JavaWorld Forums: Flaw with generics

The whole point of generics is to do compile-time type checking of polymorphic structures. But there is a simple way to get around this compile-time type checking and get a class cast exception at runtime using generics! Ouch.

The following program runs correctly if and only if you comment out the line: listObj.add("bubba");

import java.util.*;

public class TestGenerics
{

public static void main (String[] args)
{
List <Integer> listInt = new LinkedList <Integer>();

List listObj = listInt;
listObj.add("bubba"); // Comment out this line to make program work

for (int i = 1; i <= 10; i++)
listInt.add(i);

int x = listInt.get(0);
int y = listInt.get(1);
System.out.println(x + y);

printList(listInt);
}

public static void printList (List list)
{
System.out.print("{");

for (Object x: list)
System.out.print(x + " ");

System.out.print("}");
}

}

If the bad line commented out, the output is
3
{1 2 3 4 5 6 7 8 9 10 }

If the bad line is not commented out, the output is
Exception in thread "main" java.lang.ClassCastException: java.lang.String
at TestGenerics.main(TestGenerics.java:20)

This shows that you can't really trust generics since someone can always lessen the type checking. Unfortunately, that person can also then enhance the type checking like so:
List <String> listString = listObj;

Here's a program that gives another ClassCastException:
public static void main (String[] args)
{
List <String> listString = new LinkedList <String>();

for (int i = 1; i <= 10; i++)
listString.add("x" + i);

List listObj = listString;
List <Integer> listInt = listObj;

printSum(listInt);
}

public static void printSum (List <Integer> listInt)
{
int sum = 0;

for (Integer x: listInt)
sum += x;

System.out.println("The sum is " + sum);
}

All you get in either programs is a warning about type safety. However, this is not an error and does not stop the compilation. So generics don't give you compile-time type safety. They may help, but they do not solve the problem of ClassCastExceptions for polymorphic structures.

P.S.
There really should be a way to use <pre> or <code> tags in these posts. The HTML filter shouldn't filter the <code> tag because doing so causes the code to be misaligned and harder to read.

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