Question

Can anybody post the Java version of this VB.NET code?

    Public Function FetchDoc(Of T As {New, IRepoDocument})(ByVal docId As String) As IRepoDocument Implements IDocRepository.FetchDoc

        Dim repoDoc As New T

        //some code to init repoDoc

        Return repoDoc

    End Function

This function accepts and create a instance of any class implementing IRepoDocument and has a no-argument constructor.

The only way I found is:

public <T extends IRepoDocument> IRepoDocument FetchDoc(String idDoc, Class<T> clazz)
    throws InstantiationException, IllegalAccessException 
{
  return clazz.newInstance();
}

But I want to suppress Class<T> clazz as input parameter.

Answer 1

You cannot create an instance in Java without a Class instance to specify what class the instance is of — except for anonymous classes (see below). You can get that class instance as a parameter, or hide it under a rock, or fish it out of the ocean, but you have to have it at runtime. The classes associated with the generics are removed by type erasure and just aren’t there.

I don’t know VB.NET from stringbeans, but you might, in some cases, be looking for an anonymous class.

new IRepoDocument() { ......fill in code };

In this case, there’s be no <T> at all, the return type would just be IRepoDocument.