Enum can be considered to be syntax sugar for a sealed class that is instantiated only a number of times known at compile-time to define a set of constants.

A simple enum to list the different seasons would be declared as follows:

public enum Season {
    WINTER,
    SPRING,
    SUMMER,
    FALL
}

While the enum constants don’t necessarily need to be in all-caps, it is Java convention that names of constants are entirely uppercase, with words separated by underscores.

You can declare an Enum in its own file:

/**
 * This enum is declared in the Season.java file.
*/
public enum Season {
    WINTER,
    SPRING,
    SUMMER,
    FALL
}

But you can also declare it inside another class:

public class Day {

   private Season season;

   public String getSeason() {
       return season.name();
   }

   public void setSeason(String season) {
       this.season = Season.valueOf(season);
   }

   /**
    * This enum is declared inside the Day.java file and 
    * cannot be accessed outside because it's declared as private.
    */
   private enum Season {
       WINTER,
       SPRING,
       SUMMER,
       FALL
   }

}

Finally, you cannot declare an Enum inside a method body or constructor:

public class Day {

    /**
     * Constructor
    */
    public Day() {
        // Illegal. Compilation error
        enum Season {
            WINTER,
            SPRING,
            SUMMER,
            FALL
        }
    }

    public void aSimpleMethod() {
        // Legal. You can declare a primitive (or an Object) inside a method. Compile!
        int primitiveInt = 42;

        // Illegal. Compilation error.
        enum Season {
            WINTER,
            SPRING,
            SUMMER,
            FALL
        }

        Season season = Season.SPRING;
    }
    
}

Duplicate enum constants are not allowed:

public enum Season {
    WINTER,
    WINTER, //Compile Time Error : Duplicate Constants
    SPRING,
    SUMMER,
    FALL
}

Every constant of enum is public, static and final by default. As every constant is static, they can be accessed directly using the enum name.

Enum constants can be passed around as method parameters:

public static void display(Season s) {
    System.out.println(s.name());  // name() is a built-in method that gets the exact name of the enum constant
}

display(Season.WINTER);  // Prints out "WINTER"

You can get an array of the enum constants using the values() method. The values are guaranteed to be in declaration order in the returned array:

Season[] seasons = Season.values();