Java中的泛型有未知的编译错误

Question

The following compilation error makes no sense to me, and I was hoping someone would be able to elucidate:

static public void main(String args[]) throws ZipException, IOException
{
    File file = new File("C:\\temp");
    ZipFile zip_file = new ZipFile(file);
    Enumeration<ZipEntry> entries = zip_file.entries();
}   

I get the following error: Type mismatch: cannot convert from Enumeration<capture#1-of ? extends ZipEntry> to Enumeration<ZipEntry> Type mismatch: cannot convert from Enumeration<capture#1-of ? extends ZipEntry> to Enumeration<ZipEntry>

In order to get the above code to compile, I had to replace the entries declaration to use the generics <? extends ZipEntry> <? extends ZipEntry> . Why do I have to do this? Doesn't ZipEntry extend ZipEntry? (Isn't that vacuously true?) Why does the compiler complain about that?

If it helps, I'm using Java 7.03 with Eclipse Indigo 64 bit.

Answer 1

ZipFile.entries() returns an enumeration of extended ZipEntry types but is not confined to ZipEntry itself so you need to use:

Enumeration<? extends ZipEntry> entries = zip_file.entries();

This allows for other types such as JarEntry objects to be used in ZipEntry Enumerations.

Answer 2

The problem is that ZipFile.entries() returns Enumeration<? extends ZipFile> Enumeration<? extends ZipFile> .

From here :

Generics are not covariant

"While you might find it helpful to think of collections as being an abstraction of arrays, they have some special properties that collections do not. Arrays in the Java language are covariant -- which means that if Integer extends Number (which it does), then not only is an Integer also a Number, but an Integer[] is also a Number[], and you are free to pass or assign an Integer[] where a Number[] is called for. (More formally, if Number is a supertype of Integer, then Number[] is a supertype of Integer[].) You might think the same is true of generic types as well -- that List is a supertype of List, and that you can pass a List where a List is expected. Unfortunately, it doesn't work that way."

Enumeration<? extends ZipFile> Enumeration<? extends ZipFile> does not mean a collection of any type that extends ZipFile... instead it is a collection containing a specific type extending ZipFile. Even though it is legal to implicitly cast from say, JarFile to ZipFile ( ZipFile z = new JarFile(); l it is not legal to cast from Enumeration<JarFile> to Enumeration<ZipFile> .

Answer 3

If you look closely(source code) , it return Enumeration<ZipEntry> only as below:

     return new Enumeration<ZipEntry>() {
            private int i = 0;
            public boolean hasMoreElements() {
            .....
            .....

But return type is declared as public Enumeration<? extends ZipEntry> entries() { public Enumeration<? extends ZipEntry> entries() { to relax the restriction on the return type. If you wish to override entries() method through your custom subclass of Zipfile , you may want to use one of the subclasses of the ZipEntry as return type from the same method ie ZipEntry

If you don't want to use generics, you may want to write as :

  @SuppressWarnings("rawtypes")
  Enumeration entries = zip_file.entries();

Answer 4

Make sure that the file exists and is accessible, you can also try this 

Enumeration<ZipEntry> entries = (Enumeration<ZipEntry>) zip_file.entries(); 

Answer 5

Use <Z extends ZipEntry> and replace all of the ? instances in you methods if you have any, with Z . You are not limited to Z , you can use any letter you like.