java - 访问继承的类成员时出错

Question

我是java初学者。我想使用基类和派生类的对象访问基类中扩展(继承类的数据成员)。但我面临以下错误: token “bonus”上存在语法错误，当尝试使用基类和派生类的对象访问成员奖金时，在此 token 之后预期存在 VariableDeclaratorId

如果我将对象创建和访问的代码放在 public static void main 方法中，这个错误就会消失。我想了解一下这是为什么？

public class DerivedClass{
    public int bonus = 100;
    public static void main(String[] args) {
    // TODO Auto-generated method stub

    }

}
class BaseClass extends DerivedClass 
{
    BaseClass b = new BaseClass();
    DerivedClass d = new DerivedClass();
    b.bonus = 200; //error here
    d.bonus = 400; //error here
    System.out.println("bonus this year is:"+" "+ s.bonus);
}

错误消息:

Syntax error on token "bonus", VariableDeclaratorId expected after this token

Answer 1

将其包装在一个方法中，您将看到两者都有效。不允许您直接在类主体中编写代码块。

class BaseClass extends DerivedClass {    
    public static void main(String[] args) {
        BaseClass b = new BaseClass();
        DerivedClass d = new DerivedClass();
        b.bonus = 200;
        d.bonus = 400;
    }
}

有initialisers ，但它们也必须在 {} 内声明。

{
    BaseClass b = new BaseClass();
    DerivedClass d = new DerivedClass();
    b.bonus = 200;
    d.bonus = 400;
}

类体具有定义良好的结构(字段声明、方法声明、 block 声明)。您只需遵循该结构即可。

8.1.6. Class Body and Member Declarations

A class body may contain declarations of members of the class, that is, fields (§8.3), methods (§8.4), classes (§8.5), and interfaces (§8.5).

A class body may also contain instance initializers (§8.6), static initializers (§8.7), and declarations of constructors (§8.8) for the class.

ClassBody:
  { {ClassBodyDeclaration} }

ClassBodyDeclaration:
  ClassMemberDeclaration 
  InstanceInitializer 
  StaticInitializer 
  ConstructorDeclaration

ClassMemberDeclaration:
  FieldDeclaration 
  MethodDeclaration 
  ClassDeclaration 
  InterfaceDeclaration 
  ;