Chapter 5: Writing Classes

Chapter Scope

• Identifying classes and objects: Understanding how to define and use classes in Java programming.

• Structure and content of classes: Learning the components that make up a class.

• Instance data: Exploring how data is associated with instances of classes.

• Visibility modifiers: Understanding the accessibility of class members.

• Method structure: Reviewing how to define methods within classes.

• Constructors: Learning about special methods used for object instantiation.

• Relationships among classes: Exploring how classes can interact with one another.

• Static methods and data: Understanding the implications of static members in classes.

Classes and Objects Revisited

• Programs generally use classes defined in the Java API.

• Object-oriented programming focuses on creating custom classes with specific properties and functionalities.

• The class with the main method serves as the application starting point.

Classes and Objects

• Each object has a state defined by its attributes and behaviors defined by its methods.

• Attributes are the data elements associated with the object.

• Methods in the class define operations that can be performed on the object's attributes.

Class Attributes and Operations

• Example of a Student class:

  • Attributes: Name, Address, Major

  • Operations: Set address, Set major, Compute GPA

• Example of a Rectangle class:

  • Attributes: Length, Width, Color

  • Operations: Set length, Set width, Compute area

Identifying Classes and Objects

• A class groups objects with similar behaviors; often named in singular nouns (e.g., Coin, Student).

• Instancing: You can create multiple instances of a class to represent individual objects.

• Identifying potential classes based on nouns in problem descriptions is key.

Class Complexity and Responsibilities

• Classes should have an appropriate level of detail.

• For complex classes, decomposing them into smaller classes can manage responsibilities effectively.

• Assigning responsibilities or methods is essential for designing classes.

Anatomy of a Class

• Contains data (attributes) and methods (operations).

• Die Class Example: Represents a six-sided die with methods to model rolling.

• Method toString(): Common in classes to provide string representation.

Constructors

• Constructors initialize newly created objects; they share names with the class but have no return type.

• Example: The Die class constructor initializes the face value.

Data Scope

• Data scope determines where data can be referenced:

  • Class-level data can be accessed by all methods within the class.

  • Method-level data (local variables) can only be accessed within that method.

Instance Data

• Instance data refers to variables that are specific to an object instance.

• Each object has its own copy of instance variables.

UML Diagrams

• UML stands for Unified Modeling Language, used to depict class attributes and operations visually.

• Class diagrams illustrate how classes are related or interact within a program.

Encapsulation

• Encapsulation involves hiding the internal workings of an object to provide a clear interface to clients.

• Direct access to variables should be restricted; modification should occur through methods only.

Visibility Modifiers

• Java defines three visibility modifiers: public, protected, private.

• Public members are accessible anywhere; private members are limited to the defining class; default visibility permits access within the package.

Accessors and Mutators

• Accessor methods (getters) retrieve data while mutator methods (setters) modify it.

• Consistent naming conventions (getX and setX) ease readability.

Method Declarations

• Methods are defined with a standard structure containing a header and body, dictating execution flow.

• Example method header: char calc(int num1, int num2, String message).

Method Overloading

• Method overloading allows multiple methods with the same name but different parameter lists.

• Important for clarity and usability without changing method names.

Testing and Debugging

• Testing: Validates that a program behaves as expected under various conditions.

• Debugging: Locates and corrects errors in code. Tools include debuggers that provide breakpoints, variable inspection, and execution control.

• Defect Testing: Recognizes that errors can exist, employing black-box and white-box testing strategies.

• Formal Parameters: Variables listed in a method's declaration that receive values when the method is called. These are placeholders for the data the method will process.

• Actual Parameters (Arguments): The real values passed to a method when it is invoked. These correspond to the formal parameters of the method being called.

• Default Parameters: Parameters that have default values defined in the method signature, allowing the method to be called with fewer arguments than it receives by default.

• Variable-length Parameters: Using varargs (variable-length argument lists) allows a method to accept zero or more arguments of a specified type. Indicated by ... in the parameter declaration, such as void methodName(int... numbers).

• Parameter Type: Parameters can be of any data type including primitives (like int, char) and object references (like String, custom classes). The type must match the method's formal parameter type for successful execution.

Types of Methods in Java

1. Instance Methods

   • Belong to an instance of a class.

   • Can access instance variables and call other instance methods.

   • Defined without the static keyword.

2. Static Methods

   • Belong to the class rather than any instance.

   • Can access class-level data and methods but cannot access instance variables.

   • Defined with the static keyword.

3. Constructors

   • Special methods called when an object is instantiated.

   • Have the same name as the class and do not have a return type.

   • Used to initialize instance variables.

4. Accessor Methods (Getters)

   • Retrieve and return the value of a private instance variable.

   • Often prefixed with get, e.g., getName().

5. Mutator Methods (Setters)

   • Modify the value of a private instance variable.

   • Often prefixed with set, e.g., setName(String name).

6. Overloaded Methods

   • Methods that share the same name but have different parameter lists.

   • Useful for providing method variability and ease of use.

7. Default Parameters

   • Allow a method to be called with fewer parameters than are defined, utilizing default values in the method signature.

8. Variable-length Parameters (Varargs)

   • Methods that can accept zero or more arguments of a specified type, using the ... syntax.

   • For example, void methodName(int... numbers) can take any number of integer arguments.

Conclusion

Understanding these different types of methods is crucial for effective Java programming, as they enhance code organization, reusability, and clarity.