1.成员变量与构造方法
// ArrayList的默认大小为10
private static final int DEFAULT_CAPACITY = 10;
// 定义容量为0时，集合存储元素的数组
private static final Object[] EMPTY_ELEMENTDATA = {};
// 定义默认容量时，集合存储元素的数组
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
// 定义当前集合存储元素的数组
transient Object[] elementData;
// 定义集合的长度
private int size;
// 自定义容量的构造方法
public ArrayList(int initialCapacity) {
if (initialCapacity > 0) {
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == 0) {
this.elementData = EMPTY_ELEMENTDATA;
} else {
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
// 采用默认容量的构造方法
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
// 传入指定集合的构造方法
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
if ((size = elementData.length) != 0) {
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// replace with empty array.
this.elementData = EMPTY_ELEMENTDATA;
}
}
2.add()
新增方法步骤:
1.确保容量充足
2.将元素放入数组的最后一位, 集合长度+1
3.返回结果
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
private void ensureCapacityInternal(int minCapacity) {
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
计算所需容量：如果存储元素的数组为空，容量为默认值10，否则容量为(当前存储元素的数组大小+1)
private static int calculateCapacity(Object[] elementData, int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
return Math.max(DEFAULT_CAPACITY, minCapacity);
}
return minCapacity;
}
是否需要扩容：如果当前所需容量大于存储元素的数组长度，才会扩容
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
扩容方法:默认扩容0.5倍，如果新的容量还不够，会继续扩大容量，最大容量为Integer最大值(2的31次方-1)
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);
}
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
3.remove()
remove(int index)方法：
1.检查下标是否合法
2.计算移动位数
3.将删除元素之后的元素向前移动
4.存储集合长度减一，并将存储元素数组的最后以为置空
5.返回删除的元素
public E remove(int index) {
rangeCheck(index);
modCount++;
E oldValue = elementData(index);
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
return oldValue;
}
remove(Object o)方法：
1.根据值找到下标
2.计算移动位数
3.将删除元素之后的元素向前移动
4.存储集合长度减一，并将存储元素数组的最后以为置空
5.返回删除的元素
public boolean remove(Object o) {
if (o == null) {
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
fastRemove(index);
return true;
}
} else {
for (int index = 0; index < size; index++)
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
return false;
}
private void fastRemove(int index) {
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
}
4.clear()
// clear(): 将存储元素的数组的所有元素置空，集合的长度设为0
public void clear() {
modCount++;
// clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
5.addAll()
addAll:
1.将集合转成数组
2.扩容
3.将新数组的元素添加到集合数组里面
4.增加集合长度
5.返回结果
public boolean addAll(Collection<? extends E> c) {
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
}
6.retainAll()
retainAll:获取两个集合的交集
public boolean retainAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, true);
}
private boolean batchRemove(Collection<?> c, boolean complement) {
final Object[] elementData = this.elementData;
int r = 0, w = 0;
boolean modified = false;
try {
// 将公有的元素放到当前集合的数组中
for (; r < size; r++)
if (c.contains(elementData[r]) == complement)
elementData[w++] = elementData[r];
} finally {
// Preserve behavioral compatibility with AbstractCollection,
// even if c.contains() throws.
