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);
	}
}