实例化Student/Teacher对象时触发non-static variable this静态上下文引用错误

错误根源 #

你代码里的Student和Teacher是非静态内部类，这类内部类的对象必须依附于外部类Person的实例才能创建。但main是静态方法，属于类本身，没有关联的外部类实例，所以直接new Student(...)会报错。

另外，非静态内部类的构造方法里直接给外部类的personName赋值，这种写法依赖隐含的Person.this外部类实例引用，在静态方法里这个引用不存在，也会导致问题。

至于移除main的static后错误消失，是因为非静态方法属于对象，此时方法内部隐含了当前Person实例的引用，能创建非静态内部类对象，但JVM要求程序入口main必须是静态方法，所以此时找不到运行入口。

解决方案 #

方案1：将内部类改为静态内部类

给Student和Teacher加上static修饰，成为静态内部类，这样它们不依赖外部类实例，静态方法里可直接创建对象。同时静态内部类无法访问外部类非静态成员，需要自己维护属性：

public class Person
{
   String personName = "";
   String personAddress = "";

   public Person(String name, String address)
   {
      personName = name;
      personAddress = address;
   }

   // 改为静态内部类
   static class Student 
   {
      String studentName;
      String studentAddress;
      int numCourses = 0;
      String[] courses = new String[10];
      int[] grades = new int[10];
      
      Student(String name, String address)
      {
         this.studentName = name;
         this.studentAddress = address;
      }
      
   }
   
   // 改为静态内部类
   static class Teacher 
   {
      String teacherName;
      String teacherAddress;
      int numCourses = 0;
      String[] courses = new String[20];
      
      public Teacher(String name, String address)
      {
         this.teacherName = name;
         this.teacherAddress = address;
      }
      
   }
   
   public static void main(String[] args)
   {
      Student student1 = new Student("Aaron", "1234 this road S");
      Teacher teacher1 = new Teacher("Fatima", "1111 some road N");
   }
}

方案2：改用继承结构（更符合业务逻辑）

从设计角度看，Student和Teacher应该是Person的子类，用继承替代内部类更合理：

public class Person
{
   String personName;
   String personAddress;

   public Person(String name, String address)
   {
      this.personName = name;
      this.personAddress = address;
   }

   public static void main(String[] args)
   {
      Student student1 = new Student("Aaron", "1234 this road S");
      Teacher teacher1 = new Teacher("Fatima", "1111 some road N");
   }
}

class Student extends Person
{
   int numCourses = 0;
   String[] courses = new String[10];
   int[] grades = new int[10];
   
   public Student(String name, String address)
   {
      // 调用父类构造方法初始化姓名地址
      super(name, address);
   }
}

class Teacher extends Person
{
   int numCourses = 0;
   String[] courses = new String[20];
   
   public Teacher(String name, String address)
   {
      super(name, address);
   }
}

内容的提问来源于stack exchange，提问作者Frankenbtiz