Java源码分析之ArrayList

先看私有属性

//保存ArrayList中的内容 transient Object[] elementData; // non-private to simplify nested class access //表示元素的数量 private int size;

transient 关键字，就是这部分不参与序列化

构造函数

构造函数有三个

//没有参数时，构建一个空的 private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; } //当有一个参数时，就是代表开多大的空间，注意，这里并没有设size的值哦 private static final Object[] EMPTY_ELEMENTDATA = {}; 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); } } //用集合构造，转成数组后，返回给elementData，返回若不是Object[]将调用Arrays.copyOf方法将其转为Object[] 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; } }

基本的方法

get方法

//正常的get方法 public E get(int index) { rangeCheck(index); return elementData(index); } //对index的检测 private void rangeCheck(int index) { if (index >= size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } //注意这个是没有边界检测的 @SuppressWarnings("unchecked") E elementData(int index) { return (E) elementData[index]; }

set方法

//有index的检测，返回的是旧的值 public E set(int index, E element) { rangeCheck(index); E oldValue = elementData(index); elementData[index] = element; return oldValue; }

add方法

//ensureCapacityInternal稍后讲，除了为了有足够大的空间，还有其他的作用 public boolean add(E e) { ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; } //在index位置处插入element 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++; } //这里检测是否在那个区间内，个人感觉有点儿问题，比如，初始构造给了参数，接下来在某个位置add一个数，换句话说，有了空间，但该位置没数，此时该不该抛出越界异常 private void rangeCheckForAdd(int index) { if (index > size || index < 0) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); }

addAll

//加入集合中的全部，多开空间 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; } //在index位置上加入集合中的全部 public boolean addAll(int index, Collection<? extends E> c) { rangeCheckForAdd(index); Object[] a = c.toArray(); int numNew = a.length; ensureCapacityInternal(size + numNew); // Increments modCount int numMoved = size - index; if (numMoved > 0) System.arraycopy(elementData, index, elementData, index + numNew, numMoved); System.arraycopy(a, 0, elementData, index, numNew); size += numNew; return numNew != 0; }

remove方法

//先看下是否越界，再移除，返回的是旧的值 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; } //移除第一个与o相等的对象，注意remove(int index)是移除index位置的，如果想要移除的Object本身就是int该怎么办，hiahia 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; } //和remove(int index)一样，为什么不直接调用呢？ 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 } //移除fromIndex，toIndex中的元素(这个没有判断越界？ protected void removeRange(int fromIndex, int toIndex) { modCount++; int numMoved = size - toIndex; System.arraycopy(elementData, toIndex, elementData, fromIndex, numMoved); // clear to let GC do its work int newSize = size - (toIndex-fromIndex); for (int i = newSize; i < size; i++) { elementData[i] = null; } size = newSize; }

size方法

public int size() { return size; }

isEmpty()方法

public boolean isEmpty() { return size == 0; }

indexOf方法(O(n))

//返回o的第一次出现的下标，注意是从前往后依次遍历的 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; } //返回o最后一次出现的位置，是从后往前依次遍历的 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; }

contains方法

//就是判断第一次出现的位置，也是O(n)，(我还以为会用HashSet之类的呢 public boolean contains(Object o) { return indexOf(o) >= 0; }

toArray方法

public Object[] toArray() { return Arrays.copyOf(elementData, size); } @SuppressWarnings("unchecked") public <T> T[] toArray(T[] a) { if (a.length < size) // Make a new array of a's runtime type, but my contents: return (T[]) Arrays.copyOf(elementData, size, a.getClass()); System.arraycopy(elementData, 0, a, 0, size); if (a.length > size) a[size] = null; return a; }

clear

public void clear() { modCount++; // clear to let GC do its work for (int i = 0; i < size; i++) elementData[i] = null; size = 0; }

subList

public List<E> subList(int fromIndex, int toIndex) { subListRangeCheck(fromIndex, toIndex, size); return new SubList(this, 0, fromIndex, toIndex); } //这里fromIndex > toIndex并不会交换 static void subListRangeCheck(int fromIndex, int toIndex, int size) { if (fromIndex < 0) throw new IndexOutOfBoundsException("fromIndex = " + fromIndex); if (toIndex > size) throw new IndexOutOfBoundsException("toIndex = " + toIndex); if (fromIndex > toIndex) throw new IllegalArgumentException("fromIndex(" + fromIndex + ") > toIndex(" + toIndex + ")"); }

稍高级点儿的方法

add时候的空间检测

private static final int DEFAULT_CAPACITY = 10; //取当前需要的空间和10的最大值 private void ensureCapacityInternal(int minCapacity) { if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); } ensureExplicitCapacity(minCapacity); } //看element的已有空间，是否能满足 private void ensureExplicitCapacity(int minCapacity) { modCount++; // overflow-conscious code if (minCapacity - elementData.length > 0) grow(minCapacity); } //取需要的空间与原来空间1.5倍中最大值 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); } //所需空间非常大时候的判断，如果是Integer.MAX_VALUE，需要84M，注意爆栈 private static int hugeCapacity(int minCapacity) { if (minCapacity < 0) // overflow throw new OutOfMemoryError(); return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE; }

求elementData与一个集合的交集和差集

//差集 public boolean removeAll(Collection<?> c) { Objects.requireNonNull(c); return batchRemove(c, false); } //交集 public boolean retainAll(Collection<?> c) { Objects.requireNonNull(c); return batchRemove(c, true); } //设一个complement变量来判断是留或者去掉，感觉好机智啊 private boolean batchRemove(Collection<?> c, boolean complement) { final Object[] elementData = this.elementData; int r = 0, w = 0; boolean modified = false; try { 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. if (r != size) { System.arraycopy(elementData, r, elementData, w, size - r); w += size - r; } if (w != size) { // clear to let GC do its work for (int i = w; i < size; i++) elementData[i] = null; modCount += size - w; size = w; modified = true; } } return modified; }

modCount

对于这个变量，大家可能会好奇是什么用，是为了多线程中判断是否被修改时使用，举个例子

//可能很多人会问，明明上面刚刚赋值了expectedModCount ，只做了一个排序的操作，怎么可能会变了呢。在这个线程1中，当然是不会变的，注意到modCount，修改这个值的函数，大概有删除elementData中的值，修改elementData中的值，清除等，也就是说，如果其他线程2中值被修改了，modCount值就会变，那么在线程1中modCount != expectedModCount了。但这还不是线程安全的，为什么呢，因为知识抛出了异常，并没有做出什么操作(比如锁等)，所以并不是线程安全的 public void sort(Comparator<? super E> c) { final int expectedModCount = modCount; Arrays.sort((E[]) elementData, 0, size, c); if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } modCount++; }

至此，对ArrayList有了基本的了解，其中很重要的Itr没有说明，打算专门开一个说。