java - 想要静态但不能支持？

Question

我正在尝试创建一个也显示信息的随机汽车发电机。我以为我拥有一切，直到 randomCar部分。它说

'com.company.Main.this' cannot be referenced from a static context

下 return switch 中的声明.有没有想过我可能哪里出错了？

 package com.company;
 
public class Main {
 
    class Car{
        private String name;
        private boolean engine;
        private int cylinders;
        private int wheels;
 
        public Car(String name){
            this.name = name;
        }
 
        public String getName(){
            return name;
        }
 
 
        public int getCylinders() {
            if(cylinders == 0){
                System.out.println("Unknown amount of cylinders");
            }
            return cylinders;
        }
 
        public int getWheels() {
            return wheels;
        }
 
        public boolean isEngine() {
            return engine;
        }
    }
 
    class Tacoma extends Car{
 
        public Tacoma(String name) {
            super("Tacoma");
        }
 
 
        public boolean isEngine(boolean engine) {
            return true;
        }
 
 
        public int getCylinders(int cylinders) {
            return 6;
        }
 
 
        public int getWheels(int wheels) {
            return 4;
        }
    }
 
    class Motorcycle extends Car{
 
        public Motorcycle(String name) {
            super("Harley Davidson");
        }
 
 
        public boolean isEngine(boolean engine) {
            return true;
        }
 
 
        public int getCylinders(int cylinders) {
            return 2;
        }
 
 
        public int getWheels(int wheels) {
            return 2;
        }
    }
 
    class Volvo extends Car{
        public Volvo(String name) {
            super("Volvo");
        }
 
 
        public boolean isEngine(boolean engine) {
            return true;
        }
 
        public int getCylinders(int cylinders) {
            return 4;
        }
 
 
        public int getWheels(int wheels) {
            return 4;
        }
    }
 
 
 
    public static void main(String[] args) {
   for (int i = 1; i<6; i++){
       Car car = randomCar();
        System.out.println("Car # " + i + ":" + car.getName() + "\n" +
                "Number of cylinders: " + car.getCylinders() + "\n" +
                "Number of wheels: " + car.getWheels()+ "\n" +
                "Engine is: " + car.isEngine());
       }
    }
 
    private static Car randomCar() {
        int randomNumber = (int) (Math.random()*5) +1;
        System.out.println("Random number generated is: " + randomNumber);
        switch (randomNumber){
            case 1:
                return new Tacoma(); // This is where I am getting an error
            case 2:
                return new Motorcycle(); // This is where I am getting an error
            case 3:
                return new Volvo(); // This is where I am getting an error
        }
        return null;
    }
}

Answer 1

我将从这里开始阅读:https://docs.oracle.com/javase/tutorial/java/javaOO/nested.html -> 实际上所有的章节都对你有用:https://docs.oracle.com/javase/tutorial/java/javaOO/index.html
严格来说，要解决“无法从静态上下文中引用”的问题，您可以将类设为静态(汽车、塔科马、摩托车、沃尔沃)static class Car{从我的角度来看，您不需要嵌套类，只需在与 Main 类相同的包中创建类，就可以了(随意创建更多包以更好地构建类)
此外，我假设您的代码正在进行中，因为它存在多个问题:

像这样的方法没有意义public boolean isEngine(boolean engine) {return true;}您收到一个您忽略的参数并返回一个常量值:true;我假设您在这里想要做的是拥有不同类型的汽车，每种汽车都有自己的预定义特征，但为此您应该为父级 Car 中的属性设置值。为此，您可以定义 protected setter，使字段 protected ，或者最好创建采用所有值

的构造函数。

        public Car(String name, boolean engine, int cylinders, int wheels) {
            this.name = name;
            this.engine = engine;
            this.cylinders = cylinders;
            this.wheels = wheels;
        }

你可以在塔科马

        public Tacoma(String name) {
            super(name, true, 6, 4);
        }

运行你的代码我得到了 randomNumber 5 以便返回 null 并得到一个 NPE，我假设工作正在进行

在您的开关中，您正在调用默认构造函数 new Tacoma()但是，由于您定义了带参数的构造函数，因此不再可用，使用可用的构造函数或创建无参数构造函数。

关于 OOP 原则还有其他问题，所以我建议再次阅读它们，只需谷歌“java OOP 原则”然后“SOLID”......那里有很多很棒的资源，你只需要时间和耐心，你就会得到那里!