ClassFormatError in java 8?
java, java-8
Solution
Your code is valid, and the `ClassFormatError` is due to a bug in the Java 8 compiler. I think you have uncovered an edge case: loading the class file generated for a lambda expression body which contains a variable with the same name as a method that is being overriden within that body may fail.
I pasted your code into the latest versions of Intellij Idea (13.1.5), NetBeans (8.0.1) and Eclipse (Kepler SR2). The code compiled in all cases. When run, it failed with that `ClassFormatError` in NetBeans and Idea, yet it ran fine in Eclipse.
The stack trace shows the Error is arising from a failed attempt to load the class generated by the lamba expression assigned to instance variable `Foo.nodes`. The root cause is that within the lambda body assigned to `Foo.nodes` you have a variable `next` with the same name as the overriden method `next()`.
As you have already noted, if you rename the variable in the declaration `private Node next = sentinel.next;` to some unique value (e.g. `next2`) the problem goes away. Similarly, renaming it to a different overriden method (i.e. `remove` or `hasNext`) causes the problem to reappear. The message accompanying the `ClassFormatError` ("Duplicate field name&signature") exactly describes the problem, but does not explain why this should be such a precisely defined runtime error at all; the code compiled, and having the same name for a method and a variable is legal (if unwise). Prefixing next with "this." doesn't solve the problem.
Running javap on the failing Idea class file (`foo/Foo$1` in my case) shows this:
C:\Idea\Java8\Foo\out\production\Foo\foo>"C:\Program Files\Java\jdk1.8.0_20\bin\javap.exe" Foo$1.class
Compiled from "Foo.java"
class foo.Foo$1 implements java.util.Iterator<foo.Foo<E>.Node> {
final foo.Foo this$0;
foo.Foo$1(foo.Foo);
public boolean hasNext();
public foo.Foo<E>.Node next();
public void remove();
}
However, running javap on the corresponding Eclipse class which ran fine shows an extra entry:
public java.lang.Object next();
That extra method entry also magically appears in the Idea class file if the variable `next` within the lambda body is renamed to something unique, allowing the code to run. I suggest you report this as a compiler bug to JetBrains.
Also, as an unrelated issue, the `Iterator` interface implements `remove()` as a default method in Java 8. Since you never call `Iterator.remove()` you can delete your two implementations.
UPDATE: I raised a bug (JDK-8080842) for this issue with Oracle, and there will be a fix for it in release 1.8.0_60.
UPDATE TO THE UPDATE (8/27/15): Oracle have fixed this issue in all releases from 8u60 onwards. I verified the fix against 8u60. Bug JDK-8080842 refers.
Problem
I was making a class similar to `java.util.LinkedList`, and got a completely unexpected `ClassFormatError`. My IDE shows no warnings. FYI, I am using Java 8u20. Update: fixed in Java 8u60. T̶h̶i̶s̶ ̶i̶n̶c̶l̶u̶d̶e̶s̶ ̶a̶l̶l̶ ̶r̶e̶l̶e̶v̶a̶n̶t̶ ̶m̶e̶t̶h̶o̶d̶s̶:̶ Updated example as fully compilable: ``` import java.io.Serializable; import java.util.*; import java.util.function.Function; public class Foo<E> implements Deque<E>, Serializable { private static final long serialVersionUID = 0L; private final Node sentinel = sentinelInit(); private final Iterable<Node> nodes = (Iterable<Node> & Serializable) () -> new Iterator<Node>() { @SuppressWarnings("UnusedDeclaration") private static final long serialVersionUID = 0L; private Node next = sentinel.next; @Override public boolean hasNext() { return next != sentinel; } @Override public Node next() { if (!hasNext()) { throw new NoSuchElementException(); } Node old = next; next = next.next; return old; } @Override public void remove() { if (next.previous == sentinel) { throw new IllegalStateException(); } removeNode(next.previous); } }; @Override public boolean isEmpty() { return false; } @Override public Object[] toArray() { return new Object[0]; } @Override public <T> T[] toArray(T[] a) { return null; } @Override public boolean containsAll(Collection<?> c) { return false; } @Override public boolean removeAll(Collection<?> c) { return false; } @Override public boolean retainAll(Collection<?