Java容器ArrayList知識點總結

發布時間：2020-10-15 11:41:16 來源：腳本之家閱讀：191 作者：蘇生欄目：編程語言

ArrayList

底層實現是數組，訪問元素效率高 (查詢快，插入、修改、刪除元素慢)

與LinkedList相比，它效率高，但線程不安全。

ArrayList數組是一個可變數組，可以存取包括null在內的所有元素

每個ArrayList實例都有一個容量，該容量是指用來存儲列表元素的數組的大小
隨著向ArrayList中不斷增加元素，其容量自動增長
在添加大量元素前，應用程序也可以使用ensureCapacity操作來增加ArrayList實例的容量，這樣可以減少遞增式再分配的數量。
所以如果我們明確所插入元素的多少，最好指定一個初始容量值，避免過多進行擴容操作而浪費時間、效率
源碼分析

底層使用數組實現

transient Object[] elementData;

構造方法

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;
 
// 構造一個空列表
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA ;
  }
 
// 構造一個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);
    }
  }
 
 
// 構造一個指定Collection元素的列表，這些元素按照Connection元素的迭代返回順序進行排列
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;
    }
  }

存儲

// 在列表的指定位置的元素用element替代，并且返回該位置原來的元素
public E set(int index, E element) {
    rangeCheck(index); // 檢查數組容量，拋出：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); // 數組擴容
    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:源數組,srcPro:源數組中的起始位置
    // dest:目標數組，destPost:目標數組的起始位置，length:要復制的數組元素數量
       // 將當前位于該位置的元素以及所有后續元素后移一個位置
    System.arraycopy(elementData, index, elementData, index + 1,
             size - index);
    // 用element替換index位置的元素
    elementData[index] = element;
    size++;
  }
 
// 在列表的尾部添加Connection中的元素，元素順序按照Connection迭代器返回的順序
public boolean addAll(Collection<? extends E> c) {
    Object[] a = c.toArray(); // 轉化為一個數組
 
    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，元素順序按照Connection迭代器返回的順序
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;
  }

讀取

// 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;
  }
 
// 移除此列表中的某個元素
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
  }

數組擴容

每當向數組中添加元素時，都需要去檢查添加元素后元素的個數是否超出當前數組的長度，如果超出，數組將會進行擴容，以滿足添加數據的需求。

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;
  }

手寫ArrayList

public class MyArrayList /*implements List<E>*/{
 private transient Object[] elementData;
 private int size; //元素個數
 
 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;
 }
 //根據index刪掉對象
 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;
 }
 //刪掉對象
 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; //先賦值，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();
  }
 }
 //擴容
 public void ensureCapacity() {
  //數組擴容和內容拷貝
  if (size==elementData.length) {
   //elementData = new Object[size*2+1]; 這么寫原來數組里的內容丟失
   Object[] newArray = new Object[size*2+1];
   //拷貝數組里的內容
   /*for (int i = 0; i < newArray.length; i++) {
    newArray[i] = elementData[i];
   }*/
   System.arraycopy(elementData, 0, newArray, 0, elementData.length);
   elementData = newArray;
  }
 }
 // 測試
 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();
   }
  }
 }
}

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