Java: get generic type

Question

I'm writing a Java wrapper for c++ and would like to use a generic class to wrap a c++ template. Therefore I'd like to get the generic type as String so I can further pass it to JNI and instantiate a corresponding c++ object.

EDIT: this is how I implemented it, in case someone is interested:

public class A<T>
{
    private long ptr; 

        public static <E> A<E> create(Class<E> cls)
        {
            return new A<E>(cls); 
        }

    private A(Class<T> cls)
    {
        ptr = create( cls.getName() ); 

        if(ptr == 0)
        {
            throw new NullPointerException(); 
        }
     }

    private native long create(String className); 
}

well, how can I do that? I can't simply write T.getClass().toString() — Pedro
– Pedro, Commented Nov 6, 2011 at 20:25

Ted Hopp · Accepted Answer · 2013-05-30 17:44:28Z

4

Java generics don't preserve type information past compile time, due to type erasure. You need to pass an instance of T's class to your A class:

public class A<T>
{
    private Class<T> type;
    private long ptr; 

    public class A(Class<T> type)
    {
        this.type = type;
        ptr = create( getGenericType() ); 

        if(ptr == 0)
        {
            throw new NullPointerException(); 
        }
     }

    private String getGenericType()
    {
        return type.getName();
    }

    private native long create(String className); 
}

edited May 30, 2013 at 17:44

answered Nov 6, 2011 at 20:26

Ted Hopp

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3 Comments

Pedro Over a year ago

Eventually the code isn't used only by myself, so I am looking for a more elegant solution. Instantianting objects like this: A<String> a = new A<String>(String.class); isn't very user-friendly.

Sean Patrick Floyd Over a year ago

@Pedro a) Java Generics were designed that way, there's nothing we can do. b) you could improve the code by using a static factory method: A<String> a = A.ofType(String.class)

Ted Hopp Over a year ago

@Sean - the factory method is a nice suggestion.

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