ClassCastException in Java foreach loop

Question

In what circumstances can ClassCastException occur in the code below:

import java.util.Arrays;
import java.util.List;

public class Generics {

    static List getObjects() {
        return Arrays.asList(1, 2, 3);
    }

    public static void main(String[] args) {
        List<String> list = getObjects();
        for (Object o : list) { // ClassCastException?
            System.out.println(o);
        }
    }
}

We had a similar case in a production environment (bad practice, I know) and the customer provided a log with ClassCastException at the line with the comment but I can't seem to reproduce it. Any thoughts?

I know that the JVM creates an iterator in the background when using foreach, but can it create a raw Iterator in some cases and a parametrized one in other cases?

Update:I also had a look at the bytecode generated and on Windows, using JDK 1.6.0_21-b07 no checkcast was made. Interesting :)

Here is the main method:

public static void main(java.lang.String[]);
  Code:
   0:   invokestatic    #34; //Method getObjects:()Ljava/util/List;
   3:   astore_1
   4:   aload_1
   5:   invokeinterface #36,  1; //InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator;
   10:  astore_3
   11:  goto    28
   14:  aload_3
   15:  invokeinterface #42,  1; //InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;
   20:  astore_2
   21:  getstatic   #48; //Field java/lang/System.out:Ljava/io/PrintStream;
   24:  aload_2
   25:  invokevirtual   #54; //Method java/io/PrintStream.println:(Ljava/lang/Object;)V
   28:  aload_3
   29:  invokeinterface #60,  1; //InterfaceMethod java/util/Iterator.hasNext:()Z
   34:  ifne    14
   37:  return

Thanks all for the answers!

Update 2: I got mislead with Eclipse IDE that uses its own compiler so actually the bytecode above it's the one generated using the Eclipse compiler. Look here how to manually compile code with Eclipse. In conclusion Eclipse compiler generates different byte-code from the Sun compiler in some cases, regardless of the platform, the case described here being one.

Answer 1

Shouldn't that code always throw a ClassCastException? It does for me using the Sun Java 6 compiler and runtime (on Linux). You're casting Integers as Strings. The iterator created will be an Iterator<String>, but then it tries to access the first element, which is an Integer, and so it fails.

This gets clearer if you change your array like so:

return Arrays.asList("one", 2, 3);

Now the loop actually works for the first element, because the first element is a String and we see the output; then the Iterator<String> fails on the second one, because it isn't a string.

Your code works if you just use a generic List instead of a specific one:

List list = getObjects();
for (Object o : list) {
    System.out.println(o);
}

...or, of course, if you use List<Integer>, since the contents are Integers. What you're doing now triggers a compiler warning — Note: Generics.java uses unchecked or unsafe operations. — and for good reason.

This modification also works:

for (Object o : (List)list)

...presumably because at that point you're dealing with an Iterator, not an Iterator<String>.

bozho has said he doesn't see this error on Windows XP (didn't mention which compiler and runtime, but I'm guessing Sun's), and you say you're not seeing it (or not reliably), so clearly there's some implementation sensitivity here, but the bottom line is: Don't use List<String> to interact with a List of Integers. :-)

Here's the file I'm compiling:

import java.util.Arrays;
import java.util.List;

public class Generics {

    static List getObjects() {
        return Arrays.asList("one", 2, 3);
    }

    public static void main(String[] args) {
        List<String> list = getObjects();
        for (Object o : list) { // ClassCastException?
            System.out.println(o);
        }
    }
}

Here's the compilation:

tjc@forge:~/temp$ javac Generics.java 
Note: Generics.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

Here's the run:

tjc@forge:~/temp$ java Generics 
one
Exception in thread "main" java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
    at Generics.main(Generics.java:12)

Line 12 is the for statement. Note that it did output the first element, because I changed that to a String. It didn't output the others. (And before I made that change, it failed immediately.)

Here's the compiler I'm using:

tjc@forge:~/temp$ which javac
/usr/bin/javac
tjc@forge:~/temp$ ll /usr/bin/javac
lrwxrwxrwx 1 root root 23 2010-09-30 16:37 /usr/bin/javac -> /etc/alternatives/javac*
tjc@forge:~/temp$ ll /etc/alternatives/javac
lrwxrwxrwx 1 root root 33 2010-09-30 16:37 /etc/alternatives/javac -> /usr/lib/jvm/java-6-sun/bin/javac*

Here's the disassembly, which shows the checkcast:

tjc@forge:~/temp$ javap -c Generics
Compiled from "Generics.java"
public class Generics extends java.lang.Object{
public Generics();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."":()V
   4:   return

static java.util.List getObjects();
  Code:
   0:   iconst_3
   1:   anewarray   #2; //class java/io/Serializable
   4:   dup
   5:   iconst_0
   6:   ldc #3; //String one
   8:   aastore
   9:   dup
   10:  iconst_1
   11:  iconst_2
   12:  invokestatic    #4; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
   15:  aastore
   16:  dup
   17:  iconst_2
   18:  iconst_3
   19:  invokestatic    #4; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
   22:  aastore
   23:  invokestatic    #5; //Method java/util/Arrays.asList:([Ljava/lang/Object;)Ljava/util/List;
   26:  areturn

public static void main(java.lang.String[]);
  Code:
   0:   invokestatic    #6; //Method getObjects:()Ljava/util/List;
   3:   astore_1
   4:   aload_1
   5:   invokeinterface #7,  1; //InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator;
   10:  astore_2
   11:  aload_2
   12:  invokeinterface #8,  1; //InterfaceMethod java/util/Iterator.hasNext:()Z
   17:  ifeq    40
   20:  aload_2
   21:  invokeinterface #9,  1; //InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;
   26:  checkcast   #10; //class java/lang/String
   29:  astore_3
   30:  getstatic   #11; //Field java/lang/System.out:Ljava/io/PrintStream;
   33:  aload_3
   34:  invokevirtual   #12; //Method java/io/PrintStream.println:(Ljava/lang/Object;)V
   37:  goto    11
   40:  return

}

Again, though, the bottom line has to be: Don't use a List<String> to interact with a List that contains things that aren't Strings. :-)

Answer 2

I can't reproduce it either, but I spot the following mistakes that must be corrected:

• make getObjects() return List<Integer>, rather than a raw type
• Don't expect List<String>, but a List<Integer> instead
• When iterating, loop for (Integer o : list)

Answer 3

The problem is the method static List getObjects() { returns a generic (non parameterised) List. And you assign it to List<String>. This line should give a compiler warning, and that should have signalled a problem.

Answer 4

This part :

List<String> list = getObjects();
for (Object o : list) { // ClassCastException?
    System.out.println(o);
}

Will be simplified as

List<String> list = getObjects();
for (Iterator<String> iterator = list.iterator(); iterator.hasNext();) {
    Object o = iterator.next();
    System.out.println(o);
}

But the implementation of Iterator will try to cast when calling the next() method the content send by the Iterator in a String.

That's why you have a CCE.

Solutions :

Either use generics everywhere or don't use them, but it's really important to be consistant. If you had returned a List<Integer> or a List<? super Integer> you would have seen this problem at compilation time.