How to do a "new" of a generic class T extends AbstractClass (JAVA)

Question

I'm trying to instantiate a generic class called "T" extending "AbstractLauncher" and I didn't understand all topic I saw on google. Could you help me ? I've got several class called ConcretXLauncher and I would not have on my MainClass "ConcretXLauncher" but only generics who could be whatever extending AbstractLauncher...

public MainClass < T extends AbstractLauncher > {

    public MainClass(Config config){
      //T launcher = new T(config); doesnt work, I want to do new ConcretXLauncher(config)     

      T launcher = newInstance(????); 
      // code using "launcher" 
    }

    public static < T > T newInstance(Class clazz) {
        return clazz.newInstance() ;
    }

}

In other topics, I saw this function but I don't know how to call it ? What do I have to put for "clazz" argument ?

You can't instantiate a generic class like this. `new T` doesn't work. — Louis Wasserman, Apr 30 '13 at 20:00
Creating an instance of a generic type isn't currently possible, but you can implement the factory pattern. More information is available here http://en.wikipedia.org/wiki/Factory_method_pattern, and even better here http://stackoverflow.com/questions/75175/create-instance-of-generic-type-in-java. — Atlas Wegman, Apr 30 '13 at 20:09
@AtlasWegman, I don't understand what is "clazz" in the second link. How could I call the fonction (Which argument to put) ? — Redouane B., Apr 30 '13 at 20:18
Take a look at the answer with 40 votes, it makes much more sense — Atlas Wegman, Apr 30 '13 at 20:56

score 0 · Answer 1 · answered Apr 30 '13 at 20:51

Here's an example of how to define your newInstance method and call it:

public class Test {
  public static <T> T newInstance(Class<T> clazz) throws InstantiationException, IllegalAccessException {
    return clazz.newInstance();
  }

  static class A {

    @Override
    public String toString() {
      return "hello";
    }
  }

  public static void main(String[] args) throws InstantiationException, IllegalAccessException {
    A a = newInstance(A.class);
    System.out.println(a);
  }
}

What does your class A represent ? Is your class Test my "MainClass" ? — Redouane B., Apr 30 '13 at 21:10

score 0 · Answer 2 · answered May 01 '13 at 06:13

You could use the following

public class MainClass < T extends AbstractLauncher> {

    public MainClass(Config config, Class<T> clazz) throws Exception {


      T launcher = newInstance(clazz); 
      // code using "launcher" 
    }

    public static <T> T newInstance(Class<T> clazz) throws Exception {
        return clazz.newInstance() ;
    }

}

you could introduce a custom runtime exception rather than throwing Exception from the constructor

How to do a "new" of a generic class T extends AbstractClass (JAVA)

2 Answers2