使用技巧：

将ArrayList转化为int数组

int[] ret = arr.stream().mapToInt(Integer::intValue).toArray();

一、参数

1.1 默认容量

/*** Default initial capacity.*/private static final int DEFAULT_CAPACITY = 10;

1.2 空数组

/*** Shared empty array instance used for empty instances.*/private static final Object[] EMPTY_ELEMENTDATA = {};

1.3 空初始化时的数组

/*** Shared empty array instance used for default sized empty instances. We* distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when* first element is added.*/private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

1.4 存储数据的数组

/*** The array buffer into which the elements of the ArrayList are stored.* The capacity of the ArrayList is the length of this array buffer. Any* empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA* will be expanded to DEFAULT_CAPACITY when the first element is added.*/transient Object[] elementData; // non-private to simplify nested class access

1.5 数组大小

/*** The size of the ArrayList (the number of elements it contains).** @serial*/private int size;

二、方法

2.1 add（添加元素）

/*** Appends the specified element to the end of this list.** @param e element to be appended to this list* @return <tt>true</tt> (as specified by {@link Collection#add})*/public boolean add(E e) {ensureCapacityInternal(size + 1);  // Increments modCount!!elementData[size++] = e;return true;}

/*** Inserts the specified element at the specified position in this* list. Shifts the element currently at that position (if any) and* any subsequent elements to the right (adds one to their indices).** @param index index at which the specified element is to be inserted* @param element element to be inserted* @throws IndexOutOfBoundsException {@inheritDoc}*/public void add(int index, E element) {rangeCheckForAdd(index);ensureCapacityInternal(size + 1);  // Increments modCount!!System.arraycopy(elementData, index, elementData, index + 1,size - index);elementData[index] = element;size++;}

2.1.1 ensureCapacityInternal

private void ensureCapacityInternal(int minCapacity) {ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));}

2.1.1.1 ensureExplicitCapacity

private void ensureExplicitCapacity(int minCapacity) {modCount++;// overflow-conscious codeif (minCapacity - elementData.length > 0)grow(minCapacity);}

2.1.1.2 calculateCapacity

private static int calculateCapacity(Object[] elementData, int minCapacity) {if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {return Math.max(DEFAULT_CAPACITY, minCapacity);}return minCapacity;}

2.2 grow（扩大数组容量）

扩大数组容量使得数组最少可以容纳由minCapacity指定的元素个数。

    private void grow(int minCapacity) {// overflow-conscious codeint 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);}

2.2.1 hugeCapacity

private static int hugeCapacity(int minCapacity) {if (minCapacity < 0) // overflowthrow new OutOfMemoryError();return (minCapacity > MAX_ARRAY_SIZE) ?Integer.MAX_VALUE :MAX_ARRAY_SIZE;}

2.3 size

/*** Returns the number of elements in this list.** @return the number of elements in this list*/public int size() {return size;}

2.4 isEmpty

/*** Returns <tt>true</tt> if this list contains no elements.** @return <tt>true</tt> if this list contains no elements*/public boolean isEmpty() {return size == 0;}

2.5 contains

/*** Returns <tt>true</tt> if this list contains the specified element.* More formally, returns <tt>true</tt> if and only if this list contains* at least one element <tt>e</tt> such that* <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.** @param o element whose presence in this list is to be tested* @return <tt>true</tt> if this list contains the specified element*/public boolean contains(Object o) {return indexOf(o) >= 0;}

2.5.1 indexOf

/*** Returns the index of the first occurrence of the specified element* in this list, or -1 if this list does not contain the element.* More formally, returns the lowest index <tt>i</tt> such that* <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,* or -1 if there is no such index.*/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;}

2.5.2 lastIndexOf

/*** Returns the index of the last occurrence of the specified element* in this list, or -1 if this list does not contain the element.* More formally, returns the highest index <tt>i</tt> such that* <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,* or -1 if there is no such index.*/public int lastIndexOf(Object o) {if (o == null) {for (int i = size-1; i >= 0; i--)if (elementData[i]==null)return i;} else {for (int i = size-1; i >= 0; i--)if (o.equals(elementData[i]))return i;}return -1;}

2.6 toArray

/*** Returns an array containing all of the elements in this list* in proper sequence (from first to last element).** <p>The returned array will be "safe" in that no references to it are* maintained by this list.  (In other words, this method must allocate* a new array).  The caller is thus free to modify the returned array.** <p>This method acts as bridge between array-based and collection-based* APIs.** @return an array containing all of the elements in this list in*         proper sequence*/public Object[] toArray() {return Arrays.copyOf(elementData, size);}

2.7 get

获取数据的时间复杂度是O(1)

/*** 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 elementData(index);}

2.7.1 rangeCheck

/*** 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(outOfBoundsMsg(index));}

2.8 set

/*** Replaces the element at the specified position in this list with* the specified element.** @param index index of the element to replace* @param element element to be stored at the specified position* @return the element previously at the specified position* @throws IndexOutOfBoundsException {@inheritDoc}*/public E set(int index, E element) {rangeCheck(index);E oldValue = elementData(index);elementData[index] = element;return oldValue;}

2.9 remove

删除数据的开销很大，因为要重排数组中的数据（在index后的数据）

/*** 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);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 workreturn oldValue;}

/*** Removes the first occurrence of the specified element from this list,* if it is present.  If the list does not contain the element, it is* unchanged.  More formally, removes the element with the lowest index* <tt>i</tt> such that* <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>* (if such an element exists).  Returns <tt>true</tt> if this list* contained the specified element (or equivalently, if this list* changed as a result of the call).** @param o element to be removed from this list, if present* @return <tt>true</tt> if this list contained the specified element*/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;}

2.9.1 fastRemove

/** Private remove method that skips bounds checking and does not* return the value removed.*/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}

2.10 构造方法

/*** Constructs an empty list with the specified initial capacity.** @param  initialCapacity  the initial capacity of the list* @throws IllegalArgumentException if the specified initial capacity*         is negative*/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);}}

/*** Constructs an empty list with an initial capacity of ten.*/public ArrayList() {this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;}

/*** Constructs a list containing the elements of the specified* collection, in the order they are returned by the collection's* iterator.** @param c the collection whose elements are to be placed into this list* @throws NullPointerException if the specified collection is null*/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;}}

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jdk中ArrayList的实现

使用技巧：

将ArrayList转化为int数组

一、参数

1.1 默认容量

1.2 空数组

1.3 空初始化时的数组

1.4 存储数据的数组

1.5 数组大小

二、方法

2.1 add（添加元素）

2.1.1 ensureCapacityInternal

2.1.1.1 ensureExplicitCapacity

2.1.1.2 calculateCapacity

2.2 grow（扩大数组容量）

2.2.1 hugeCapacity

2.3 size

2.4 isEmpty

2.5 contains

2.5.1 indexOf

2.5.2 lastIndexOf

2.6 toArray

2.7 get

2.7.1 rangeCheck

2.8 set

2.9 remove

2.9.1 fastRemove

2.10 构造方法

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