Question

我有这个枚举单例类：

enum C1 {
    INSTANCE("");

    C1(String s) {
        System.out.println("with param = " +s);
    }
    C1() {
        System.out.println("without param");
    }   
    public void g() {
        System.out.println("inside g");
    }
}

public class Main {
    public static void main(String s[]) {
        C1.INSTANCE.g();
        C1.INSTANCE.g();

    }
}

如何通过传递自定义参数使用C1(String s)来调用INSTANCE构造函数？

Answer 1

你可以像那样吃东西

    enum C1 {
        WITH_PARAM("value"),
        EMPTY();

        private String value;
        C1(String s) {
            System.out.println("with param = " +s);
            value=s;
        }
        C1() {
            System.out.println("without param");
        }
        public void g() {
            System.out.println("inside g, value is "+value);
        }
    }

        public static void main(String s[]) {
            C1.EMPTY.g();
            C1.WITH_PARAM.g();

        }

Answer 2

通常，您可以使用类类型，而不是枚举类型。枚举旨在为静态的和最终的。枚举构造函数的想法是封装多个值，以便您可以在以后只设置一种枚举类型就可以使用它们。

public enum Vehicle {
    CAR(4, 2),
    TRUCK(6, 20);

    private Integer numOfTyres;
    private Integer maxWeight;

    Vehicle(Integer numOfTyres, Integer maxWeight) {
        this.numOfTyres = numOfTyres;
        this.maxWeight = maxWeight;
        System.out.println("Number of Tyres = " + numOfTyres);
        System.out.println("Max weight = " + maxWeight);
    }


    public Integer getMaxWeight() {
         return maxWeight;
    }

    public Integer getNumberOfTyres() {
        return numOfTyres;
    }

}


public class Main {
    public static void main(String s[]) {
        Vehicle.CAR.getNumberOfTyres();

        for (Vehicle v : Vehicle.values()) {
            System.out.println();

            StringBuilder sb = new StringBuilder();
            sb.append("Vehicle ");
            sb.append(v.name());
            sb.append(" has ");
            sb.append(v.getNumberOfTyres());
            sb.append(" number of tyres and ");
            sb.append(v.getMaxWeight());
            sb.append(" max weight ");


            System.out.println("-----------------");
            System.out.println(sb.toString());
            System.out.println("-----------------");
        }

    }

}

Answer 3

您可能知道-枚举类型的构造函数必须是程序包私有或私有访问。

它会自动调用并创建在枚举主体开头定义的常量。

您不能自己调用枚举构造函数。

以下是一些可能有用的示例。

public enum MySingleton {
    // if you have multiple constants here - it's not Singleton
    INSTANCE;

    // if you need to provide some info to your INSTANCE,
    // you can add params to its methods
    public void someMethod(String value) {
        System.out.println("inside someMethod, value is " +value);
    }
}

如果您需要实例具有某种状态，则可以添加字段和构造函数：

public enum MySingleton {
    INSTANCE("Hello");

    private String message;

    MySingleton(String msg) {
        this.message = msg;
    }

    // you can even have getter/setter for it
    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }
}

主要方法示例：

public static void main(String[] args) {
    System.out.println(MySingleton.INSTANCE.getMessage()); // prints Hello

    MySingleton.INSTANCE.setMessage("Bye");

    System.out.println(MySingleton .INSTANCE.getMessage()); // prints Bye
}

在枚举单例中调用参数化构造函数？

3 个答案: