详解Java List的扩容机制原理及应用

public class ArrayList<E> implements List<E> {
private static final int DEFAULT_CAPACITY = 10;
private Object[] elementData;
private int size;
public ArrayList() {
this.elementData = new Object[DEFAULT_CAPACITY];
this.size = 0;
}
public void add(E e) {
ensureCapacity(size + 1);
elementData[size++] = e;
}
private void ensureCapacity(int minCapacity) {
if (minCapacity > elementData.length) {
int newCapacity = elementData.length + (elementData.length >> 1);
if (newCapacity < minCapacity)
newCapacity = minCapacity;
elementData = Arrays.copyOf(elementData, newCapacity);
}
}
// 其他方法省略...
}

import java.util.ArrayList;
public class ArrayListResizeDemo {
public static void main(String[] args) {
ArrayList<Integer> list = new ArrayList<>(5);
// 添加6个元素，触发扩容
for (int i = 1; i <= 6; i++) {
list.add(i);
}
System.out.println("List size: " + list.size()); // 输出：6
System.out.println("List capacity: " + getArrayListCapacity(list)); // 输出：10
}
// 获取ArrayList的容量
private static int getArrayListCapacity(ArrayList<?> list) {
try {
java.lang.reflect.Field capacityField = ArrayList.class.getDeclaredField("elementData");
capacityField.setAccessible(true);
return ((Object[]) capacityField.get(list)).length;
} catch (NoSuchFieldException | IllegalAccessException e) {
e.printStackTrace();
return -1;
}
}
}