addAll(Collection<? extends E> c)的方法源码

	public class ArrayList<E> extends AbstractList<E>
			implements List<E>, RandomAccess, Cloneable, java.io.Serializable{

			// 长度为0的空数组
			private static final Object[] EMPTY_ELEMENTDATA = {};

			// 默认容量为空的数组
			private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

			// 存储元素的数组
			transient Object[] elementData;

			// 数组的长度
			private int size;

			// 数组的默认容量为10
			private static final int DEFAULT_CAPACITY = 10;

			public boolean addAll(Collection<? extends E> c) {
				Object[] a = c.toArray();  // 将有数据的集合转换为数组

				int numNew = a.length; // 将有数据的集合长度赋给numNew

				ensureCapacityInternal(size + numNew);  // 校验并对list2进行扩容默认容量为10

				System.arraycopy(a, 0, elementData, size, numNew);  // 将旧数组a的元素拷贝到elementData新数组中

				size += numNew;
				return numNew != 0;
			}
	}

	public class Main {
		public static void main(String[] args) {
			ArrayList<String> list1 = new ArrayList<>();
			list1.add("1");
			list1.add("2");
			list1.add("3");

			ArrayList<String> list2 = new ArrayList<>();
			list2.addAll(list1);

			System.out.println(list2);
		}
	}