A switch expression can be used to return a value or just execute a set of statements, like the traditional switch statement.

When you are using a switch expression to return a value that is used to assign a value to a variable, its cases must be exhaustive. This essentially means that, whatever value you pass to the switch argument, it must be able to find an appropriate branch to execute. A switch expression can accept arguments of the byte, short, int, Byte, Short, Integer, or String types or enums. Of these, only an enum has exhaustive values.

In the following example, a switch expression is being used to assign a value to the damage variable. Since there is no matching branch to execute for the PLATE value, this code won't compile:

class Planet { 
    private static long damage; 
    public void use(SingleUsePlastic plastic) { 
        damage += switch(plastic) { 
            case SPOON, FORK, KNIFE -> 7; 
        }; 
    } 
} 

To compile the preceding code, you can either add the switch branch with the case label, PLATE, or add a default branch, as follows:

class Planet { 
    private static long damage; 
    public void use(SingleUsePlastic plastic) { 
        damage += switch(plastic) { 
            case SPOON, FORK, KNIFE -> 7; 
            // Adding (1) or (2), or both will enable the code to 
            // compile
            case PLATE -> 10;     // line number (1)
            default -> 100;       // line number (2)
        }; 
    } 
}

For switch arguments such as primitive types, wrapper classes, or String classes, which don't have an exhaustive list of values, you must define a default case label (if you are returning a value from switch expressions), as follows:

String getBook(String name) {
    String bookName = switch(name) { 
        case "Shreya" -> "Harry Potter";
        case "Paul" -> "Management tips";
        case "Harry" -> "Life of Pi";
        default -> "Design Patters - everyone needs this";   
    };
    return bookName;
} 

Let's work with the second situation where you are not using switch expressions to return a value. The following code modifies the getBook() method from the preceding code. Though switch expressions use the new syntax (using -> to define the code to execute), it is not returning a value. In such a case, the cases of a switch expression need not be exhaustive:

 String getBook(String name) {
    String bookName = null;
    switch(name) {                                    // NOT returning 
                                                     // a value
        case "Shreya" -> bookName = "Harry Potter";
        case "Paul" -> bookName = "Management tips";
        case "Harry" -> bookName = "Life of Pi";      // default case 
                                                    // not included
    }
    return bookName;
}