ArrayList在整个常用的集合体系中的位置:
jdk 7ArrayList
ArrayList list = new ArrayList();//底层创建了长度是10的Object[]数组elementData
list.add(111);//elementData[0] = new Integer(111);
...
list.add(11);//如果此次的添加导致底层elementData数组容量不够,则扩容。
默认情况下,扩容为原来的容量的1.5倍,同时需要将原有数组中的数据复制到新的数组中。
结论:建议开发中使用带参的构造器(在已知道需要存储多少数据的情况下):ArrayList list = new ArrayList(int capacity)
public ArrayList() {
this(10); // 调用 1 个int类型参数的构造器, 初始状态容量是10
}
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity]; // initialCapacity 为10
}
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
private void ensureCapacityInternal(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity); // 扩容
}
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1); // 1.5 倍
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); // 拷贝原来数据
}
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JDK8ArrayList
ArrayList list = new ArrayList();//底层Object[] elementData初始化为{}.并没有创建长度为10的数组
list.add(123);//第一次调用add()时,底层才创建了长度10的数组,并将数据123添加到elementData[0]
...
后续的添加和扩容操作与jdk 7 无异。
2.3 小结:jdk7中的ArrayList的对象的创建类似于单例的饿汉式,而jdk8中的ArrayList的对象
的创建类似于单例的懒汉式,延迟了数组的创建,节省内存。
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
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;
}
private void ensureCapacityInternal(int minCapacity) {
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
private static final int DEFAULT_CAPACITY = 10;
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);
}