// 将公有的元素放到当前集合的数组中
if (r != size) {
System.arraycopy(elementData, r,
elementData, w,
size - r);
w += size - r;
}
// 公有的元素的个数与当前集合的个数不相等，返回true，否则返回false
if (w != size) {
// clear to let GC do its work
// 将当前集合数组的剩余位置的值置空
for (int i = w; i < size; i++)
elementData[i] = null;
modCount += size - w;
size = w;
modified = true;
}
}
return modified;
}
7.contains(), indexOf(), lastIndexOf()
// ArrayList的contains(), indexOf(), lastIndexOf()是在for循环里面一个一个找，数据量大时，效率比较低，尽量不要使用
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
public int indexOf(Object o) {
if (o == null) {
for (int i = 0; i < size; i++)
if (elementData[i]==null)
return i;
} else {
for (int i = 0; i < size; i++)
if (o.equals(elementData[i]))
return i;
}
return -1;
}
8.clone()
// ArrayList重写clone()，属于深度克隆
public Object clone() {
try {
ArrayList<?> v = (ArrayList<?>) super.clone();
v.elementData = Arrays.copyOf(elementData, size);
v.modCount = 0;
return v;
} catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError(e);
}
}
9.toArray()
// toArray(),实际是将当前集合的数组属性复制了一份
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
10.set()
// set()返回的是原来的值
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
11.iterator(), listIterator()
public Iterator<E> iterator() {
return new Itr();
}
public ListIterator<E> listIterator() {
return new ListItr(0);
}
private class Itr implements Iterator<E> {
int cursor; // index of next element to return
int lastRet = -1; // index of last element returned; -1 if no such
int expectedModCount = modCount;
Itr() {}
// 判断是否有下一个元素
public boolean hasNext() {
return cursor != size;
}
// 获取下一个元素
@SuppressWarnings("unchecked")
public E next() {
checkForComodification();
int i = cursor;
if (i >= size)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
}
// 调用remove()前，必须调用next(),否则lastRet < 0, 从而导致会报IllegalStateException
public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
ArrayList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
// 为每个剩余元素执行给定的操作,直到所有的元素都已经被处理或抛出异常
@Override
@SuppressWarnings("unchecked")
public void forEachRemaining(Consumer<? super E> consumer) {
Objects.requireNonNull(consumer);
final int size = ArrayList.this.size;
int i = cursor;
if (i >= size) {
return;
}
final Object[] elementData = ArrayList.this.elementData;
// 如果当前游标大于集合长度，会抛出ConcurrentModificationException
if (i >= elementData.length) {
throw new ConcurrentModificationException();
}
while (i != size && modCount == expectedModCount) {
consumer.accept((E) elementData[i++]);
}
// update once at end of iteration to reduce heap write traffic
cursor = i;
lastRet = i - 1;
// 检查处理次数是否正常，如果不正常，抛出ConcurrentModificationException
checkForComodification();
}
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
private class ListItr extends Itr implements ListIterator<E> {
ListItr(int index) {
super();
cursor = index;
}
// 是否有上一个元素
public boolean hasPrevious() {
return cursor != 0;
}
// 返回下一个元素的索引
public int nextIndex() {
return cursor;
}
// 返回返回上一个元素的索引
public int previousIndex() {
return cursor - 1;
}
// 返回上一个元素
@SuppressWarnings("unchecked")
public E previous() {
checkForComodification();
int i = cursor - 1;
// 如果上一个元素不存在，抛出NoSuchElementException
if (i < 0)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
// 如果上一个元素的索引大于等于集合长度，抛出ConcurrentModificationException， 由于多线程并发导致获取的索引有问题
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i;
return (E) elementData[lastRet = i];
}
// 替换元素
public void set(E e) {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
ArrayList.this.set(lastRet, e);
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
// 新增元素
public void add(E e) {
checkForComodification();
try {
int i = cursor;
ArrayList.this.add(i, e);
cursor = i + 1;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
}
Iterator与ListIterator的区别:
1. Iterator有remove(), forEachRemaining(), ListIterator没有
2.ListIterator有add(), set(), 能获取上一个元素，Iterator不能
3.ListIterator可以通过previousIndex(), nextIndex()获取索引，Iterator不能
12.forEach()
public void forEach(Consumer<? super E> action) {
Objects.requireNonNull(action);
final int expectedModCount = modCount;
