Question

I have this demo, which I dont need a particular solution with a redrawn architecture, but just understanding why behaves like that and any thing I am missing for avoiding it. I am wondering why:

• Compiler allows the insertion of an element that its not the type of the list into the list

• The ClassCast exception is thrown when we try to get the element instead of when pushing it

  import Test.*; //Inner classes
  import java.util.List;
  import java.util.ArrayList;
  
  public class Test<E extends Object> {
  
      private List<E> list = new ArrayList<E>();
      public Test() {}
  
      public static void main(String[] args) {
          Test<String> a = new Test<String>();
          a.addElement(new String());
          a.addElement(new Integer(0)); // No complain in comp/run-time, I dont understand why CastException is not thrown here
          String class1 = a.getElement(0); 
          String class2 = a.getElement(1); //No complain in comp but ClassCastException: Integer cannot be cast to String
          //Integer class3 = a.getElement(1); //Compilation error
      }
  
      public void addElement (Object node) { list.add((E) node); } //No complain in comp/run-time
      public E getElement(int index)       { return list.get(index); }
  }
  

What could be a solution? Notice the line addElement where I need to pass a SuperClass, instead of type E. This is needed for my architecture, this is just a simple mock demo. But anyway the casting to type E should act as desired and throw a CastExeption in runtime, should not?

Answer 1

The problem is (E) node doesn’t do run-time type check due to type erasure. All the information about generic types is lost after compilation.

Java compiler should warn you about it:

Type safety: Unchecked cast from Object to E

You can do this check manually by using reference to java.lang.Class:

private Class<E> clazz;

public Test(Class clazz) {
    this.clazz = clazz;
}

public void addElement (Object node) {
    list.add(clazz.cast(node));
}

public static void main(String[] args) {
    Test<String> test = new Test(String.class);
    test.addElement("test");
    test.addElement(Integer.intValue(10));
}