問題描述

鑒于以下代碼，我如何迭代 ProfileCollection 類型的對象?

Given the following code, how can I iterate over an object of type ProfileCollection?

public class ProfileCollection implements Iterable {    
    private ArrayList<Profile> m_Profiles;

    public Iterator<Profile> iterator() {        
        Iterator<Profile> iprof = m_Profiles.iterator();
        return iprof; 
    }

    ...

    public Profile GetActiveProfile() {
        return (Profile)m_Profiles.get(m_ActiveProfile);
    }
}

public static void main(String[] args) {
     m_PC = new ProfileCollection("profiles.xml");

     // properly outputs a profile:
     System.out.println(m_PC.GetActiveProfile()); 

     // not actually outputting any profiles:
     for(Iterator i = m_PC.iterator();i.hasNext();) {
        System.out.println(i.next());
     }

     // how I actually want this to work, but won't even compile:
     for(Profile prof: m_PC) {
        System.out.println(prof);
     }
}

推薦答案

Iterable 是一個泛型接口.您可能遇到的一個問題(您實際上并沒有說您遇到了什么問題，如果有的話)是，如果您使用泛型接口/類而不指定類型參數，您可以刪除不相關的泛型類型的類型班內.這方面的一個例子是 對泛型類的非泛型引用導致非泛型返回類型.

Iterable is a generic interface. A problem you might be having (you haven't actually said what problem you're having, if any) is that if you use a generic interface/class without specifying the type argument(s) you can erase the types of unrelated generic types within the class. An example of this is in Non-generic reference to generic class results in non-generic return types.

所以我至少會把它改成:

So I would at least change it to:

public class ProfileCollection implements Iterable<Profile> { 
    private ArrayList<Profile> m_Profiles;

    public Iterator<Profile> iterator() {        
        Iterator<Profile> iprof = m_Profiles.iterator();
        return iprof; 
    }

    ...

    public Profile GetActiveProfile() {
        return (Profile)m_Profiles.get(m_ActiveProfile);
    }
}

這應該有效:

for (Profile profile : m_PC) {
    // do stuff
}

如果沒有 Iterable 上的類型參數，迭代器可能會被簡化為 Object 類型，所以只有這樣才能工作:

Without the type argument on Iterable, the iterator may be reduced to being type Object so only this will work:

for (Object profile : m_PC) {
    // do stuff
}

這是 Java 泛型的一個非常晦澀的極端案例.

This is a pretty obscure corner case of Java generics.

如果沒有，請提供更多關于發生了什么的信息.

If not, please provide some more info about what's going on.

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