从静态上下文引用非静态变量

Question

我正在阅读Kathy和Bert SCJP 1.6并遇到以下代码:

class Bar {
int barNum = 28;
}

class Foo {
  Bar myBar = new Bar();
  void changeIt(Bar myBar) {
    myBar.barNum = 99;
    System.out.println("myBar.barNum in changeIt is " + myBar.barNum);
    myBar = new Bar();
    myBar.barNum = 420;
    System.out.println("myBar.barNum in changeIt is now " + myBar.barNum);
 }
  public static void main (String [] args) {
    Foo f = new Foo();
    System.out.println("f.myBar.barNum is " + f.myBar.barNum);
    f.changeIt(f.myBar);
    System.out.println("f.myBar.barNum after changeIt is "
    + f.myBar.barNum);
 }
}

虽然它是阴影变量的主题,但我无法理解如何在main()方法(静态)中引用非静态变量myBar？

Answer 1

诀窍是myBar访问的上下文不是静态的.

通过编写f.myBar而不仅仅是简单地myBar,您可以在存储在f(局部变量)中的实例的上下文中访问它.

这是启动应用程序的常用模式的基础.

public class Main {
   private final Foo param1;
   private final Bar param2;       


   private Main( Foo initParam1, Bar initParam2 ) {
      //initialise the fields 
   }

   private void run() {
      // execute the application
   }

   public static void main( String [] args ) {
      // parse the command line arguments
      Foo parsedParam1 = ...
      Bar parsedParam2 = ...
      Main main = new Main( parsedParam1, parsedParam2 );
      main.run();
   }
}