如何在Java中创建现有对象的数据类型列表

Question

Well the question says it all. Heres an Example of what i want

String s = new String();
List<s.type> ls = new ArrayList<s.type>();

Something like that, or any elegant way to achieve this, please no multiple if conditions or condition logic, as there may be unlimited data types, i have used string here only as an example, it can be anything.

Answer 1

If you want to get the runtime type of s and use it in a generic, this is impossible. Generics in Java are used only in compile time for type-checking and type inference. The generic's information is erased in the runtime, so the runtime can't tell the differense between List<String> and List<Integer> .

If you just want to make a generic with the compile-time-known type for s, than it does not make sense, because you already know the type of s and you can write a class name for it.

Answer 2

If I understood you correctly, you want to know the class ( type ) of an instance ( object ) in runtime ( existing ), which has nothing to do with generics though, as Petr explained.

public class Types {    
 public static void main(String[] args) {
     String s = "";
     Runtime r = Runtime.getRuntime();
     Types t = new Types();

     List<Class> types = new LinkedList<Class>();         
     types.add(Integer.class);
     types.add(t.getClass());
     types.add(r.getClass());
     types.add(s.getClass());

     for (Class type :types) {
         System.out.println(type.getSimpleName());
     }         
 }    
}

which outputs:

Integer
Types
Runtime
String

However, such approach is rarely needed, so rethink is there a simpler solution to your problem. If you still want to tackle it like this:

• What is reflection, and why is it useful?
• official tutorial

EDIT:

The other interpretation of your question leads to a dynamic generic list as answer:

public static <T> List<T> createListOfType() {
    return new ArrayList<T>();
}

public static void main(String[] args) {
    List<String> a = createListOfType();    
    List<Integer> b = createListOfType();
    a.add("");
    b.add(2);
    a.add(2); // remove this line to compile
    b.add(""); // remove this line to compile       
}