Use enumerated types for a fixed set of constants.
Enums, short for enumerated types, are a special data type in Java that enables a variable to be a set of predefined constants. Before enums were introduced, the common practice was to use integer constants (e.g., `public static final int MONDAY = 1;`), but this approach is not type-safe. You could pass any integer to a method expecting a day, leading to potential bugs. Enums solve this problem by providing type-safety. You declare an enum using the `enum` keyword. For example, `public enum Day { SUNDAY, MONDAY, TUESDAY, ... }`. Now, if a method expects a `Day`, you can only pass it one of the predefined constants like `Day.MONDAY`, and the compiler will enforce this. Under the hood, Java enums are much more powerful than simple constants; they are a special type of class that implicitly extends the `java.lang.Enum` class. This means enums can have constructors, instance variables, and methods, just like regular classes. For example, you could add a field to your `Day` enum to indicate whether it's a weekday or a weekend and a method to check this. Enums are commonly used for things like days of the week, months of the year, states in a state machine, or any scenario where you have a fixed, known set of values. They improve code readability, reduce errors, and make the code more maintainable.