Question

The lastIndexOf() method of List interface accepts a parameter which is of type Object.

However, the add() method accepts a parameter of type E (which is the generic-type of the List which is defined at the time of creating a List)
Since add() accepts only E, this prevents the developer (or user) to add any incompatible object to the list at compile-time itself.

Now, Java doc says that lastIndexOf() can throw ClassCastException if the object passed is incompatible. However, when I run the following code in Eclipse Helios I don’t get any Exception :-

package scjp.collection.list;

import java.util.ArrayList;
import java.util.List;

public class LastIndexOf {
public static void main(String[] args) {
    List<String> list = new ArrayList<String>();
    
    list.add("some");
    list.add("thing");
    list.add("at");
    list.add("last");
    list.add("and");
    list.add("at");
    list.add("again");
    
    
    
    System.out.println(list.lastIndexOf("at"));
    System.out.println(list.lastIndexOf(10));                    // # 1
    System.out.println(list.lastIndexOf(new LastIndexOf()));     // # 2
}
}

At Line 1 and Line 2, I have passed incompatible objects to my List which is of type String.
However, the output that I get is :-

5
-1
-1

I get no ClassCastException.

Had the lastIndexOf() method been accepting objects of type E rather than objects of type Object, this would have been prevented at compile-time only. Why is this not done??

Java creators must have thought some problem that could occur if it accept E (and not Object). What harm would it be??

Answer 1

Here’s an example that might help. Note that the equals method used by ArrayList<T> will return true if passed a LinkedList<T> with equal elements in the same order. So, if you have a List<ArrayList<Integer>>, it would be valid to call lastIndexOf on it, passing a LinkedList<Integer>, in order to find an equal ArrayList<Integer>. But if lastIndexOf were defined in the way you describe, this wouldn’t be possible.