ArrayList

初始化：

1. 如果呼叫空構造方法，預設elementData為10；

2. 擴容倍數為1.5倍 oldCapacity +oldCapacity >>1 //oldCapacity =10

3. 可以指定大小，如果指定大小以後，oldCapacity 為設定的大小

>>1表示除以2

新建arraylist，無參初始化：（先走classloader和hashcode，不做記錄）

public ArrayList() {
  this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

public synchronized boolean add(E e) {
  modCount++;
  ensureCapacityHelper(elementCount + 1);
  elementData[elementCount++] = e;
  return true;
}
 

private void ensureCapacityHelper(int minCapacity) {
  // overflow-conscious code
  if (minCapacity - elementData.length > 0)
    grow(minCapacity);
}

如果elmentcount+1 -陣列長度 >0，也就是下一個元素將會越界時候，進行擴容

public synchronized boolean add(E e) {
  modCount++;
  ensureCapacityHelper(elementCount + 1);
  elementData[elementCount++] = e;
  return true;
}
 

賦值。

看看新增第一個元素結果：

arrayList.add(101);

public static Integer valueOf(int i) {
  if (i >= IntegerCache.low && i <= IntegerCache.high)
    return IntegerCache.cache[i + (-IntegerCache.low)];
  return new Integer(i);
}

先轉型成Integer

public boolean add(E e) {
  ensureCapacityInternal(size + 1); // Increments modCount!!
  elementData[size++] = e;
  return true;
}
 

一樣會先判斷是否需要擴容，

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

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

計算，此處DEFAULTCAPACITY_EMPTY_ELEMENTDATA ={}

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

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

此處minCapacity為10，elementData.length為0，所以需要擴容

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);
}

本質就是copy一個新陣列

public static <T> T[] copyOf(T[] original, int newLength) {
  return (T[]) copyOf(original, newLength, original.getClass());
}

擴容完成後返回，將值寫入新陣列，over

public boolean add(E e) {
  ensureCapacityInternal(size + 1); // Increments modCount!!
  elementData[size++] = e;
  return true;
}