分享知识-快乐自己：都说新的Arraylist 扩容是(1.5倍+1) 看了1.8的源代码发现不是这么回事
都说新的Arraylist 扩容是(1.5倍+1) 看了1.8的源代码发现不是这么回事
就用下面这段代码在jdk的三个版本运行看了下效果：
import java.lang.reflect.Field;
import java.util.ArrayList;
public class ArrayListDemo {
public static void main(String[] args) {
ArrayList<Integer> list1 = new ArrayList<Integer>();
Integer capacity = getCapacity(list1);// 获取容量
int size = list1.size();
System.out.println("list1的容量:" + capacity);
System.out.println("list1的大小:" + size);
System.out.println("----------------------------");
ArrayList<Integer> list2 = new ArrayList<Integer>();
list2.add(1);
capacity = getCapacity(list2);// 获取容量,arraylist初始化容量是10
size = list2.size();
System.out.println("list2的容量:" + capacity);
System.out.println("list2的大小:" + size);
System.out.println("----------------------------");
ArrayList<Integer> list3 = new ArrayList<Integer>();
capacity = getCapacity(list3);// 获取容量,arraylist初始化容量是10
for (int i = 0; i < 10; i++) {
list3.add(i);
}
capacity = getCapacity(list3);
size = list3.size();
System.out.println("list3的容量:" + capacity);
System.out.println("list3的大小:" + size);
System.out.println("----------------------------");
ArrayList<Integer> list4 = new ArrayList<Integer>();
for (int i = 0; i < 11; i++) {
list4.add(i);
}
capacity = getCapacity(list4);// 获取容量
size = list4.size();
System.out.println("list4的容量:" + capacity);
System.out.println("list4的大小:" + size);
}
// 获取list容量
public static Integer getCapacity(ArrayList list) {
Integer length = null;
Class clazz = list.getClass();
Field field;
try {
field = clazz.getDeclaredField("elementData");
field.setAccessible(true);
Object[] object = (Object[]) field.get(list);
length = object.length;
return length;
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return length;
}
}
jdk1.6：运行结果
list1的容量:10
list1的大小:0
----------------------------
list2的容量:10
list2的大小:1
----------------------------
list3的容量:10
list3的大小:10
----------------------------
list4的容量:16
list4的大小:11
部分源代码：
public void ensureCapacity(int var1) {
++this.modCount;
int var2 = this.elementData.length;
if (var1 > var2) {
Object[] var3 = this.elementData;
int var4 = var2 * 3 / 2 + 1;
if (var4 < var1) {
var4 = var1;
}
this.elementData = Arrays.copyOf(this.elementData, var4);
}
}
jdk1.7 ：运行结果
list1的容量:0
list1的大小:0
----------------------------
list2的容量:10
list2的大小:1
----------------------------
list3的容量:10
list3的大小:10
----------------------------
list4的容量:15
list4的大小:11
部分源代码：
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
private void ensureCapacityInternal(int minCapacity) {
if (elementData == 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);
}
jdk1.8：运行结果
list1的容量:0
list1的大小:0
----------------------------
list2的容量:10
list2的大小:1
----------------------------
list3的容量:10
list3的大小:10
----------------------------
list4的容量:15
list4的大小:11
部分源码：
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 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);
}
总结：
发现不同jdk是不一样的，关于（1.5倍+1)出现在jdk1.6，其他1.7和1.8都是（1.5倍扩容）。
关于 详细代码和原理可以参考 3。参考1和2也是不错的！
参考 1： ArrayList扩容1.5倍
