在Java中，方法签名枚举有什么作用 <? extends ZipEntry> 意思？

Question

In Java, what does does the method signature Enumeration<? extends ZipEntry> Enumeration<? extends ZipEntry> means?

It is defining a generic enumeration type. I just don't know what the ? means. It seems that it's saying that a new type is being defined and all the compiler needs to know is that the new type extends ZipEntry . Is that accurate?

This is the context of the code:

for (Enumeration<? extends ZipEntry> e = zipfile.entries(); 
        e.hasMoreElements(); fileNumber++) 
{
    // ...etc...
}

If I were to speak Enumeration<? extends ZipEntry> Enumeration<? extends ZipEntry> in english, what would be said?

Thanks!

Answer 1

It means "an Enumeration of some definite, but unspecified type that extends ZipEntry ".

See: Upper Bounded Wildcards in Oracle's tutorial about generics, and Angelika Langer's Java Generics FAQ .