The default keyword in Java serves various purposes across different contexts, offering flexibility and enhancing code readability. Here, we explore three key scenarios where the default keyword is employed:

1. Specify Default Values in a Switch Case Statement:

Consider the following method, where the default keyword is utilized within a switch case statement to provide a default value for unspecified cases:

public static int getDaysOfMonth(int month) {
    switch (month) {
        case 2:
            return 28;
        case 4:
        case 6:
        case 9:
        case 11:
            return 30;
        default:
            return 31;
    }
}

This method efficiently returns the number of days in a month, defaulting to 31 for cases not explicitly handled.

2. Declare Default Values in a Java Annotation:

In the next example, the default keyword is used within a custom annotation class to declare default values for annotation attributes:

public @interface Editable {
    boolean value() default false;
    String name() default "Untitled";
}

This Editable annotation showcases default values for boolean and String attributes, providing a fallback when not explicitly specified.

3. Declare Default Methods in an Interface:

The default keyword also finds utility in interfaces, allowing the declaration of default methods. In the interface example below, a default method sleep() is added:

public interface Animal {
    void eat();
    void move();
    default void sleep() {
        // Implementation goes here...
    }
}

The primary purpose of a default method in an interface is to introduce new methods without breaking existing subtypes.

By understanding the diverse applications of the default keyword, Java developers gain a powerful tool for managing switch cases, custom annotations, and interface evolution.