JDK源码之AbstractCollection

public abstract class AbstractCollection<E> implements Collection<E> {
/**
* Sole constructor.  (For invocation by subclass constructors, typically
* implicit.)
*/
protected AbstractCollection() {
}

// Query Operations

/**
* Returns an iterator over the elements contained in this collection.
*
* @return an iterator over the elements contained in this collection
*/
public abstract Iterator<E> iterator();

public abstract int size();

/**
* {@inheritDoc}
*
* <p>This implementation returns <tt>size() == 0</tt>.
*/
public boolean isEmpty() {
return size() == 0;
}

/**
* {@inheritDoc}
*
* <p>This implementation iterates over the elements in the collection,
* checking each element in turn for equality with the specified element.
*
* @throws ClassCastException   {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public boolean contains(Object o) {
Iterator<E> it = iterator();
if (o==null) {
while (it.hasNext())
if (it.next()==null)
return true;
} else {
while (it.hasNext())
if (o.equals(it.next()))
return true;
}
return false;
}

/**
* {@inheritDoc}
*
* <p>This implementation returns an array containing all the elements
* returned by this collection's iterator, in the same order, stored in
* consecutive elements of the array, starting with index {@code 0}.
* The length of the returned array is equal to the number of elements
* returned by the iterator, even if the size of this collection changes
* during iteration, as might happen if the collection permits
* concurrent modification during iteration.  The {@code size} method is
* called only as an optimization hint; the correct result is returned
* even if the iterator returns a different number of elements.
*
* <p>This method is equivalent to:
*
*  <pre> {@code
* List<E> list = new ArrayList<E>(size());
* for (E e : this)
*     list.add(e);
* return list.toArray();
* }</pre>
*/
public Object[] toArray() {
// Estimate size of array; be prepared to see more or fewer elements
Object[] r = new Object[size()];
Iterator<E> it = iterator();
for (int i = 0; i < r.length; i++) {
if (! it.hasNext()) // fewer elements than expected
return Arrays.copyOf(r, i);
r[i] = it.next();
}
　　　　　//如果有更多的元素，调用finishToArray, 否则直接返回
return it.hasNext() ? finishToArray(r, it) : r;
}

/**
* {@inheritDoc}
*
* <p>This implementation returns an array containing all the elements
* returned by this collection's iterator in the same order, stored in
* consecutive elements of the array, starting with index {@code 0}.
* If the number of elements returned by the iterator is too large to
* fit into the specified array, then the elements are returned in a
* newly allocated array with length equal to the number of elements
* returned by the iterator, even if the size of this collection
* changes during iteration, as might happen if the collection permits
* concurrent modification during iteration.  The {@code size} method is
* called only as an optimization hint; the correct result is returned
* even if the iterator returns a different number of elements.
*
* <p>This method is equivalent to:
*
*  <pre> {@code
* List<E> list = new ArrayList<E>(size());
* for (E e : this)
*     list.add(e);
* return list.toArray(a);
* }</pre>
*
* @throws ArrayStoreException  {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public <T> T[] toArray(T[] a) {
// Estimate size of array; be prepared to see more or fewer elements
int size = size();
T[] r = a.length >= size ? a :
(T[])java.lang.reflect.Array
.newInstance(a.getClass().getComponentType(), size);
Iterator<E> it = iterator();

for (int i = 0; i < r.length; i++) {
if (! it.hasNext()) { // fewer elements than expected
if (a == r) {
r[i] = null; // null-terminate
} else if (a.length < i) {
return Arrays.copyOf(r, i);
} else {
System.arraycopy(r, 0, a, 0, i);
if (a.length > i) {
a[i] = null;
}
}
return a;
}
r[i] = (T)it.next();
}
// more elements than expected
return it.hasNext() ? finishToArray(r, it) : r;
}

/**
* The maximum size of array to allocate.
* Some VMs reserve some header words in an array.
* Attempts to allocate larger arrays may result in
* OutOfMemoryError: Requested array size exceeds VM limit
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

/**
* Reallocates the array being used within toArray when the iterator
* returned more elements than expected, and finishes filling it from
* the iterator.
*
* @param r the array, replete with previously stored elements
* @param it the in-progress iterator over this collection
* @return array containing the elements in the given array, plus any
*         further elements returned by the iterator, trimmed to size
*/
private static <T> T[] finishToArray(T[] r, Iterator<?> it) {
int i = r.length;  // 从原有数组最尾位置开始添加
while (it.hasNext()) {
int cap = r.length;
if (i == cap) { // 先扩充现在数组的容量
int newCap = cap + (cap >> 1) + 1;  // 新容量大小， 3*cap+1
// overflow-conscious code
if (newCap - MAX_ARRAY_SIZE > 0) // 超过整型最大值时
newCap = hugeCapacity(cap + 1);
r = Arrays.copyOf(r, newCap); // 将原有数据复制到新的数组中
}
r[i++] = (T)it.next(); // 从原有数组最尾位置开始添加
}
// trim if overallocated
return (i == r.length) ? r : Arrays.copyOf(r, i);
}

private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError
("Required array size too large");
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}

// Modification Operations

/**
* {@inheritDoc}
*
* <p>This implementation always throws an
* <tt>UnsupportedOperationException</tt>.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException            {@inheritDoc}
* @throws NullPointerException          {@inheritDoc}
* @throws IllegalArgumentException      {@inheritDoc}
* @throws IllegalStateException         {@inheritDoc}
*/
public boolean add(E e) {
throw new UnsupportedOperationException();
}

