Question

Here is the whole code :

import java.util.ArrayList;

    public class Test<E extends Comparable<E>>{

    ThreadLocal<ArrayList<E>>arraylist=new ThreadLocal<ArrayList<E>>(){
        @Override
        protected ArrayList<E> initialValue() {
            // TODO Auto-generated method stub
            //return super.initialValue();
            ArrayList<E>arraylist=new ArrayList<E>();
            for(int i=0;i<=20;i++)
            arraylist.add((E) new Integer(i));
            return arraylist;
        }
    };


    class MyRunnable implements Runnable{

        private Test mytest;

        public MyRunnable(Test test){
            mytest=test;
            // TODO Auto-generated constructor stub
        }
        @Override
        public void run() {
                System.out.println("before"+mytest.arraylist.toString());
                ArrayList<E>myarraylist=(ArrayList<E>) mytest.arraylist.get();
                myarraylist.add((E) new Double(Math.random()));
                mytest.arraylist.set(myarraylist);
                System.out.println("after"+mytest.arraylist.toString());
            }

            // TODO Auto-generated method stub

        }
    public static void main(String[] args){

        Test test=new Test<Double>();

        System.out.println(test.arraylist.toString());

        new Thread(new MyRunnable(test)).start();

        new Thread(new MyRunnable(test)).start();

        System.out.println(arraylist.toString());

    }

}

my questions are:

1. Why new Thread(new MyRunnable(test)).start(); cause the error:
   Cannot make a static reference to the non-static type MyRunnable?
2. What does the term “static reference” refer to?

Answer 1

Question 1: Why new Thread(new MyRunnable(test)).start(); cause the error:
Cannot make a static reference to the non-static type MyRunnable?

Answer 1: Because you can not instantiate inner class in static context directly.main method is always static.
To Avoid this error you can use outer class reference to initialize inner class which binds that inner class to specified reference.

 new Thread(test.new MyRunnable(test)).start();//Use test object to create new

Question 2: What does the term “static reference” refer to?

Answer 2: new MyRunnable(test) is not static you will have to make MyRunnable static in order to access like this.

You are using generics in most inefficient way 🙂

Declare MyRunnable<E> Because Generics in static context is different than in normal object context

If I understand you correctly you want MyRunnable class to understand E that is passed to Test class.
This is not possible like you are doing.

You will have to make MyRunnable to understand E and then you can access it.

static class MyRunnable<E extends Comparable<E>> implements Runnable {

    private Test<E> mytest;

    public MyRunnable(Test<E> test) {
        mytest = test;
        // TODO Auto-generated constructor stub
    }

    @Override
    public void run() {
        Test<Double> test = new Test<Double>();
        ArrayList<Double> doubles = test.arraylist.get();
        doubles.add(new Double(Math.random()));//No type cast needed
    }
    // TODO Auto-generated method stub

 }