> c) { return false; } @Override public void clear() { } @Override public void addFirst(E e) { } @Override public void addLast(E e) { } @Override public boolean offerLast(E e) { return false; } @Override public E removeFirst() { return null; } @Override public E removeLast() { return null; } @Override public E pollFirst() { return null; } @Override public E getFirst() { return null; } @Override public E getLast() { return null; } @Override public E peekFirst() { return null; } @Override public boolean removeFirstOccurrence(Object o) { return false; } @Override public boolean removeLastOccurrence(Object o) { return false; } @Override public E remove() { return null; } @Override public E element() { return null; } @Override public void push(E e) { } @Override public E pop() { return null; } @Override public boolean contains(Object o) { return false; } @Override public boolean offerFirst(E e) { return false; } @Override public E pollLast() { return null; } @Override public E peekLast() { return null; } @Override public boolean offer(E e) { Node node = new Node(e); sentinel.previous.next = node; node.previous = sentinel.previous; sentinel.previous = node; node.next = sentinel; return true; } @Override public E poll() { return null; } @Override public E peek() { return null; } @Override public boolean remove(Object o) { for (Node node : nodes) { if (node.value.equals(o)) { removeNode(node); return true; } } return false; } @Override public int size() { return 0; } @Override public Iterator<E> descendingIterator() { return null; } @Override public Iterator<E> iterator() { return new Iterator<E>() { private final Iterator<Node> backingIter = nodes.iterator(); @Override public boolean hasNext() { return backingIter.hasNext(); } @Override public E next() { return backingIter.next().value; } @Override public void remove() { backingIter.remove(); } }; } private Node sentinelInit() { Node sentinel = new Node(); sentinel.next = sentinel; sentinel.previous = sentinel; return sentinel; } private void removeNode(Node node) { node.previous.next = node.next; node.next.previous = node.previous; } private class Node implements Serializable { private static final long serialVersionUID = 0L; public E value; public Node next; public Node previous; public Node(E value) { this.value = value; } public Node() { this(null); } } public static <I, O> List<O> map(Function<? super I, O> function, Iterable<I> objects) { ArrayList<O> returned = new ArrayList<>(); for (I obj : objects) { returned.add(function.apply(obj)); } return returned; } @Override public boolean addAll(Collection<? extends E> c) { boolean ret = false; for (boolean changed : map(this::add, c)) { if (changed) { ret = true; } } return ret; } @Override public boolean add(E e) { if (!offer(e)) { throw new IllegalStateException(); } return true; } public static void main(String[] args) { Foo<String> list = new Foo<>(); System.out.println("Constructed list"); list.addAll(Arrays.asList("a", "B", "c")); System.out.println("Added a, B and c."); list.forEach(System.out::println); list.remove("B"); list.forEach(System.out::println); } } ``` Here is the output: ``` Constructed list Added a, B and c. Exception in thread "main" java.lang.ClassFormatError: Duplicate field name&signature in class file uk/org/me/Foo$1 at java.lang.ClassLoader.defineClass1(Native Method) at java.lang.ClassLoader.defineClass(ClassLoader.java:760) at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142) at java.net.URLClassLoader.defineClass(URLClassLoader.java:455) at java.net.URLClassLoader.access$100(URLClassLoader.java:73) at java.net.URLClassLoader$1.run(URLClassLoader.java:367) at java.net.URLClassLoader$1.run(URLClassLoader.java:361) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:360) at java.lang.ClassLoader.loadClass(ClassLoader.java:424) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308) at java.lang.ClassLoader.loadClass(ClassLoader.java:357) at uk.org.me.Foo.lambda$new$c83cc381$1(Foo.java:18) at uk.org.me.Foo$$Lambda$1/1392838282.iterator(Unknown Source) at uk.org.me.Foo$2.<init>(Foo.java:222) at uk.org.me.Foo.iterator(Foo.java:221) at java.lang.Iterable.forEach(Iterable.java:74) at uk.org.me.Foo.main(Foo.java:300) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:483) at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134) ```