Java 简明教程

Java - Enum String

Java enum 是一个特殊结构，用于表示一组预定义的常量 strings，并且在用作应用程序代码中的常量时在代码中提供清晰性。默认情况下，enum string 表示法与其声明相同。考虑以下示例：

Java enum is a special construct to represents a group of pre-defined constant strings and provides clarity in code while used as constants in application code. By default, an enum string representation is the same as its declaration. Consider the following example:

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

如果我们使用 enum 直接打印上述枚举的字符串表示，使用 toString() 或使用 name() 方法，它会打印与声明相同的字符串。

If we print the string representation of the above enum using the enum directly, using toString(), or using the name() method, it will print the same string as declared.

System.out.println(WEEKDAY.MONDAY);
System.out.println(WEEKDAY.MONDAY.toString());
System.out.println(WEEKDAY.MONDAY.name());

它会按如下所示打印结果：

It will print the result as shown below:

MONDAY
MONDAY
MONDAY

Overriding Enum toString() Method

现在，如果我们希望将默认字符串表示法更改为枚举的字符串表示法，我们可以为 enum constructor 的每个值创建重写的 toString() 方法，如下所示：

Now in case, we want to change the default string representation to the enum’s string representation, we can create an overridden toString() method for each value of the enum constructor as shown below:

enum WEEKDAY {
	MONDAY{
      // overridden toString() method per value
      public String toString() {
        return "Day 1 of the Week: Monday";
      }
	};

	// or override toString() per enum
	// priority will be given to value level toString() method.
    public String toString() {
       return "Day 1 of the Week: Monday";
    }
}

在此情况下，我们重写枚举的默认 toString() 方法，以提供自定义描述。

In this case, we’re overriding a default toString() method of the enum to give a custom description.

Example: Overriding toString() Method in Java

在此示例中，我们创建了枚举 WEEKDAY。使用 toString() 方法，我们设置枚举值的自定义描述。

In this example, we’ve created an enum WEEKDAY. Using toString() method, we’re setting a custom description of enum value.

package com.tutorialspoint;

enum WEEKDAY {
   // enum value constants
   MONDAY, TUESDAY, WEDNESDAY, THRUSDAY, FRIDAY, SATURDAY, SUNDAY;

   // override the toString() method for custom description
   @Override
   public String toString() {
      return switch(this) {
         case MONDAY: yield "Day 1";
         case TUESDAY:yield "Day 2";
         case WEDNESDAY:yield "Day 3";
         case THRUSDAY:yield "Day 4";
         case FRIDAY:yield "Day 5";
         case SATURDAY:yield "DAY 6";
         case SUNDAY: yield "Day 7";
      };
   }
}
public class Tester {
   public static void main(String[] args) {
      // invoke toString() internally
      System.out.println(WEEKDAY.MONDAY);
      // invoke toString explicitly
      System.out.println(WEEKDAY.TUESDAY.toString());
      // invoke name() method to get the default name
      System.out.println(WEEKDAY.WEDNESDAY.name());
   }
}

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
Day 2
WEDNESDAY

Example: Overriding toString() Method Per Value in Java

在此示例中，我们重写了此枚举 WEEKDAY 每个值的 toString() 方法。这种方式，我们也可以自定义字符串表示。

In this example, we’ve overridden toString() methods per value of this enum WEEKDAY. This way we can customize string representation per value in this way as well.

package com.tutorialspoint;

enum WEEKDAY {

   // override the toString() method for custom description
   MONDAY{
      @Override
      public String toString() {
         return "Day 1";
      }
   },
   TUESDAY{
      @Override
      public String toString() {
         return "Day 2";
      }
   },
   WEDNESDAY{
      @Override
      public String toString() {
         return "Day 3";
      }
   },
   THRUSDAY{
      @Override
      public String toString() {
         return "Day 4";
      }
   },
   FRIDAY{
      @Override
      public String toString() {
         return "Day 5";
      }
   },
   SATURDAY{
      @Override
      public String toString() {
         return "Day 6";
      }
   },
   SUNDAY{
      @Override
      public String toString() {
         return "Day 7";
      }
   };
}
public class Tester {
   public static void main(String[] args) {
      // invoke toString() internally
      System.out.println(WEEKDAY.MONDAY);
      // invoke toString explicitly
      System.out.println(WEEKDAY.TUESDAY.toString());
      // invoke name() method to get the default name
      System.out.println(WEEKDAY.WEDNESDAY.name());
   }
}

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
Day 2
WEDNESDAY

enum name() 方法是最终的，不能被重写。当枚举的字符串表示被 toString() 方法重写时，它可用于获取枚举的默认名称。

enum name() method is final and cannot be overridden. It can be used to get the default name of the enum while string representation of enum is overridden by toString() method.