An ArrayList that reuses a single iterator

core/java/src/net/i2p/util/CachedIteratorArrayList.java

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+package net.i2p.util;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.ConcurrentModificationException;
+import java.util.Iterator;
+import java.util.NoSuchElementException;
+
+/**
+ * ArrayList that uses a single iterator.  Useful to avoid object churn
+ * while keeping the conveniences of an iterator.
+ * 
+ * @since 0.9.4
+ * 
+ * @author zab
+ */
+public class CachedIteratorArrayList<E> extends ArrayList<E> {
+
+    private static final long serialVersionUID = 4863212596318574111L;
+    
+    private final CachedIterator iterator = new CachedIterator();
+
+    public CachedIteratorArrayList() {
+        super();
+    }
+
+    public CachedIteratorArrayList(Collection<? extends E> c) {
+        super(c);
+    }
+
+    public CachedIteratorArrayList(int initialCapacity) {
+        super(initialCapacity);
+    }
+    
+    @Override
+    public Iterator<E> iterator() {
+        iterator.reset();
+        return iterator;
+    }
+
+    private class CachedIterator implements Iterator<E> {
+        /**
+         * Index of element to be returned by subsequent call to next.
+         */
+        int cursor = 0;
+
+        /**
+         * Index of element returned by most recent call to next or
+         * previous.  Reset to -1 if this element is deleted by a call
+         * to remove.
+         */
+        int lastRet = -1;
+
+        /**
+         * The modCount value that the iterator believes that the backing
+         * List should have.  If this expectation is violated, the iterator
+         * has detected concurrent modification.
+         */
+        int expectedModCount = modCount;
+        
+        void reset() {
+            cursor = 0;
+            lastRet = -1;
+            expectedModCount = modCount;
+        }
+
+        public boolean hasNext() {
+            return cursor != size();
+        }
+
+        public E next() {
+            checkForComodification();
+            try {
+                int i = cursor;
+                E next = get(i);
+                lastRet = i;
+                cursor = i + 1;
+                return next;
+            } catch (IndexOutOfBoundsException e) {
+                checkForComodification();
+                throw new NoSuchElementException();
+            }
+        }
+
+        public void remove() {
+            if (lastRet < 0)
+                throw new IllegalStateException();
+            checkForComodification();
+
+            try {
+                CachedIteratorArrayList.this.remove(lastRet);
+                if (lastRet < cursor)
+                    cursor--;
+                lastRet = -1;
+                expectedModCount = modCount;
+            } catch (IndexOutOfBoundsException e) {
+                throw new ConcurrentModificationException();
+            }
+        }
+
+        final void checkForComodification() {
+            if (modCount != expectedModCount)
+                throw new ConcurrentModificationException();
+        }
+
+    }
+    
+}

core/java/test/junit/net/i2p/util/CachedIteratorArrayListTest.java

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+package net.i2p.util;
+
+import static org.junit.Assert.*;
+
+import java.util.Iterator;
+import java.util.List;
+
+import org.junit.Before;
+import org.junit.Test;
+
+public class CachedIteratorArrayListTest {
+
+    private List<Integer> l;
+    
+    @Before
+    public void setUp() {
+        l = new CachedIteratorArrayList<Integer>();
+        l.add(1);
+        l.add(2);
+        l.add(3);
+    }
+    
+    @Test
+    public void test() {
+        // test for-each worx
+        int total = 0;
+        for (int i : l)
+            total += i;
+        assertEquals(6, total);
+        for (int i : l)
+            total += i;
+        assertEquals(12, total);
+        
+    }
+    
+    @Test
+    public void testSameness() {
+        Iterator<Integer> two = l.iterator();
+        Iterator<Integer> one = l.iterator();
+        assertSame(one, two);
+    }
+
+}