java - 创建对象时"Non-static variable this cannot be referenced from a static context"

Question

我编写了下面的代码来测试 Java 中类和对象的概念。

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);
    }
}

为什么在编译过程中会出现以下错误？

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

Answer 1

使 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two types of nested classes :“静态嵌套类”和 "Inner class" 。如果没有 static 关键字，它是一个内部类，您将需要 ShowBike 的实例来访问 ShowBike.Bicycle:

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

<小时/>

静态嵌套类和普通(非嵌套)类在功能上几乎相同，只是分组方式不同。但是，当使用静态嵌套类时，不能将它们的定义放在单独的文件中，这将导致单个文件包含大量类定义。