如何实现 Iterable 接口?

2022-01-10 00:00:00 iterator java

鉴于以下代码，我如何迭代 ProfileCollection 类型的对象?

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);
     }
}

推荐答案

Iterable 是一个泛型接口.您可能遇到的一个问题(您实际上并没有说您遇到了什么问题，如果有的话)是，如果您使用泛型接口/类而不指定类型参数，您可以删除不相关的泛型类型的类型班内.这方面的一个例子是 对泛型类的非泛型引用导致非泛型返回类型.

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.

所以我至少会把它改成:

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);
    }
}

这应该有效:

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

如果没有 Iterable 上的类型参数，迭代器可能会被简化为 Object 类型，所以只有这样才能工作:

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
}

这是 Java 泛型的一个非常晦涩的极端案例.

This is a pretty obscure corner case of Java generics.

如果没有，请提供更多关于发生了什么的信息.

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

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