Question

By extending class Vector, Java's designers were able to create class Stack quickly. What are the negative aspects of this use of inheritance, particularly for class Stack?

Short answer

Inheriting Vector for Stack violates encapsulation and may lead to maintenance issues.

Step-by-step solution

Understanding Inheritance in Java

Inheritance in Java is a mechanism where a new class (subclass) is created from an existing class (superclass). The subclass inherits fields and methods from the superclass, allowing reuse of code.

Defining Vector and Stack Classes

The `Vector` class in Java is part of the Java Collections Framework and represents a dynamic array. The `Stack` class extends `Vector` to utilize its functionalities for stack operations like push, pop, and peek.

Overuse of Inheritance

Inheritance should represent an 'is-a' relationship. The Stack class being a 'type of' Vector doesn't accurately represent the relationship, as a stack is a specific structure with operations limited to LIFO (Last-In, First-Out) which aren't all natural fits with Vector's API.

Nullifying Encapsulation

Using inheritance exposes all the public methods of `Vector`, including ones that aren't appropriate for a stack, like inserting or accessing elements arbitrarily. This breaks encapsulation since it provides access to methods that shouldn't be available in a strict stack context.

Potential Maintenance Issues

Future changes to the `Vector` class could unintentionally affect the `Stack` class, leading to maintenance issues. If new features or changes are added to `Vector`, they might not make sense for `Stack`, leading to potential bugs or unintended behavior.

Key concepts

Java Collections Framework

The Java Collections Framework is a set of classes and interfaces in Java that provides a comprehensive architecture to store and manipulate groups of objects. It is crucial for developers because it offers a ready-made set of data structures and algorithms. Within this framework, you'll find collections like Lists, Sets, Maps, and Queues. Each serves a distinct purpose and is designed for efficient data manipulation.

Collections offer useful features such as:

• Dynamic resizing of arrays.
• Easy access to elements by indices.
• Advanced data processing with built-in methods.

The `Vector` and `Stack` classes belong to this framework. `Vector` manages dynamic arrays, while `Stack` utilizes these features to implement LIFO operations. However, directly extending `Vector` for a `Stack` can lead to problems, as it exposes unnecessary functionality not suitable for a strict stack, contributing to design inefficiencies.

Encapsulation in Java

Encapsulation is a fundamental concept in Java that involves bundling the data (variables) and the code (methods) that manipulates this data into a single unit or class. It restricts direct access to some of an object's components and can prevent the accidental modification of data. A well-encapsulated class typically follows these principles:

• Class variables are always private and can only be accessed through methods.
• Methods that provide access to variables act as a barrier, controlling how the variables are accessed or modified.

With respect to `Stack` extending `Vector`, this inheritance violates encapsulation. By inheriting from `Vector`, all the `Vector` methods become accessible in `Stack`, even those that should not be publicly available in a stack. This leads to improperly accessible methods, breaking the intended design and limitations of the stack structure.

Subclass and Superclass

In Java, the terms subclass and superclass refer to the hierarchy established through inheritance. A subclass, or child class, inherits properties and behaviors (methods) from another class known as the superclass, or parent class. When creating subclasses, the key rules include:

• A subclass is a specialized version of a superclass and should logically represent an 'is-a' relationship.
• The subclass inherits all accessible fields and methods of the superclass, but it can also have its own unique fields and methods.
• Method overriding allows a subclass to provide a specific implementation of a method already defined in its superclass.

With `Stack` extending from `Vector`, the relationship is flawed as a `Stack` doesn’t truly qualify as a `Vector`, complicating the 'is-a' aspect. The subclass should align more closely with its specific functional requirements, rather than duplicating broader capabilities.

LIFO Data Structure

LIFO stands for Last-In-First-Out. It is a method of handling data structures where the last element added to the structure is the first one to be removed. Stacks are prime examples of LIFO structures and are used in situations such as undo mechanisms in applications, parsing expressions, or managing method calls in programming. The key operations of a stack are:

• Push: Adding an element to the top of the stack.
• Pop: Removing the top element from the stack.
• Peek: Viewing the top element without removing it.

The LIFO principle ensures that all operations apply to the 'top' of the stack. However, when the `Stack` class unnecessarily exposes unrelated `Vector` methods, this principle is significantly diluted. It allows operations that undermine the LIFO concept by, for instance, allowing arbitrary element access, which is counterproductive to the stack's primary use case.