1、抽象函数定义，子类继承
/**
* 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();2、空集合定义，判断size是否为0
/**
* {@inheritDoc}
*
* <p>This implementation returns <tt>size() == 0</tt>.
*/
public boolean isEmpty() {
return size() == 0;
}3、是否包含某个变量，迭代器判断是否相等，如果变量为null，则判断集合中是否存在null值
/**
* {@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;
}4、toArray，转为数组。通过迭代器的方式，如果期间集合的值添加了，也会返回添加后的值。如果减少了，则返回减少后的值。
/**
* {@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;
}5、带数组参数的toArray。需要注意的是会改变传入数组的值，新数组也不是原有集合的长度，而是与传入数组的长度相关，当传入数组的长度小于等于集合的长度时，以集合长度为准，否则以数组长度为准。返回数组中，如果输出超出集合部分的第一个值为null，其后值不变。具体看测试结果。
/**
* {@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}
*/
@SuppressWarnings("unchecked")
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;
}测试代码：
public static void main(String[] args) {
List<Long> list = new ArrayList<Long>();
list.add(1L);
list.add(2L);
list.add(3L);
Long[] array = new Long[] {4L, 5L, 6L, 7L, 8L};
Long[] newArray = list.toArray(array);
for (int i = 0; i < array.length; i++) {
System.out.print(array[i] + " ");
}
System.out.println();
for (int i = 0; i < newArray.length; i++) {
System.out.print(newArray[i] + " ");
}
}测试结果：
1 2 3 null 8
1 2 3 null 8
6、集合的最大长度：2的31次方 - 8
/**
* 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;7、将迭代器的值设置到数组中并返回
/**
* 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);
}8、最大容量：hugeCapacity
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;
}9、remove：删除集合中第一个出现的值，删除成功返回true。不存在则返回false
/**
* {@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;
}10、containsAll：判断每个值是否都存在
/**
* {@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;
}11、addAll:添加集合全部的值，添加成功返回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;
}12、差集：removeAll：删除集合全部的值，存在删除一个则返回true
/**
* {@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;
}13、交集：集合中存在的不删
/**
* {@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;
}14、clear：一个一个删除
/**
* {@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();
}
}
