例子

addAll()

@Testpublic void CopyListTest(){Student student = Student.builder().id(1).name("张三").age(23).classId(1).build();Student student2 = Student.builder().id(2).name("李四").age(22).classId(1).build();List students = new ArrayList();students.add(student);students.add(student2);System.out.println("旧的list");for (Student stu : students){System.out.println(stu);}List list2 = new ArrayList();list2.addAll(students);list2.stream().forEach(t->t.setClassId(2));System.out.println("新的list");for (Student stu : list2){System.out.println("班级id"+stu.getClassId());}System.out.println("旧的list");for (Student stu : students){System.out.println("班级id"+stu.getClassId());}}

旧的list的值被改变

直接用newList = oldList是引用传递,复制的是地址
改变oldList里的内容同样也会影响到newList

newList.addAll(oldList)是值传递

参考:http://t.csdnimg.cn/EGcYC

但 用allAll() 原来的值也被改变了

解决

使用深度复制

public static  List deepCopy(List sourceList) throws IOException, ClassNotFoundException{ByteArrayOutputStream bo= new ByteArrayOutputStream();ObjectOutputStream oos= new ObjectOutputStream(bo);oos.writeObject(sourceList);ByteArrayInputStream bi= new ByteArrayInputStream(bo.toByteArray());ObjectInputStream ois=new ObjectInputStream(bi);@SuppressWarnings("unchecked")List dest = (List)ois.readObject();return dest;}

所复制的元素如果是对象,需要实现序列化。

参考:http://t.csdnimg.cn/EGcYC

如果所复制的元素是字符

List newList=new ArrayList(oldList);