- 可以看到我们创建
ArrayLis的时候指定了大小，会创建一个默认指定大小的数组
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
}
- 无参的会创建一个空数组
public ArrayList() {
super();
this.elementData = EMPTY_ELEMENTDATA;
}
- 转换成数组并且copy到elementData数组中去
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}
- 测量存放数据数组的大小
public void ensureCapacity(int minCapacity) {
如果是空数组就是0，否则minExpand =10
int minExpand = (elementData != EMPTY_ELEMENTDATA)
// any size if real element table
? 0
// larger than default for empty table. It's already supposed to be
// at default size.
: DEFAULT_CAPACITY; 默认大小是10
if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity);
}
}
- 进入到
ensureExplicitCapacity
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
private void grow(int minCapacity) {
之前存放的数据大小
int oldCapacity = elementData.length;、
新增大小为=之前存放的数据大小+oldCapacity/2
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)
新增之后的数据大小还小于计算之后newCapacity大小，就以传递的大小为准
newCapacity = minCapacity;
判断当前增加的大小是否大于 Integer.MAX_VALUE - 8
if (newCapacity - MAX_ARRAY_SIZE > 0)
重新计算返回Integer.MAX_VALUE或者 Integer.MAX_VALUE - 8
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
拷贝扩大当前数据容量
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;
}
contains判断可以里看出，是2个循环，至于这里为什么不用加强for因为Arratlisr实现了RandomAccess
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;
}
- get方法实现
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
private void rangeCheck(int index) {
if (index >= size)
这个异常就是大家熟悉的角标越界
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
可以看到直接根据数组的角标返回因此查询比较快
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}
- set方法实现
public E set(int index, `E element) {
rangeCheck(index);
如果有数据就会返回当前位置的数据并且替换
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
- add方法
public boolean add(E e) {
测量数组是否能存放不能就扩大
ensureCapacityInternal(size + 1); // Increments modCount!!
设置当前位置的数据并且集合size++
elementData[size++] = e;
return true;
}
测量大小方法
private void ensureCapacityInternal(int minCapacity) {
如果之前没有添加数据，是空数组那就设置数组的大小默认为10
if (elementData == EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
下面这个就是上面分析的扩大数组大小的方法
ensureExplicitCapacity(minCapacity);
}
` System.arraycopy和Arrays.copyOf我们查看源码可以发现 Arrays.copyOf 底层还是 System.arraycopy实现拷贝的
- remove方法
public boolean remove(Object o) {
if (o == null) {
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
清空的方法具体实现
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)
重新拷贝个新的数组，从当前要删除的位置的下一位开始拷贝，此处可见删除只是重新copay的一份新的数组
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
}
- clean方法
public void clear() {
modCount++;
// clear to let GC do its work 清空数组，等待GC回收
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
