Chapter THREE
Working with Records and Enums

Answers

1. The correct answer is C.

Explanation:

• A) The Employee record explicitly defines a public constructor that initializes its fields.
  • This option is incorrect because the record Employee does not explicitly define a public constructor. Records automatically generate a public constructor with the same parameters as the record’s declaration.
• B) The fields name and age can be reassigned to new values after an Employee object is created.
  • This option is incorrect as the fields within a record are final, which means they cannot be reassigned to new values after an Employee object has been created. This immutability is one of the key characteristics of records.
• C) The Employee record implicitly creates a public constructor and private final fields for name and age.
  • This is the correct option. Records implicitly create a public constructor for the record’s fields and also make these fields private and final. This means you don’t have to manually write boilerplate code for constructor, getters, or to ensure immutability.
• D) It is mandatory to define getters for the fields name and age in the Employee record.
  • This option is incorrect because records automatically generate public methods to access the fields, known as accessor methods, which essentially act as getters. Therefore, it is not mandatory (or even possible) to define separate getters for the fields.

2. The correct answer is B.

Explanation:

• A) The balance field can be modified using a public setter method within the Account record.
  • This option is incorrect because records in Java do not support public setter methods for their fields. The fields of a record are final and cannot be modified after the object’s construction, which is a key aspect of their design to enforce immutability.
• B) Once an Account object is created, its id and balance cannot be changed.
  • This is the correct option. Records are immutable by design, meaning that once a record object is created, the values of its fields (id and balance in this case) cannot be changed. This immutability is ensured by making the fields private and final, and by not providing setter methods.
• C) Immutability of records can be bypassed by you define custom setter methods for the id and balance fields.
  • This option is incorrect. Custom setter methods cannot be defined for the record fields because records do not allow defining mutators for their components.
• D) Records allow field values to be modified if accessed directly, without using setter methods.
  • This option is incorrect because the fields in a record are implicitly final and private, which means they cannot be modified directly or through setter methods. The design of records enforces this immutability to ensure that instances of records act as true carriers of immutable data.

3. The correct answer is D.

Explanation:

• A) Product p = new Product();
  • This option is incorrect because the default constructor without parameters does not exist for records in Java. Records require all their fields to be specified at the time of instantiation.
• B) Product p = Product(101, "Coffee", 15.99);
  • This option is incorrect because the syntax used here is not valid for creating a new instance of a record in Java. The correct syntax for instantiating a record involves using the new keyword followed by the record name and the parameters in parentheses.
• C) Product p = {101, "Coffee", 15.99};
  • This option is incorrect as it mistakenly uses the syntax for array initialization. In Java, objects, including records, cannot be instantiated using curly braces without the new keyword and proper constructor.
• D) Product p = new Product(101, "Coffee", 15.99);
  • This is the correct option. Records in Java are instantiated using the new keyword followed by the record’s constructor, which requires passing all the fields defined in the record. This syntax correctly creates a new Product record with the given id, name, and price.

4. The correct answer is B.

Explanation:

• A) Records cannot implement interfaces because they are final and immutable by design, which prevents any form of behavior customization.
  • This option is incorrect. Records in Java can implement interfaces. The finality and immutability of records do not preclude them from implementing interfaces, which can be used to add behaviors or contractual obligations to a record.
• B) This record correctly implements the Comparable interface, allowing Item objects to be sorted based on their price.
  • This is the correct option. The provided record definition correctly implements the Comparable<Item> interface by overriding the compareTo method. This customization allows instances of the Item record to be sorted based on the price field, demonstrating that records can indeed implement interfaces and override their methods as needed.
• C) Implementing interfaces in records is restricted only to functional interfaces due to their immutable nature.
  • This option is incorrect. There is no such restriction that limits records to implementing only functional interfaces. Records can implement any interface, including those with multiple abstract methods, as long as the record provides implementations for the abstract methods defined in the interface.
• D) The compareTo method cannot be overridden in records because method overriding is not supported in record types.
  • This option is incorrect. Records can override methods from the interfaces they implement, including the compareTo method from the Comparable interface in this example. Method overriding is a key aspect of implementing interfaces and is fully supported by record types in Java.

5. The correct answers are A and D.

Explanation:

• A)

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

  • This option is correct. It demonstrates a valid declaration of an enum in Java. Enums are used to define a set of named constants, and this syntax is the standard way to declare them. The public access modifier makes this enum accessible from any other class.
• B)

  enum Month {
    private JANUARY, FEBRUARY, MARCH, APRIL, MAY, JUNE, JULY, AUGUST, SEPTEMBER, OCTOBER, NOVEMBER, DECEMBER;
  }
  

  • This option is incorrect. Enums cannot have private access modifiers for their constants. Enum constants are implicitly public, static, and final and should be declared without access modifiers.
• C)

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

  • This option is incorrect because enums cannot be declared with protected or private access levels. Enums are implicitly public if they are defined outside of a class. If defined within a class, they can have any access level, but the protected keyword cannot be used at the enum level itself.
• D)

  enum Status {
    ACTIVE, INACTIVE, DELETED;
  
    public void printStatus() {
        System.out.println("Current status: " + this);
    }
  }
  

  • This option is correct. It shows an enum Status with a method printStatus(). Enums in Java can contain methods, fields, constructors, and implement interfaces. This demonstrates the ability of enums to have methods, making this declaration valid.

6. The correct answer is A.

Explanation:

• A) 1
  • This option is correct. The ordinal() method returns the ordinal of this enumeration constant (its position in its enum declaration, where the initial constant is assigned an ordinal of zero). Since GREEN is the second enum constant declared in the Color enum, its ordinal value is 1.
• B) 2
  • This option is incorrect. The ordinal value of BLUE would be 2, not GREEN, because BLUE is the third declared constant in the Color enum.
• C) 0
  • This option is incorrect. The ordinal value of RED is 0, as it is the first declared constant in the Color enum.
• D) Color.GREEN
  • This option is incorrect. The ordinal() method returns an integer representing the position of the enum constant in the declaration, not the enum constant itself.

7. The correct answer is D.

Explanation:

• A)

  public enum Size {
    SMALL, MEDIUM, LARGE;
    public static void printSize() {
        System.out.println("The size is " + this.name());
    }
  }
  

  • This option is incorrect because the method printSize() is defined as static, which means it cannot access the this reference. Static methods in enums can’t directly access the enum constants without specifying the constant explicitly or being passed a reference.
• B)

  enum Flavor {
    CHOCOLATE, VANILLA, STRAWBERRY;
    void printFlavor() {
        System.out.println("Flavor: " + Flavor.name);
    }
  }
  

  • This option is incorrect because the name property of an enum constant is private. You can only access it using the this reference and the name() method (this.name()).
• C)

  protected enum Direction {
    NORTH, SOUTH, EAST, WEST;
    private printDirection() {
        System.out.println("Going " + this.toString());
    }
  }
  

  • This option is incorrect for two reasons. First, protected is not a valid access modifier for a top-level enum, top-level enums can only be public or package-private (no modifier). Second, printDirection() method is missing a return type (e.g., void).
• D)

  public enum Season {
    WINTER, SPRING, SUMMER, FALL;
    public void printSeason() {
        System.out.println("The season is " + this.name());
    }
  }
  

  • This is the correct option. The printSeason() method is properly defined: it’s public, non-static, and utilizes the this reference to access the name of the current enum constant. This method correctly provides custom behavior for each enum constant, allowing it to print a message indicating the current season.