包Java文件命名限制

Question

实际上，我正在浏览Java语言规范，发现了一个奇怪的句子，我无法从中得出结论。因为我不是以英语为母语的人

If and only if packages are stored in a file system (§7.2), the host system may choose to enforce the restriction that it is a compile-time error if a type is not found in a file under a name composed of the type name plus an extension (such as .java or .jav) if either of the following is true:
• The type is referred to by code in other compilation units of the package in which the type is declared.
• The type is declared public (and therefore is potentially accessible from code in other packages).

有人能举例说明用粗体标出的那一行吗。提前谢谢你

Answer 1

在包com.example中有类CustomObject

package com.example;
public class CustomObject {
    String firstName;
    String lastName;
}

这个类可以从同一个包中的另一个类访问，注意，因为CustomObject是公共类，所以它可以从项目中的任何类访问。如果它被声明为class CustomObject，那么它只能在包com.example中访问

package com.example;

public class Test {
    
    public CustomObject myObject; 
    
    public static void main(String[] args) {
        myObject = new CustomObject();
        
    }

}

Answer 2

在Java中，编译单元本质上是一个文件

如果声明的类型（例如类）在另一个文件中使用，而不是在中声明（通常是这种情况），那么如果找不到以类型名+扩展名命名的文件，则是编译错误

例如：当导入类或其他类型时，Java必须能够将其包+名称转换为必须存在的文件名