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newListnewList.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);