"创建对象时,无法从静态上下文引用非静态变量"

Question

我编写了下面的代码来测试Java中的类和对象的概念.

public class ShowBike {
    private class Bicycle {
        public int gear = 0;
        public Bicycle(int v) {
            gear = v;
        }
    }

    public static void main() {
        Bicycle bike = new Bicycle(5);
        System.out.println(bike.gear);
    }
}

为什么这会在编译过程中给出以下错误？

ShowBike.java:12: non-static variable this cannot be referenced from a static context
        Bicycle bike = new Bicycle(5);
                       ^

Answer 1

制作ShowBike.Bicycle静态.

public class ShowBike {

    private static class Bicycle {
        public int gear = 0;
        public Bicycle(int v) {
            gear = v;
        }

    }

    public static void main() {
        Bicycle bike = new Bicycle(5);
        System.out.println(bike.gear);
    }
}

在Java中,有两种类型的嵌套类:"静态嵌套类"和"内部类".没有static关键字,它是一个内部类,您将需要一个ShowBike访问实例ShowBike.Bicycle:

ShowBike showBike = new ShowBike();
Bicycle bike = showBike.new Bicycle(5);

静态嵌套类和普通(非嵌套)类在功能上几乎相同,它只是不同的分组方式.但是,在使用静态嵌套类时,不能将它们的定义放在单独的文件中,这将导致包含大量类定义的单个文件.

Answer 2

Bicycle是一个内部类,因此您需要外部类实例来创建内部类实例,如:

ShowBike sBike = new ShowBike();
Bicycle bike = sBike.new Bicycle(5);

或者您可以简单地声明Bicycle 类static以使您当前的方式工作.