-
扩容:把原来的数组复制到另一个内存空间更大的数组中。
添加元素:把新元素添加到扩容以后的数组中。
2、源码分析
2.1 ArrayList里面的属性
/**
* 默认初始化容量.
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* Shared empty array instance used for default sized empty instances. We
* distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
* first element is added.
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
/**
* The array buffer into which the elements of the ArrayList are stored.
* The capacity of the ArrayList is the length of this array buffer. Any
* empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
* will be expanded to DEFAULT_CAPACITY when the first element is added.
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
* ArrayList的大小 (它包含的元素个数).
*
* @serial
*/
private int size;
2.2 构造方法
分析之前我们先看一下ArrayList的两个构造方法
/**
* Constructs an empty list with the specified initial capacity.
*
* @param initialCapacity the initial capacity of the list
* @throws IllegalArgumentException if the specified initial capacity
* is negative
*/
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);
}
}
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
2.3 扩容
add方法就两步,
第一步:增加长度。
第二步:添加元素到数组,第二步就是将数据放到新数组最后一位。
第二步没什么好讲的,下面主要讲第一步。
/**
* Appends the specified element to the end of this list.
*
* @param e element to be appended to this list
* @return <tt>true</tt> (as specified by {@link Collection#add})
*/
public boolean add(E e) {
// 增加长度
ensureCapacityInternal(size + 1); // Increments modCount!!
// 添加元素
elementData[size++] = e;
return true;
}
我们看看ensureCapacityInternal(int minCapacity)这个增加长度的方法
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
这个地方我们看到了,如果在添加的时候远数组是空的,就直接给一个10的长度,否则的话就是size+1。
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
通过这个地方是真正的增加长度,当需要的长度大于原来数组长度的时候就需要扩容了,相反的则不需要扩容。
/**
* Increases the capacity to ensure that it can hold at least the
* number of elements specified by the minimum capacity argument.
*
* @param minCapacity the desired minimum capacity
*/
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);
}
扩容实现:
扩容结束。