--- jsr166/src/main/java/util/ArrayDeque.java 2016/11/13 02:10:09 1.113 +++ jsr166/src/main/java/util/ArrayDeque.java 2017/05/31 19:01:08 1.129 @@ -50,7 +50,7 @@ import java.util.function.UnaryOperator; * Iterator} interfaces. * *

This class is a member of the - * + * * Java Collections Framework. * * @author Josh Bloch and Doug Lea @@ -68,13 +68,15 @@ public class ArrayDeque extends Abstr * * Because in a circular array, elements are in general stored in * two disjoint such slices, we help the VM by writing unusual - * nested loops for all traversals over the elements. + * nested loops for all traversals over the elements. Having only + * one hot inner loop body instead of two or three eases human + * maintenance and encourages VM loop inlining into the caller. */ /** * The array in which the elements of the deque are stored. - * We guarantee that all array cells not holding deque elements - * are always null. + * All array cells not holding deque elements are always null. + * The array always has at least one null slot (at tail). */ transient Object[] elements; @@ -88,7 +90,8 @@ public class ArrayDeque extends Abstr /** * The index at which the next element would be added to the tail - * of the deque (via addLast(E), add(E), or push(E)). + * of the deque (via addLast(E), add(E), or push(E)); + * elements[tail] is always null. */ transient int tail; @@ -114,16 +117,16 @@ public class ArrayDeque extends Abstr if (jump < needed || (newCapacity = (oldCapacity + jump)) - MAX_ARRAY_SIZE > 0) newCapacity = newCapacity(needed, jump); - elements = Arrays.copyOf(elements, newCapacity); + final Object[] es = elements = Arrays.copyOf(elements, newCapacity); // Exceptionally, here tail == head needs to be disambiguated - if (tail < head || (tail == head && elements[head] != null)) { + if (tail < head || (tail == head && es[head] != null)) { // wrap around; slide first leg forward to end of array int newSpace = newCapacity - oldCapacity; - System.arraycopy(elements, head, - elements, head + newSpace, + System.arraycopy(es, head, + es, head + newSpace, oldCapacity - head); - Arrays.fill(elements, head, head + newSpace, null); - head += newSpace; + for (int i = head, to = (head += newSpace); i < to; i++) + es[i] = null; } // checkInvariants(); } @@ -187,7 +190,10 @@ public class ArrayDeque extends Abstr * @param numElements lower bound on initial capacity of the deque */ public ArrayDeque(int numElements) { - elements = new Object[Math.max(1, numElements + 1)]; + elements = + new Object[(numElements < 1) ? 1 : + (numElements == Integer.MAX_VALUE) ? Integer.MAX_VALUE : + numElements + 1]; } /** @@ -201,7 +207,7 @@ public class ArrayDeque extends Abstr * @throws NullPointerException if the specified collection is null */ public ArrayDeque(Collection c) { - elements = new Object[c.size() + 1]; + this(c.size()); addAll(c); } @@ -224,19 +230,21 @@ public class ArrayDeque extends Abstr } /** - * Adds i and j, mod modulus. - * Precondition and postcondition: 0 <= i < modulus, 0 <= j <= modulus. + * Circularly adds the given distance to index i, mod modulus. + * Precondition: 0 <= i < modulus, 0 <= distance <= modulus. + * @return index 0 <= i < modulus */ - static final int add(int i, int j, int modulus) { - if ((i += j) - modulus >= 0) i -= modulus; + static final int add(int i, int distance, int modulus) { + if ((i += distance) - modulus >= 0) i -= modulus; return i; } /** * Subtracts j from i, mod modulus. - * Index i must be logically ahead of j. - * Returns the "circular distance" from j to i. - * Precondition and postcondition: 0 <= i < modulus, 0 <= j < modulus. + * Index i must be logically ahead of index j. + * Precondition: 0 <= i < modulus, 0 <= j < modulus. + * @return the "circular distance" from j to i; corner case i == j + * is diambiguated to "empty", returning 0. */ static final int sub(int i, int j, int modulus) { if ((i -= j) < 0) i += modulus; @@ -305,8 +313,7 @@ public class ArrayDeque extends Abstr /** * Adds all of the elements in the specified collection at the end * of this deque, as if by calling {@link #addLast} on each one, - * in the order that they are returned by the collection's - * iterator. + * in the order that they are returned by the collection's iterator. * * @param c the elements to be inserted into this deque * @return {@code true} if this deque changed as a result of the call @@ -314,10 +321,10 @@ public class ArrayDeque extends Abstr * of its elements are null */ public boolean addAll(Collection c) { - final int s = size(), needed; - if ((needed = s + c.size() - elements.length + 1) > 0) + final int s, needed; + if ((needed = (s = size()) + c.size() + 1 - elements.length) > 0) grow(needed); - c.forEach(e -> addLast(e)); + c.forEach(this::addLast); // checkInvariants(); return size() > s; } @@ -512,8 +519,8 @@ public class ArrayDeque extends Abstr /** * Retrieves and removes the head of the queue represented by this deque. * - * This method differs from {@link #poll poll} only in that it throws an - * exception if this deque is empty. + * This method differs from {@link #poll() poll()} only in that it + * throws an exception if this deque is empty. * *

