Java容器ArrayList知识点总结

ArrayList

底层实现是数组，访问元素效率高 (查询快，插入、修改、删除元素慢)

与LinkedList相比，它效率高，但线程不安全。

ArrayList数组是一个可变数组，可以存取包括null在内的所有元素

• 每个ArrayList实例都有一个容量，该容量是指用来存储列表元素的数组的大小
• 随着向ArrayList中不断增加元素，其容量自动增长
• 在添加大量元素前，应用程序也可以使用ensureCapacity操作来增加ArrayList实例的容量，这样可以减少递增式再分配的数量。
• 所以如果我们明确所插入元素的多少，最好指定一个初始容量值，避免过多进行扩容操作而浪费时间、效率
• 源码分析

底层使用数组实现

transient Object[] elementData;

构造方法

private static final int DEFAULT_CAPACITY = 10;
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
transient Object[] elementData;
private int size;

// 构造一个空列表
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA ;
  }

// 构造一个指定初始容量的空列表
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);
    }
  }


// 构造一个指定Collection元素的列表，这些元素按照Connection元素的迭代返回顺序进行排列
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;
    }
  }

存储

// 在列表的指定位置的元素用element替代，并且返回该位置原来的元素
public E set(int index, E element) {
    rangeCheck(index); // 检查数组容量，抛出：IndexOutOfBoundsException

    E oldValue = elementData(index);
    elementData[index] = element;
    return oldValue;
  }

// 在列表的尾部添加指定元素
public boolean add(E e) {
    ensureCapacityInternal(size + 1); // 数组扩容
    elementData[size++] = e;
    return true;
  }

// 在列表的指定位置添加元素
public void add(int index, E element) {
    rangeCheckForAdd(index);

    ensureCapacityInternal(size + 1); // Increments modCount!!

    // src:源数组,srcPro:源数组中的起始位置
    // dest:目标数组，destPost:目标数组的起始位置，length:要复制的数组元素数量
       // 将当前位于该位置的元素以及所有后续元素后移一个位置
    System.arraycopy(elementData, index, elementData, index + 1,
             size - index);
    // 用element替换index位置的元素
    elementData[index] = element;
    size++;
  }

// 在列表的尾部添加Connection中的元素，元素顺序按照Connection迭代器返回的顺序
public boolean addAll(Collection<? extends E> c) {
    Object[] a = c.toArray(); // 转化为一个数组

    int numNew = a.length;
    ensureCapacityInternal(size + numNew); // Increments modCount

    // Increments modCount!!
    System.arraycopy(a, 0, elementData, size, numNew);
    size += numNew;
    return numNew != 0;
  }

// 在列表的指定位置添加Connection中的元素，元素顺序按照Connection迭代器返回的顺序
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;
  }

读取

// 移除此列表指定位置上的元素
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;
  }

// 移除此列表中的某个元素
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;
  }

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
  }

数组扩容

每当向数组中添加元素时，都需要去检查添加元素后元素的个数是否超出当前数组的长度，如果超出，数组将会进行扩容，以满足添加数据的需求。

public void ensureCapacity(int minCapacity) {
    int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA )
      ? 0 : DEFAULT_CAPACITY;

    if (minCapacity > minExpand) {
      ensureExplicitCapacity(minCapacity);
    }
  }

private void ensureCapacityInternal(int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA ) {
      minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
    }

    ensureExplicitCapacity(minCapacity);
  }

private void ensureExplicitCapacity(int minCapacity) {
    modCount++;

    // overflow-conscious code
    if (minCapacity - elementData.length > 0)
      grow(minCapacity);
  }

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);
  }

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

手写ArrayList

public class MyArrayList /*implements List<E>*/{
 private transient Object[] elementData;
 private int size; //元素个数

 public MyArrayList(){
  this(10);
 }

 public MyArrayList(int initialCapacity) {
  if (initialCapacity<0) {
   try {
    throw new Exception();
   } catch (Exception e) {
    e.printStackTrace();
   }
  }
  elementData = new Object[initialCapacity];
 }

 public int size() {
  return size;
 }

 public boolean isEmpty(){
  return size == 0;
 }
 //根据index删掉对象
 public void remove(int index) throws Exception {
  rangeCheck(index);
  int numMoved = size-index-1;
  if (numMoved > 0) {
   System.arraycopy(elementData, index+1, elementData, index, numMoved);
  }
  elementData[--size] = null;
 }
 //删掉对象
 public boolean remove(Object obj) throws Exception {
  for (int i = 0; i < size; i++) {
   if (get(i).equals(obj)) {
    remove(i);
   }
   return true;
  }
  return true;
 }
 //修改元素
 public Object set(int index , Object obj ) throws Exception{
  rangeCheck(index);
  Object oldValue = elementData[index];
  elementData[index] = obj;
  return oldValue;
 }
 //在指定位置插入元素
 public void add(int index,Object obj) throws Exception {
  rangeCheck(index);
  ensureCapacity();
  System.arraycopy(elementData, index, elementData, index+1, size-index);
  elementData[index] = obj;
  size ++;
 }
 public void add(Object object) {
  ensureCapacity();
  /*elementData[size] = object;
  size ++;*/
  elementData[size++] = object; //先赋值，后自增
 }

 public Object get(int index) throws Exception {
  rangeCheck(index);
  return elementData[index];
 }
 public void rangeCheck(int index) throws Exception {
  if (index<0 || index >=size) {
   throw new Exception();
  }
 }
 //扩容
 public void ensureCapacity() {
  //数组扩容和内容拷贝
  if (size==elementData.length) {
   //elementData = new Object[size*2+1]; 这么写原来数组里的内容丢失
   Object[] newArray = new Object[size*2+1];
   //拷贝数组里的内容
   /*for (int i = 0; i < newArray.length; i++) {
    newArray[i] = elementData[i];
   }*/
   System.arraycopy(elementData, 0, newArray, 0, elementData.length);
   elementData = newArray;
  }
 }
 // 测试
 public static void main(String[] args) {
  MyArrayList myArrayList = new MyArrayList(3);
  myArrayList.add("111");
  myArrayList.add("222");
  myArrayList.add("333");
  myArrayList.add("444");
  myArrayList.add("555");

  try {
   myArrayList.remove(2);
   myArrayList.add(3, "新值");
   myArrayList.set(1, "修改");
  } catch (Exception e1) {
   // TODO Auto-generated catch block
   e1.printStackTrace();
  }
  System.out.println(myArrayList.size());
  for (int i = 0; i < myArrayList.size(); i++) {
   try {
    System.out.println(myArrayList.get(i));
   } catch (Exception e) {
    e.printStackTrace();
   }
  }
 }
}

原文链接：https://segmentfault.com/a/1190000015015669