标签：扩容 code Vector int JDK14 elementData length growth
JDK14中Vector类的动态扩容
Vector类的动态扩容
- Vector与ArrayList一样，也是长度可变的动态数组。
- ArrayList无参构造方法实际上是创建空数组，在第一次调用add()或addAll()时会扩容成长度为10；而Vector在调用无参构造方法时，就创建了一个长度为10的空数组。
- 当集合中的元素数量大于当前集合的长度时，Vector会扩容为原来的2倍，而ArrayList只会扩容为原来的1.5倍。
源码分析
无参构造方法
- 注意Vector在调用无参构造方法时，就创建了一个长度为10的空数组。
/**
* Constructs an empty vector so that its internal data array
* has size {@code 10} and its standard capacity increment is
* zero.
*/
public Vector() {
this(10);
}
扩容
- 调用add()方法需要扩容时，前面的处理都和ArrayList动态扩容的方法相同，具体分析可以看这篇博客，直到grow()方法。
/**
* Appends the specified element to the end of this Vector.
*
* @param e element to be appended to this Vector
* @return {@code true} (as specified by {@link Collection#add})
* @since 1.2
*/
public synchronized boolean add(E e) {
modCount++;
add(e, elementData, elementCount);
return true;
}
/**
* This helper method split out from add(E) to keep method
* bytecode size under 35 (the -XX:MaxInlineSize default value),
* which helps when add(E) is called in a C1-compiled loop.
*/
private void add(E e, Object[] elementData, int s) {
if (s == elementData.length)
elementData = grow();
elementData[s] = e;
elementCount = s + 1;
}
private Object[] grow() {
return grow(elementCount + 1);
}
- grow()
- capacityIncrement是增长因子。
- 当增长因为大于0时，Vector会增加capacityIncrement的量；
- 否则，Vector的容量加倍，即为原来的2倍。
/**
* 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
* @throws OutOfMemoryError if minCapacity is less than zero
*/
private Object[] grow(int minCapacity) {
int oldCapacity = elementData.length;
int newCapacity = ArraysSupport.newLength(oldCapacity,
minCapacity - oldCapacity, /* minimum growth */
capacityIncrement > 0 ? capacityIncrement : oldCapacity
/* preferred growth */);
return elementData = Arrays.copyOf(elementData, newCapacity);
}
/**
* Calculates a new array length given an array's current length, a preferred
* growth value, and a minimum growth value. If the preferred growth value
* is less than the minimum growth value, the minimum growth value is used in
* its place. If the sum of the current length and the preferred growth
* value does not exceed {@link #MAX_ARRAY_LENGTH}, that sum is returned.
* If the sum of the current length and the minimum growth value does not
* exceed {@code MAX_ARRAY_LENGTH}, then {@code MAX_ARRAY_LENGTH} is returned.
* If the sum does not overflow an int, then {@code Integer.MAX_VALUE} is
* returned. Otherwise, {@code OutOfMemoryError} is thrown.
*
* @param oldLength current length of the array (must be non negative)
* @param minGrowth minimum required growth of the array length (must be
* positive)
* @param prefGrowth preferred growth of the array length (ignored, if less
* then {@code minGrowth})
* @return the new length of the array
* @throws OutOfMemoryError if increasing {@code oldLength} by
* {@code minGrowth} overflows.
*/
public static int newLength(int oldLength, int minGrowth, int prefGrowth) {
// assert oldLength >= 0
// assert minGrowth > 0
int newLength = Math.max(minGrowth, prefGrowth) + oldLength;
if (newLength - MAX_ARRAY_LENGTH <= 0) {
return newLength;
}
return hugeLength(oldLength, minGrowth);
}
Vector类和ArrayList类的区别
- vector是线程（Thread）同步（Synchronized）的，所以它也是线程安全的；而Arraylist是线程异步（ASynchronized）的，是不安全的。如果不考虑到线程的安全因素，一般用Arraylist效率比较高。
- 当集合中的元素数量大于当前集合的长度时，Vector会扩容为原来的2倍，而ArrayList只会扩容为原来的1.5倍。如果在集合中使用数据量比较大的数据，用vector有一定的优势。
标签：扩容,code,Vector,int,JDK14,elementData,length,growth 来源： https://blog.csdn.net/Caroline_13/article/details/119250571
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