No Instance

A type parameter cannot be used to instantiate its object inside a method.

public static <T> void add(Box<T> box) {
   //compiler error
   //Cannot instantiate the type T
   //T item = new T();  
   //box.add(item);
}

To achieve such functionality, use reflection.

public static <T> void add(Box<T> box, Class<T> clazz) 
   throws InstantiationException, IllegalAccessException{
   T item = clazz.newInstance();   // OK
   box.add(item);
   System.out.println("Item added.");
}

Example

package com.tutorialspoint;

public class GenericsTester {
   public static void main(String[] args) 
  throws InstantiationException, IllegalAccessException {
  Box<String> stringBox = new Box<String>();
  add(stringBox, String.class);
   }  

   public static <T> void add(Box<T> box) {
  //compiler error
  //Cannot instantiate the type T
  //T item = new T();  
  //box.add(item);
   }

   public static <T> void add(Box<T> box, Class<T> clazz) 
  throws InstantiationException, IllegalAccessException{
  T item = clazz.newInstance();   // OK
  box.add(item);
  System.out.println("Item added.");
   }   
}

class Box<T> {
   private T t;

   public void add(T t) {
  this.t = t;
   }

   public T get() {
  return t;
   }   
}

This will produce the following result −

Item added.