Javaの「Enum」タイプは何ですか？

Java中的枚举（enum）是一种特殊的类，用于表示固定数量的常量值。1. 使用enum关键字定义；2. 每个枚举值都是该枚举类型的公共静态最终实例；3. 可以包含字段、构造函数和方法，为每个常量添加行为；4. 可在switch语句中使用，支持直接比较，并提供name()、ordinal()、values()和valueOf()等内置方法；5. 枚举可提升代码的类型安全性、可读性和灵活性，适用于状态码、颜色或星期等有限集合场景。

Enums in Java are a special type of class that represent a fixed number of constant values. They're commonly used when you have a variable that should only hold one of a small set of possible values—like days of the week, colors, or status codes.

How to Define an Enum

You define an enum using the enum keyword. Here's a basic example:

public enum Day {
    MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
}

Each value (like MONDAY, TUESDAY, etc.) is an instance of the Day enum. Behind the scenes, Java creates a class that extends java.lang.Enum, so enums come with some built-in methods.

Some key points:

• Enums can be declared outside or inside a class, but not inside a method.
• Each enum constant is implicitly public static final.

Common Uses of Enums

Enums are more than just named constants. You can add fields, constructors, and methods to them.

For example, let’s say you want each day to have a "type":

public enum Day {
    MONDAY("Weekday"),
    TUESDAY("Weekday"),
    WEDNESDAY("Weekday"),
    THURSDAY("Weekday"),
    FRIDAY("Weekday"),
    SATURDAY("Weekend"),
    SUNDAY("Weekend");

    private String type;

    Day(String type) {
        this.type = type;
    }

    public String getType() {
        return type;
    }
}

Now, each day has additional behavior. This makes enums very useful for grouping related constants with shared logic.

Working with Enums in Code

You can use enums in switch statements, compare them directly, and get their names or ordinal positions.

Here’s how you might use it:

Day today = Day.MONDAY;

System.out.println(today); // Output: MONDAY
System.out.println(today.getType()); // Output: Weekday

if (today.getType().equals("Weekend")) {
    System.out.println("Time to relax!");
} else {
    System.out.println("Time to work.");
}

Useful methods you'll often see:

• name() – returns the name of the enum constant.
• ordinal() – returns the position in the enum declaration (starts at 0).
• values() – returns an array of all enum constants.
• valueOf(String) – returns the enum constant matching the given name.

Enums give you a clean way to model fixed sets of related data. They’re type-safe, readable, and flexible enough to handle many real-world scenarios without needing extra classes or constants.基本上就这些。

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