能正常运行的代码突然开始报错了

这样的提示是怎么回事：

Test.java:1: error: class User is public, should be declared in a file named User.java
public class User{
      ^
User.java:1: error: class UserManager is public, should be declared in a file named UserManager.java
public class UserManager{
      ^
UserManager.java:1: error: class Test is public, should be declared in a file named Test.java
public class Test{
      ^
3 errors

代码上次是正确执行的，然后我关了浏览器再次进入后，就开始提示这个东西。