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)
