介绍
AbstractCollection是对Collection的一个主要实现，如果要实现不可修改的collection，程序需要去扩展这个类，实现iterator和size方法, 而Iterator需要额外实现hasNext() 和next()方法；如果要实现可修改的collection，必须额外实现add方法，并且Iterator需要额外实现remove 方法。
方法
iterator()
public abstract Iterator<E> iterator();
返回一个迭代器对象
size()
public abstract int size();
返回集合大小
isEmpty()
public boolean isEmpty() { return size() == 0;}
是否为空
contains(Object o)
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;
}
是否包含指定元素
Obejct[] toArray()
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;
}
把集合转成数组
T[] toArray(T[] a)
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;
}
把集合转换为指定数组
T[] finishToArray(T[] r, Iterator it)
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;
}
这里其实就做了一个扩容然后赋值的操作，只要是下标i和数组长度r.length相等时，就进行一次扩容，最后返回数组元素数量大小的一个数组。
add(E e)
public boolean add(E e) {
throw new UnsupportedOperationException();
}
子类如果可以添加元素，需要重写add方法
remove(Object o)
public boolean remove(Object o) {
Iterator<E> it = iterator();
if (o==null) {
while (it.hasNext()) {
if (it.next()==null) {
it.remove();
return true;
}
}
应用迭代器，移除某个元素
containsAll(Collection c)
public boolean containsAll(Collection<?> c) {
for (Object e : c)
if (!contains(e))
return false;
return true;
}
是否包含指定数组的所有元素
addAll(Collection c)
public boolean addAll(Collection<? extends E> c) {
boolean modified = false;
for (E e : c)
if (add(e))
modified = true;
return modified;
}
循环调用add方法
removeAll(Collection c)
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;
}
移除与入参集合相同的元素
retainAll(Collection c)
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;
}
保留与入参集合中元素相同的元素
clear()
public void clear() {
Iterator<E> it = iterator();
while (it.hasNext()) {
it.next();
it.remove();
}
}
清空集合所有元素
toString()
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(' ');
}
}
重写了toString方法
总结
凡是通过Iterator能实现的，AbstractCollection 都尽最大努力实现了，并整体使用了抽象模板设计模式。
