Question

(What Does Each Code Snippet Do?) a) Assume that the following method call is located in an overridden earnings method in a subclass: super .earnings() b) Assume that the following line of code appears before a method declaration: QOverride c) Assume that the following line of code appears as the first statement in a constructor's body: super(firstArgument, secondArgument);

Short answer

a) Calls the 'earnings' method from the superclass. b) Indicates the method overrides one from a superclass. c) Calls the superclass constructor with two arguments.

Step-by-step solution

Understanding Code Snippet a

The code snippet 'super.earnings();' is calling the 'earnings' method from the superclass of the current class. This is typically done when the current class is overriding the 'earnings' method but still needs to use the functionality of the 'earnings' method from the superclass.

Understanding Code Snippet b

The annotation '@Override' before a method declaration indicates that the method is intended to override a method from a superclass. This annotation is used by the compiler to ensure that you are indeed overriding a method that exists in the superclass with the same signature.

Understanding Code Snippet c

The code line 'super(firstArgument, secondArgument);' appears in a constructor and is calling the constructor of the superclass with the specified arguments. This is done to initialize the part of an object that comes from the superclass with particular values.

Key concepts

Super Keyword Usage

The super keyword in Java has a special role in the context of inheritance. It refers to the superclass (parent class) from which the current class is derived. By using super, a subclass can directly access methods and variables of its superclass.

For instance, in the exercise snippet super.earnings();, super is used within an overridden earnings method of a subclass. The purpose here is to call the original earnings method defined in the superclass. This is essential when the subclass wants to extend or modify the behavior of the superclass method, rather than completely replacing it. It's a way to build upon the existing functionality, ensuring that the inheritance hierarchy is respected and the DRY (Don't Repeat Yourself) principle is upheld.

Moreover, constructors in a subclass often invoke a constructor of the superclass using super to ensure that the inherited attributes are properly initialized. For example, super(firstArgument, secondArgument); calls the matching constructor in the superclass, passing the specified arguments. This is generally the first line in a subclass constructor, demonstrating how Java ensures a chain of construction, starting from the top-most superclass down to the current class.

@Override Annotation

The @Override annotation in Java is a form of metadata that provides specific instructions to the compiler about the associated method. It declares that a method is meant to override a method in a superclass. This serves two main functions:

Firstly, it is a declarative tool for the programmer to indicate their intention to override a method, which improves code readability and clarity of intention.

Secondly, it acts as a safeguard during compilation. The compiler checks that there is indeed a method with the same signature in the superclass. If the method doesn't exist, or if there is a mismatch in the signature, the compiler will throw an error. This helps catch typographical errors, such as a method name being slightly misspelled, or issues with the parameter list that could lead to bugs in the program.

For example, in the code snippet @Override before a method declaration, the annotation implies that the subsequent method overrides one from its superclass, and the compiler will verify this. If, for some reason, the superclass method is removed or changed, the subclass method with the @Override annotation will cause a compile-time error, prompting the developer to review their code.

Subclass Method Overriding

Method overriding is a fundamental concept in Java's inheritance model. It allows a subclass to provide a specific implementation for a method that already exists in its superclass. In simple terms, the subclass reshapes the behavior of the superclass method to suit its unique requirements.

This is done by defining a method in the subclass with the same name, return type, and parameters as the method in the superclass. A key aspect of method overriding is that the polymorphic nature of Java ensures that when a method call is made, the overridden method in the actual object's class is executed, not the superclass's version.

Overriding is beneficial because it allows for dynamic method dispatch – the decision of which method to invoke is made at runtime, which is a cornerstone of polymorphism. Moreover, it adheres to the open/closed principle, allowing classes to be open for extension but closed for modification. With method overriding, new behavior can be defined in a subclass without altering the superclass, fostering reusability and maintainability of code.

However, certain rules apply to overriding: the access modifier of the overriding method can be more permissive but not less, and the overriding method can throw the same or narrower checked exceptions, but not broader ones or new unchecked exceptions.