Question

I'm trying to learn the Java class Properties. I want to print out the list of properties's key. However, the IDE displayed the following error:

Enumeration<Object> eo1 = p2.propertyNames();
//error, cast...propertyNames(...) to Enumeration

I made the following change:

Enumeration<Object> eo1 = (Enumeration<Object>) p2.propertyNames();

And everything works fine. However, I was just wondering why I need to cast propertyNames() to Enumeration<Object>. I know p2.propertyNames() return a object of Enumeration, but the syntax is very confusing for a beginner like me.

Properties p1 = new Properties();

try (OutputStream os1 = new FileOutputStream("random.txt")){
    p1.setProperty("1", "one");
    p1.setProperty("2", "two");
    p1.setProperty("3", "three");
    p1.store(os1, "comment");
} catch(IOException e){
    e.printStackTrace();
}

Properties p2 = new Properties();
try (InputStream is1 = new FileInputStream("random.txt")){
    p2.load(is1);
    System.out.println(p2.getProperty("2"));
} catch (IOException e){
    e.printStackTrace();
}

Enumeration<Object> eo1 = p2.propertyNames();
while (eo1.hasMoreElements()){
    System.out.println(eo1.nextElement());
}

Answer 1

检查propertyNames（）的返回类型。它不是枚举。它的public Enumeration<?> propertyNames()。两者都不同。详细了解generics

in Enumeration<object> eo1 = p2.propertyNames(), why is it necessary to cast p2.propertyName() to Enumeration<object>?

1 个答案: