在枚举单例中调用参数化构造函数？

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调用C1(String s)构造函数？INSTANCE

Answer 1

你可以有这样的东西    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

通常你可以使用类类型，而不是枚举类型。枚举旨在是静态的和最终的。枚举构造函数的想法是封装几个值，以便您可以通过仅设置一种类型的枚举来使用它们。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包私有或私有访问。它被自动调用并创建在枚举体开头定义的常量。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，您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}