// 获取当前集合存储元素的数组
@SuppressWarnings("unchecked")
final E[] elementData = (E[]) this.elementData;
final int size = this.size;
// 消费者迭代消费数组的元素
for (int i=0; modCount == expectedModCount && i < size; i++) {
action.accept(elementData[i]);
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
List<String> list = Arrays.asList("1", "2", "3");
// 输出
list.forEach(System.out::println);
list.forEach((a) -> {
// 业务逻辑代码
});
13.asList()的坑
public static <T> List<T> asList(T... a) {
return new ArrayList<>(a);
}
private static class ArrayList<E> extends AbstractList<E>
implements RandomAccess, java.io.Serializable
{
private static final long serialVersionUID = -2764017481108945198L;
private final E[] a;
ArrayList(E[] array) {
a = Objects.requireNonNull(array);
}
@Override
public int size() {
return a.length;
}
@Override
public Object[] toArray() {
return a.clone();
}
@Override
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
int size = size();
if (a.length < size)
return Arrays.copyOf(this.a, size,
(Class<? extends T[]>) a.getClass());
System.arraycopy(this.a, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
@Override
public E get(int index) {
return a[index];
}
@Override
public E set(int index, E element) {
E oldValue = a[index];
a[index] = element;
return oldValue;
}
@Override
public int indexOf(Object o) {
E[] a = this.a;
if (o == null) {
for (int i = 0; i < a.length; i++)
if (a[i] == null)
return i;
} else {
for (int i = 0; i < a.length; i++)
if (o.equals(a[i]))
return i;
}
return -1;
}
@Override
public boolean contains(Object o) {
return indexOf(o) != -1;
}
@Override
public Spliterator<E> spliterator() {
return Spliterators.spliterator(a, Spliterator.ORDERED);
}
@Override
public void forEach(Consumer<? super E> action) {
Objects.requireNonNull(action);
for (E e : a) {
action.accept(e);
}
}
@Override
public void replaceAll(UnaryOperator<E> operator) {
Objects.requireNonNull(operator);
E[] a = this.a;
for (int i = 0; i < a.length; i++) {
a[i] = operator.apply(a[i]);
}
}
@Override
public void sort(Comparator<? super E> c) {
Arrays.sort(a, c);
}
}
// AbstractList中的remove(),add()
public E remove(int index) {
throw new UnsupportedOperationException();
}
public boolean add(E e) {
add(size(), e);
return true;
}
public void add(int index, E element) {
throw new UnsupportedOperationException();
}
总结:Arrays中的ArrayList继承AbstractList,没有重写remove()和add(),当list1调用remove和add实际执行的是AbstractList中的remove和add,因此会报UnsupportedOperationException,可以通过下面方式替换:
List<String> list2 = Arrays.stream(new String[]{"1", "2"}).collect(Collectors.toList());
14.subList的坑
ArrayList<String> list = new ArrayList<>();
list.add("1");
list.add("2");
list.add("3");
List<String> list1 = list.subList(0, 1);
list1.add("4");
list.forEach(System.out::println); // 结果: 1 2 3 4
list.add("5"); // 报ConcurrentModificationException
public List<E> subList(int fromIndex, int toIndex) {
subListRangeCheck(fromIndex, toIndex, size);
return new SubList(this, 0, fromIndex, toIndex);
}
private class SubList extends AbstractList<E> implements RandomAccess {
private final AbstractList<E> parent;
private final int parentOffset;
private final int offset;
int size;
SubList(AbstractList<E> parent,
int offset, int fromIndex, int toIndex) {
this.parent = parent;
this.parentOffset = fromIndex;
this.offset = offset + fromIndex;
this.size = toIndex - fromIndex;
this.modCount = ArrayList.this.modCount;
}
public E set(int index, E e) {
rangeCheck(index);
checkForComodification();
E oldValue = ArrayList.this.elementData(offset + index);
ArrayList.this.elementData[offset + index] = e;
return oldValue;
}
public E get(int index) {
rangeCheck(index);
checkForComodification();
return ArrayList.this.elementData(offset + index);
}
public int size() {
checkForComodification();
return this.size;
}
.....
}
总结:使用subList, 操作获得的list会影响原来的list,操作原来的list会报ConcurrentModificationException,因为sublist只是获取原来list部分元素的引用，而不是产生新的list,可以通过下面方式替换:
List<String> newList = list.stream().skip(2).collect(Collectors.toList());
List<String> newList2 = list.stream().limit(2).collect(Collectors.toList());
