1. ArrayList概要： public ArrayList() { this(10);
}
- 底層采用數(shù)組存儲：private transient Object[] elementData;
transient表示數(shù)組elementData不需要通過serialization序列化傳輸
- 線程不安全，在多線程場景會出現(xiàn)問題，可以考慮使用Vector或者Collections.synchronizedList同步該容器
- 查詢效率比較高，比較適用于查詢
- 擴(kuò)容的長度為初始長度的一半，建議初始化的時候設(shè)置已知的長度，免得容器自己去擴(kuò)容，浪費空間以及效率
2. ArrayList代碼詳解： public boolean add(E e) { ensureCapacity(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
首先檢查數(shù)組是否已滿，如果滿了就開始擴(kuò)容，擴(kuò)容后的長度為原長度的1.5倍。
/** * Increases the capacity of this <tt>ArrayList</tt> instance, if
* necessary, to ensure that it can hold at least the number of elements
* specified by the minimum capacity argument.
*
* @param minCapacity the desired minimum capacity
*/
public void ensureCapacity(int minCapacity) {
modCount++;//modCount表示數(shù)組的操作計數(shù)，用于iterator的時候與Count比較，防止迭代操作對add,remove等操作影響迭代操作
int oldCapacity = elementData.length;
if (minCapacity > oldCapacity) { //新插入元素后的長度大于老的長度，數(shù)組開始擴(kuò)容
Object oldData[] = elementData;
int newCapacity = (oldCapacity * 3)/2 + 1;//新空間為原長度的1.5倍，等于是擴(kuò)容了50%
if (newCapacity < minCapacity)
newCapacity = minCapacity;
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);//把之前的元素拷貝到新的數(shù)組 }
}
/**
* Removes the element at the specified position in this list.
* Shifts any subsequent elements to the left (subtracts one from their
* indices).
*
* @param index the index of the element to be removed
* @return the element that was removed from the list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E remove(int index) {
RangeCheck(index); //檢查索引是否溢出
modCount++; //操作計數(shù)
E oldValue = (E) elementData[index];
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,//復(fù)制原數(shù)組制定index+1到length-1的元素到elementData的index的索引位
numMoved);
elementData[--size] = null; // Let gc do its work//最后一位設(shè)置為null
return oldValue;
}
/**
* Checks if the given index is in range. If not, throws an appropriate
* runtime exception. This method does *not* check if the index is
* negative: It is always used immediately prior to an array access,
* which throws an ArrayIndexOutOfBoundsException if index is negative.
*/
private void RangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(
"Index: "+index+", Size: "+size);
}
/**
* Returns the element at the specified position in this list.
*
* @param index index of the element to return
* @return the element at the specified position in this list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E get(int index) {
RangeCheck(index);
return (E) elementData[index]; //直接獲取數(shù)組的索引位
}