Eclipse给我一个如下形式的警告:
类型安全:未检查从对象转换到HashMap
这是从一个API调用，我无法控制返回对象:
HashMap<String, String> getItems(javax.servlet.http.HttpSession session) {
HashMap<String, String> theHash = (HashMap<String, String>)session.getAttribute("attributeKey");
return theHash;
}
如果可能的话，我希望避免使用Eclipse警告，因为理论上它们至少表明存在潜在的代码问题。不过，我还没有找到消除这个问题的好方法。我可以将所涉及的单行单独提取到一个方法中，并向该方法添加@SuppressWarnings(“unchecked”)，从而限制忽略警告的代码块的影响。有更好的选择吗?我不想在Eclipse中关闭这些警告。
在我接触代码之前，它更简单，但仍然引起了警告:
HashMap getItems(javax.servlet.http.HttpSession session) {
HashMap theHash = (HashMap)session.getAttribute("attributeKey");
return theHash;
}
问题是在其他地方，当你试图使用散列时，你会得到警告:
HashMap items = getItems(session);
items.put("this", "that");
Type safety: The method put(Object, Object) belongs to the raw type HashMap. References to generic type HashMap<K,V> should be parameterized.
Two ways, one which avoids the tag completely, the other using a naughty but nice utility method.
The problem is pre-genericised Collections...
I believe the rule of thumb is: "cast objects one thing at a time" - what this means when trying to use raw classes in a genericised world is that because you don't know what is in this Map<?, ?> (and indeed the JVM might even find that it isn't even a Map!), it obvious when you think about it that you can't cast it. If you had a Map<String, ?> map2 then HashSet<String> keys = (HashSet<String>)map2.keySet() does not give you a warning, despite this being an "act of faith" for the compiler (because it might turn out to be a TreeSet)... but it is only a single act of faith.
PS to the objection that iterating as in my first way "is boring" and "takes time", the answer is "no pain no gain": a genericised collection is guaranteed to contain Map.Entry<String, String>s, and nothing else. You have to pay for this guarantee. When using generics systematically this payment, beautifully, takes the form of coding compliance, not machine time!
One school of thought might say that you should set Eclipse's settings to make such unchecked casts errors, rather than warnings. In that case you would have to use my first way.
package scratchpad;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Vector;
public class YellowMouse {
// First way
Map<String, String> getHashMapStudiouslyAvoidingSuppressTag(HttpSession session) {
Map<?, ?> theHash = (Map<?, ?>)session.getAttribute("attributeKey");
Map<String, String> yellowMouse = new HashMap<String, String>();
for( Map.Entry<?, ?> entry : theHash.entrySet() ){
yellowMouse.put( (String)entry.getKey(), (String)entry.getValue() );
}
return yellowMouse;
}
// Second way
Map<String, String> getHashMapUsingNaughtyButNiceUtilityMethod(HttpSession session) {
return uncheckedCast( session.getAttribute("attributeKey") );
}
// NB this is a utility method which should be kept in your utility library. If you do that it will
// be the *only* time in your entire life that you will have to use this particular tag!!
@SuppressWarnings({ "unchecked" })
public static synchronized <T> T uncheckedCast(Object obj) {
return (T) obj;
}
}
您可以创建如下所示的实用程序类，并使用它来抑制未检查的警告。
public class Objects {
/**
* Helps to avoid using {@code @SuppressWarnings({"unchecked"})} when casting to a generic type.
*/
@SuppressWarnings({"unchecked"})
public static <T> T uncheckedCast(Object obj) {
return (T) obj;
}
}
你可以这样使用它:
import static Objects.uncheckedCast;
...
HashMap<String, String> getItems(javax.servlet.http.HttpSession session) {
return uncheckedCast(session.getAttribute("attributeKey"));
}
这里有更多关于这个问题的讨论:
http://cleveralias.blogs.com/thought_spearmints/2006/01/suppresswarning.html