This method is equivalent to {@link #removeFirst}. * @@ -608,10 +615,11 @@ public class ArrayDeque extends Abstr // checkInvariants(); final Object[] es = elements; final int capacity = es.length; - final int h = head; + final int h, t; // number of elements before to-be-deleted elt - final int front = sub(i, h, capacity); - final int back = size() - front - 1; // number of elements after + final int front = sub(i, h = head, capacity); + // number of elements after to-be-deleted elt + final int back = sub(t = tail, i, capacity) - 1; if (front < back) { // move front elements forwards if (h <= i) { @@ -627,14 +635,13 @@ public class ArrayDeque extends Abstr return false; } else { // move back elements backwards - tail = dec(tail, capacity); + tail = dec(t, capacity); if (i <= tail) { System.arraycopy(es, i + 1, es, i, back); } else { // Wrap around - int firstLeg = capacity - (i + 1); - System.arraycopy(es, i + 1, es, i, firstLeg); + System.arraycopy(es, i + 1, es, i, capacity - (i + 1)); es[capacity - 1] = es[0]; - System.arraycopy(es, 1, es, 0, back - firstLeg - 1); + System.arraycopy(es, 1, es, 0, t - 1); } es[tail] = null; // checkInvariants(); @@ -702,8 +709,7 @@ public class ArrayDeque extends Abstr throw new NoSuchElementException(); final Object[] es = elements; E e = nonNullElementAt(es, cursor); - lastRet = cursor; - cursor = inc(cursor, es.length); + cursor = inc(lastRet = cursor, es.length); remaining--; return e; } @@ -751,8 +757,7 @@ public class ArrayDeque extends Abstr throw new NoSuchElementException(); final Object[] es = elements; E e = nonNullElementAt(es, cursor); - lastRet = cursor; - cursor = dec(cursor, es.length); + cursor = dec(lastRet = cursor, es.length); remaining--; return e; } @@ -814,6 +819,8 @@ public class ArrayDeque extends Abstr /** Constructs spliterator over the given range. */ DeqSpliterator(int origin, int fence) { + // assert 0 <= origin && origin < elements.length; + // assert 0 <= fence && fence < elements.length; this.cursor = origin; this.fence = fence; } @@ -856,15 +863,15 @@ public class ArrayDeque extends Abstr } public boolean tryAdvance(Consumer action) { - if (action == null) - throw new NullPointerException(); - int t, i; - if ((t = fence) < 0) t = getFence(); - if (t == (i = cursor)) + Objects.requireNonNull(action); + final Object[] es = elements; + if (fence < 0) { fence = tail; cursor = head; } // late-binding + final int i; + if ((i = cursor) == fence) return false; - final Object[] es; - action.accept(nonNullElementAt(es = elements, i)); + E e = nonNullElementAt(es, i); cursor = inc(i, es.length); + action.accept(e); return true; } @@ -880,6 +887,9 @@ public class ArrayDeque extends Abstr } } + /** + * @throws NullPointerException {@inheritDoc} + */ public void forEach(Consumer action) { Objects.requireNonNull(action); final Object[] es = elements; @@ -1068,11 +1078,14 @@ public class ArrayDeque extends Abstr /** * Nulls out slots starting at array index i, upto index end. + * Condition i == end means "empty" - nothing to do. */ private static void circularClear(Object[] es, int i, int end) { + // assert 0 <= i && i < es.length; + // assert 0 <= end && end < es.length; for (int to = (i <= end) ? end : es.length; ; i = 0, to = end) { - Arrays.fill(es, i, to, null); + for (; i < to; i++) es[i] = null; if (to == end) break; } } @@ -1097,17 +1110,17 @@ public class ArrayDeque extends Abstr private T[] toArray(Class klazz) { final Object[] es = elements; final T[] a; - final int size = size(), head = this.head, end; - final int len = Math.min(size, es.length - head); - if ((end = head + size) >= 0) { + final int head = this.head, tail = this.tail, end; + if ((end = tail + ((head <= tail) ? 0 : es.length)) >= 0) { + // Uses null extension feature of copyOfRange a = Arrays.copyOfRange(es, head, end, klazz); } else { // integer overflow! - a = Arrays.copyOfRange(es, 0, size, klazz); - System.arraycopy(es, head, a, 0, len); + a = Arrays.copyOfRange(es, 0, end - head, klazz); + System.arraycopy(es, head, a, 0, es.length - head); } - if (tail < head) - System.arraycopy(es, 0, a, len, tail); + if (end != tail) + System.arraycopy(es, 0, a, es.length - head, tail); return a; } @@ -1232,10 +1245,12 @@ public class ArrayDeque extends Abstr /** debugging */ void checkInvariants() { + // Use head and tail fields with empty slot at tail strategy. + // head == tail disambiguates to "empty". try { int capacity = elements.length; - // assert head >= 0 && head < capacity; - // assert tail >= 0 && tail < capacity; + // assert 0 <= head && head < capacity; + // assert 0 <= tail && tail < capacity; // assert capacity > 0; // assert size() < capacity; // assert head == tail || elements[head] != null;