Java 简明教程

Java - Enum Constructor

Java enum是 class 的一种特殊类型，它表示一组预定义的常量值，可用于 switch expressions 进行比较，作为应用程序代码中的常量使用。

Java enum is a special kind of class that represents a group of pre-defined constant values and can be used in switch expressions for comparison, to be used as constants in application code.

默认情况下，一个 enum 不需要任何构造函数，其默认值与其声明相同。考虑以下示例：

By default, an enum does not require any constructor and its default value is the same as its declaration. Consider the following example:

enum WEEKDAY { MONDAY, TUESDAY, WEDNESDAY, THRUSDAY, FRIDAY, SATURDAY, SUNDAY }

如果我们打印任何上述枚举的值，它将按照声明打印相同的字符串。

If we print the any of the value of above enum, it will print the same string as declared.

System.out.println(WEEKDAY.MONDAY);

它会按如下所示打印结果：

It will print the result as shown below:

MONDAY

Use of Enum Constructor

在我们需要为枚举分配默认值的情况下，我们可以创建下述枚举结构器：

Now in case, we want to assign a default value to enum, we can create a enum constructor as shown below:

enum WEEKDAY {
	MONDAY("Day 1");
	private final String description;

	WEEKDAY(String description) {
		this.description = description;
	}
}

在这种情况下，我们使用结构器为枚举分配一个默认描述。

In this case, we’re assigning a default description to the enum using the constructor.

Scope of Enum Constructor

由于枚举是一个特殊的类，因此枚举结构器只能具有 private 或 package-private 修饰符。设置 public 标识符会导致枚举结构器出现编译时间错误。

As enum is a special class, an enum constructor can only have private or package-private modifier. Setting a public idenifier to enum constructor will attract a compile time error.

enum WEEKDAY {
	MONDAY("Day 1");
	private final String description;

	// compiler error: illegal modifier, only private is permitted.
	public WEEKDAY(String description) {
		this.description = description;
	}
}

以下是枚举结构器的有效声明

Following are valid declarations of enum constructor

Enum with Private Constructor

我们可以按照下述方式为枚举创建 private 结构器。

We can create a private constructor for an enum as shown below.

enum WEEKDAY {
	MONDAY("Day 1");
	private final String description;

	// valid declaration
	private WEEKDAY(String description) {
		this.description = description;
	}
}

Example

在这个例子中，我们使用 private 结构器创建了一个枚举 WEEKDAY。使用 private 结构器，我们使用其结构器设置枚举描述的值。

In this example, we’ve created an enum WEEKDAY with a private constructor. Using private constructor, we’re setting the value of enum description using its constructor.

package com.tutorialspoint;

// create the enum
enum WEEKDAY {

	// create values of enum
	MONDAY("Day 1"),
	TUESDAY("Day 2"),
	WEDNESDAY("Day 3"),
	THRUSDAY("Day 4"),
	FRIDAY("Day 5"),
	SATURDAY("Day 6"),
	SUNDAY("Day 7");

	private final String description;
	// private construtor to set default value
	private WEEKDAY(String description) {
		this.description = description;
	}
	// getter method to get the description
	public String getDescription () {
		return this.description;
	}
}
public class Tester {
   public static void main(String[] args) {
	   System.out.println(WEEKDAY.MONDAY.getDescription());
	   System.out.println(WEEKDAY.MONDAY);
   }
}

让我们编译并运行上述程序，这将生成以下结果 −

Let us compile and run the above program, this will produce the following result −

Day 1
MONDAY

Enum with Package-private Constructor

我们可以按照下述方式为枚举创建 package-private 结构器。

We can create a package private constructor for an enum as shown below.

enum WEEKDAY {
	MONDAY("Day 1");
	private final String description;

	// valid declaration
    WEEKDAY(String description) {
		this.description = description;
	}
}

Example

在这个例子中，我们创建了一个枚举 WEEKDAY，因为它未传递任何修饰符，因此它是一个 package-private 结构器。使用这个结构器，我们使用其结构器设置枚举描述的值。

In this example, we’ve created an enum WEEKDAY with a package-private constructor as no modifier is passed. Using this constructor, we’re setting the value of enum description using its constructor.

package com.tutorialspoint;

// create the enum
enum WEEKDAY {

	// create values of enum
	MONDAY("Day 1"),
	TUESDAY("Day 2"),
	WEDNESDAY("Day 3"),
	THRUSDAY("Day 4"),
	FRIDAY("Day 5"),
	SATURDAY("Day 6"),
	SUNDAY("Day 7");

	private final String description;
	// private construtor to set default value
	WEEKDAY(String description) {
		this.description = description;
	}
	// getter method to get the description
	public String getDescription () {
		return this.description;
	}
}
public class Tester {
   public static void main(String[] args) {
	   System.out.println(WEEKDAY.MONDAY.getDescription());
	   System.out.println(WEEKDAY.MONDAY);
   }
}

让我们编译并运行上述程序，这将生成以下结果 −

Let us compile and run the above program, this will produce the following result −

Day 1
MONDAY