Abstract Class - java.util.AbstractCollection
Created by : Mr Dk.
2019 / 11 / 04 11:07
Nanjing, Jiangsu, China
Definition
public abstract class AbstractCollection<E> implements Collection<E> {
}
实现 Collection 接口的基本抽象类
- 为了实现不可修改的集合，必须实现
iterator()和size()函数，返回的迭代器必须实现hasNext()和next() - 为了实现可修改的集合，必须额外 override
add()函数，返回的迭代器必须实现remove()函数
/**
* This class provides a skeletal implementation of the <tt>Collection</tt>
* interface, to minimize the effort required to implement this interface. <p>
*
* To implement an unmodifiable collection, the programmer needs only to
* extend this class and provide implementations for the <tt>iterator</tt> and
* <tt>size</tt> methods. (The iterator returned by the <tt>iterator</tt>
* method must implement <tt>hasNext</tt> and <tt>next</tt>.)<p>
*
* To implement a modifiable collection, the programmer must additionally
* override this class's <tt>add</tt> method (which otherwise throws an
* <tt>UnsupportedOperationException</tt>), and the iterator returned by the
* <tt>iterator</tt> method must additionally implement its <tt>remove</tt>
* method.<p>
*
* The programmer should generally provide a void (no argument) and
* <tt>Collection</tt> constructor, as per the recommendation in the
* <tt>Collection</tt> interface specification.<p>
*
* The documentation for each non-abstract method in this class describes its
* implementation in detail. Each of these methods may be overridden if
* the collection being implemented admits a more efficient implementation.<p>
*
* This class is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @author Josh Bloch
* @author Neal Gafter
* @see Collection
* @since 1.2
*/
Constructor
/**
* Sole constructor. (For invocation by subclass constructors, typically
* implicit.)
*/
protected AbstractCollection() {
}
Iterator
/**
* 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();
Size
public abstract int size();
public boolean isEmpty() {
return size() == 0;
}
Contains
遍历集合中的每个元素，查看是否存在相同的元素。注意比较用的是 equals()，默认比较内存地址 (是否是同一个对象)——如果想要比较内容，需要 override equals()：
/**
* {@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;
}
To Array
返回与迭代器遍历顺序相同的元素。
首先根据集合的大小，分配好对应大小的空间；同时做好准备：实际的元素可能少或更多。
- 如果数量与实际相同，则返回
r - 如果实际元素数量少了，就只返回这么多元素
- 如果实际元素数量多了，就还要在
finishToArray()中把迭代器遍历完，返回一个更大的数组
/**
* {@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();
}
return it.hasNext() ? finishToArray(r, it) : r;
}
cap + (cap >> 1) + 1 是将数组扩容到当前长度的 1.5 倍？
数组的扩容大小不能超过上限，否则会溢出。重新分配完毕后，开始将迭代器未迭代到的元素加入数组。迭代完毕后，如果数组后还有多余空间，则放弃多余空间后返回。
/**
* 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
*/
@SuppressWarnings("unchecked")
private static <T> T[] finishToArray(T[] r, Iterator<?> it) {
int i = r.length;
while (it.hasNext()) {
int cap = r.length;
if (i == cap) {
int newCap = cap + (cap >> 1) + 1;
// overflow-conscious code
if (newCap - MAX_ARRAY_SIZE > 0)
newCap = hugeCapacity(cap + 1);
r = Arrays.copyOf(r, newCap);
}
r[i++] = (T)it.next();
}
// 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;
}
Add / Remove
可修改集合必须实现该函数，否则异常。
public boolean add(E e) {
throw new UnsupportedOperationException();
}
对集合进行迭代，如果找到对应元素，调用迭代器的 remove() 函数删掉元素。
所以实际删除元素的操作是由迭代器来做的。难怪用迭代器迭代时，不能对集合进行修改，不然相当于两个迭代器在并发对集合进行操作，可能有安全问题。
/**
* {@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;
}
集合操作
迭代输入集合中的每一个元素。如果有本集合不存在的元素，则立刻退出。
/**
* {@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;
}
迭代输入集合中的每个元素，并接入到本集合中。
/**
* {@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;
}
迭代本集合中的每个元素。如果出现在了输入集合中，就用迭代器的 remove() 函数在本集合中删掉该元素。
/**
* {@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) {
Objects.requireNonNull(c);
boolean modified = false;
Iterator<?> it = iterator();
while (it.hasNext()) {
if (c.contains(it.next())) {
it.remove();
modified = true;
}
}
return modified;
}
用迭代器迭代本集合。删除所有不出现在输入集合 c 中的元素。
/**
* {@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) {
Objects.requireNonNull(c);
boolean modified = false;
Iterator<E> it = iterator();
while (it.hasNext()) {
if (!c.contains(it.next())) {
it.remove();
modified = true;
}
}
return modified;
}
用迭代器迭代每一个元素并删除。具体实现类应该会覆盖该函数，以得到更好的性能。
/**
* {@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();
}
}
To String
用 [] 和 , 将集合用字符串表示。
那个
(this Collection)是啥意思？？？
/**
* 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(' ');
}
}