);
        }
    }
}

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();

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

Any advice?

解决方案

In your class definition, you have

public class MySet<Klant> {

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 {

and you should be good.

这篇关于问题与HashSet的迭代器的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！