list复制出新的list后修改元素，也更改了旧的list？

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例子

addAll()

@Test
public 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<Student> students = new ArrayList<>();
    students.add(student);
    students.add(student2);
    System.out.println("旧的list");
    for (Student stu : students){
        System.out.println(stu);
    }
    List<Student> 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

解决

使用深度复制

  public static <T> List<T> deepCopy(List<T> 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<T> dest = (List<T>)ois.readObject();
        return dest;
    }

所复制的元素如果是对象，需要实现序列化。