问题
When I try to declare a Dictionary as such:
private Dictionary<String, int> map;
The compiler gives me the following error:
Syntax error on token "int", Dimensions expected after this token
But it works fine with Integer. I'm vaguely aware that Java treats int / Integer differently (I come from a .NET background), but I was hoping someone could give me a full explanation on why I can't use primitives in a Dictionary<>
回答1:
In Java primitives aren't objects, so you can't use them in place of objects. However Java will automatically box/unbox primitives (aka autoboxing) into objects so you can do things like:
List<Integer> intList = new LinkedList<Integer>();
intList.add(1);
intList.add(new Integer(2));
...
Integer first = intList.get(0);
int second = intList.get(1);
But this is really just the compiler automatically converting types for you.
回答2:
In .Net, "primitive" types are backed by objects. In Java, there's a hard distinction between primitive types and Objects. Java 5 introduced autoboxing, which can coerce between the two in certain situations. However, because the Java generics system uses type-erasure, there isn't enough information to autobox in this case.
回答3:
Java collections only allow references not primitives. You need to use the wrapper classes (in this case java.lang.Integer) to do what you are after:
private Dictionary<String, Integer> map;
they you can do things like:
int foo = map.get("hello");
and
map.put("world", 42);
and Java uses autoboxing/unboxing to deal with the details of the conversion for you.
Here is a little description on it.
回答4:
To expand on TofuBeer's answer.
int is a primitive
Integer is an Object.
Generics does not support primitives.
回答5:
@XmlJavaTypeAdapter(value=MyAdapter.class, type=int.class)
Thats the trick specify type to make it work with primitives
In your adapter
using the same in package-info will mean you do it globally for that package
Found this after experimenting.
public class MyAdapter extends XmlAdapter<String, Integer> {
回答6:
Because in Java the primitives are truely primitives. In Java int will pass by value, while Integer will pass a reference. In .NET int or Int32 etc. are just different names.
来源：https://stackoverflow.com/questions/2001755/using-int-as-a-type-parameter-for-java-util-dictionary