【java源码】ArrayList

题目：【java源码】ArrayListArrayList 常用功能：构造函数、增、批量增、删、批量删、批量保留ArrayList 属性： // 默认数组长度（数组，而不是数据个数） private static final int DEFAULT_CAPACITY = 10; // 空数据 private static final Object[] EMPTY_ELEMENTDATA = {}; private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; // 实际用于存放数据的地方 transient Object[] elementData; // 数据个数 private int size; 1、构造函数 ①public ArrayList();  只干了一件事：this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; ②public ArrayList(int initialCapacity);  指定初始化elementData数组的初始大小。this.elementData = new Object[initialCapacity]; ③public 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、增：就是数组中插一个元素操作思路 ①public boolean add(E e);  1、确保elementData数组能够装下   首先判断原来数组长度是否为0，如果为零，那么新数组长度为Math.max(DEFAULT_CAPACITY, 加入后数组最小应该长度)；   判断原来数组是否还装的下，如果需要扩容那么：   private void grow(int minCapacity) { // minCapacity：加入后数组最小应该长度    // 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);   }  2、elementData[size++] = e;return true; ②public void add(int index, E element);  1、rangeCheckForAdd(index); // 检测index是否越接 if (index > size || index < 0)  2、确保elementData数组能够装下  3、System.arraycopy(elementData, index, elementData, index + 1, size - index); // 数组挪位  4、elementData[index] = element;  5、size++;3、批量增：就是数组中插多个元素操作思路 ①public boolean addAll(Collection<? extends E> c);  public boolean addAll(Collection<? extends E> c) {   Object[] a = c.toArray();   int numNew = a.length;   ensureCapacityInternal(size + numNew); // 同上：确保elementData数组能够装下   System.arraycopy(a, 0, elementData, size, numNew);   size += numNew;   return numNew != 0;  } ②public boolean addAll(int index, Collection<? extends E> c)  public boolean addAll(int index, Collection<? extends E> c) {   rangeCheckForAdd(index);   Object[] a = c.toArray();   int numNew = a.length;   ensureCapacityInternal(size + numNew); // // 同上：确保elementData数组能够装下   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;  }4、删：三点注意①按内容删，只删除第一个；②删的是equals为真的；③注意看fastRemove(index)源码 ①public boolean remove(Object o);  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;  } ②public E remove(int index); // 同理 ③private void fastRemove(int index);  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  }5、批量删、批量保留：一点注意：batchRemove方法。 ①public boolean removeAll(Collection<?> c);  public boolean removeAll(Collection<?> c) {   Objects.requireNonNull(c);   return batchRemove(c, false);  } ②public boolean retainAll(Collection<?> c);  public boolean retainAll(Collection<?> c) {   Objects.requireNonNull(c);   return batchRemove(c, true);  } ③private boolean batchRemove(Collection<?> c, boolean complement);  // 思想：维护两个指针。一个读指针(r)，一个写指针(w)。读指针从0遍历到数组尾，在遍历中，如果符合条件就elementData[w++] = elementData[r];  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;  }  总结：1、ArrayList通过内置一个Object数组实现顺序表功能。通过grow函数实现动态增长，最大长度可在源码中找答案。2、删除功能通过fastRemove函数实现对一个删除，通过batchRemove函数实现对多个删除。通过elementData[i] = null；或elementData[--size] = null; 的方式，让JVM垃圾回收，自动回收。 private void grow(int minCapacity) { // minCapacity：加入后数组最小应该长度  // 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 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 }   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; }

 

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题目：【java源码】ArrayListArrayList常用功能：构造函数、增、批量增、删、批量删、批量保留ArrayList属性：//默认数组长度（数组，而不是数据个数）privatestaticfinalintDEFAULT_CAPACITY=10;//空数据privatestaticfinalObject[]EMPTY_ELEMENTDATA={};privatestaticfinalOb
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