/**
* {@inheritDoc}
*
* <p>This implementation iterates over the collection looking for the
* specified element.  If it finds the element, it removes the element
* from the collection using the iterator's remove method.
*
* <p>Note that this implementation throws an
* <tt>UnsupportedOperationException</tt> if the iterator returned by this
* collection's iterator method does not implement the <tt>remove</tt>
* method and this collection contains the specified object.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException            {@inheritDoc}
* @throws NullPointerException          {@inheritDoc}
*/
public boolean remove(Object o) {
Iterator<E> it = iterator();
if (o==null) {
while (it.hasNext()) {
if (it.next()==null) {
it.remove();
return true;
}
}
} else {
while (it.hasNext()) {
if (o.equals(it.next())) {
it.remove();
return true;
}
}
}
return false;
}

// Bulk Operations

/**
* {@inheritDoc}
*
* <p>This implementation iterates over the specified collection,
* checking each element returned by the iterator in turn to see
* if it's contained in this collection.  If all elements are so
* contained <tt>true</tt> is returned, otherwise <tt>false</tt>.
*
* @throws ClassCastException            {@inheritDoc}
* @throws NullPointerException          {@inheritDoc}
* @see #contains(Object)
*/
public boolean containsAll(Collection<?> c) {
for (Object e : c)
if (!contains(e))
return false;
return true;
}

/**
* {@inheritDoc}
*
* <p>This implementation iterates over the specified collection, and adds
* each object returned by the iterator to this collection, in turn.
*
* <p>Note that this implementation will throw an
* <tt>UnsupportedOperationException</tt> unless <tt>add</tt> is
* overridden (assuming the specified collection is non-empty).
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException            {@inheritDoc}
* @throws NullPointerException          {@inheritDoc}
* @throws IllegalArgumentException      {@inheritDoc}
* @throws IllegalStateException         {@inheritDoc}
*
* @see #add(Object)
*/
public boolean addAll(Collection<? extends E> c) {
boolean modified = false;
for (E e : c)
if (add(e))
modified = true;
return modified;
}

/**
* {@inheritDoc}
*
* <p>This implementation iterates over this collection, checking each
* element returned by the iterator in turn to see if it's contained
* in the specified collection.  If it's so contained, it's removed from
* this collection with the iterator's <tt>remove</tt> method.
*
* <p>Note that this implementation will throw an
* <tt>UnsupportedOperationException</tt> if the iterator returned by the
* <tt>iterator</tt> method does not implement the <tt>remove</tt> method
* and this collection contains one or more elements in common with the
* specified collection.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException            {@inheritDoc}
* @throws NullPointerException          {@inheritDoc}
*
* @see #remove(Object)
* @see #contains(Object)
*/
public boolean removeAll(Collection<?> c) {
boolean modified = false;
Iterator<?> it = iterator();
while (it.hasNext()) {
if (c.contains(it.next())) {
it.remove();
modified = true;
}
}
return modified;
}

/**
* {@inheritDoc}
*
* <p>This implementation iterates over this collection, checking each
* element returned by the iterator in turn to see if it's contained
* in the specified collection.  If it's not so contained, it's removed
* from this collection with the iterator's <tt>remove</tt> method.
*
* <p>Note that this implementation will throw an
* <tt>UnsupportedOperationException</tt> if the iterator returned by the
* <tt>iterator</tt> method does not implement the <tt>remove</tt> method
* and this collection contains one or more elements not present in the
* specified collection.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException            {@inheritDoc}
* @throws NullPointerException          {@inheritDoc}
*
* @see #remove(Object)
* @see #contains(Object)
*/
public boolean retainAll(Collection<?> c) {
boolean modified = false;
Iterator<E> it = iterator();
while (it.hasNext()) {
if (!c.contains(it.next())) {
it.remove();
modified = true;
}
}
return modified;
}

/**
* {@inheritDoc}
*
* <p>This implementation iterates over this collection, removing each
* element using the <tt>Iterator.remove</tt> operation.  Most
* implementations will probably choose to override this method for
* efficiency.
*
* <p>Note that this implementation will throw an
* <tt>UnsupportedOperationException</tt> if the iterator returned by this
* collection's <tt>iterator</tt> method does not implement the
* <tt>remove</tt> method and this collection is non-empty.
*
* @throws UnsupportedOperationException {@inheritDoc}
*/
public void clear() {
Iterator<E> it = iterator();
while (it.hasNext()) {
it.next();
it.remove();
}
}

//  String conversion

/**
* Returns a string representation of this collection.  The string
* representation consists of a list of the collection's elements in the
* order they are returned by its iterator, enclosed in square brackets
* (<tt>"[]"</tt>).  Adjacent elements are separated by the characters
* <tt>", "</tt> (comma and space).  Elements are converted to strings as
* by {@link String#valueOf(Object)}.
*
* @return a string representation of this collection
*/
public String toString() {
Iterator<E> it = iterator();
if (! it.hasNext())
return "[]";

StringBuilder sb = new StringBuilder();
sb.append('[');
for (;;) {
E e = it.next();
sb.append(e == this ? "(this Collection)" : e);
if (! it.hasNext())
return sb.append(']').toString();
sb.append(',').append(' ');
}
}

}