A7 - Classes

Classes Introduction

• Definition: A class is a blueprint for what an Object stores and how it behaves.

• Purpose: Defines the type of data an Object will store and its methods.

• Key Concept: Encapsulation - combining multiple data into a single class.

Classes Overview

Files

• Each class written in its own file.

• File name must match the class name.

  • Example: A class named Person should be in a file called Person.java.

Parts of a Class

1. Attributes: Store data for objects (e.g., first name, last name).

2. Constructors: Special methods for building objects and assigning initial values to attributes.

3. Methods: Perform actions on attributes.

Attributes

• Attributes are not mandatory but are typically included.

• Access Levels: Determines visibility of attributes/methods:

  • public: Accessible outside the class.

  • private: Not accessible outside the class.

  • protected: Accessible by subclasses or classes in the same package.

• Modifiers:

  • final: Value cannot be changed after initialization.

  • static: Shared among all instances (class attribute).

• 
  

  Format for Attributes:accessLevel modifiers type attributeName;

  • Examples:

    • public static final double PI = 3.14;

    • private String name;

    • private int age = 0;

Constructors

• Rules for creating constructors:

  • Must be public.

  • Cannot have a return type.

  • Can receive parameters.

  • Can be overloaded.

• A default constructor is one that receives no parameters; it initializes attributes to default values.

• 
  

  Format for Constructor:public className(parameters) { //Code }

Methods

Getters

• Allow public access to private attributes.

• Format for Getter: public returnType getVariableName() { return variableName; }

  • Example: public int getAge() { return age; }

Setters

• Allow changing private attributes publicly.

• Format for Setter: public void setVariableName(variableType variableName) { this.variableName = variableName; }

  • Example: public void setAge(int age) { this.age = age; }

Other Methods

• Classes can have various methods beyond accessors/mutators.

• Format for Methods: accessLevel modifiers returnType methodName(parameters) { //code }

Working with Instances

• Creating an Instance: className variableName = new className(parameters);

• Accessing Public Attributes: variableName.attributeName

• Changing Public Attributes: variableName.attributeName = newValue

• Accessing Private Attributes: variableName.accessor()

• Changing Private Attributes: variableName.mutator(newValue)

• Accessing Public Class Methods: variableName.methodName(parameters)

Class Attributes & Methods

• Accessing class attributes and methods can be done with or without instances.

• Accessing Class Attributes: className.attributeName

• Changing Class Attributes: className.attributeName = newValue

• Accessing Class Methods: className.methodName(parameters)

Keywords

this

• Keyword for accessing the current instance within a class.

  • Usage: this.attributeName

  • To call another constructor: this(parameter);

null

• Represents a variable that does not reference any Object.

Built-in Class Methods

equals

• Tests equality of two Objects (returns true or false).

• By default compares memory addresses:

  • Example scenario demonstrating default behavior.

Fixing Equals Method

• Custom implementation of equals required:

  • 
    

    Format:public boolean equals(Object o) { // Logic for comparison }

  • Example provided for comparing object attributes.

toString

• Returns a String representation of an Object.

• Default implementation provides type and memory address.

Fixing toString Method

• Custom implementation of toString required:

  • 
    

    Format:public String toString() { // Create and return a text version }

Memory Addresses

• Variables store memory addresses, not object data directly.

• == operator compares memory addresses instead of actual data values.

Reference Revisited

• Objects are passed by reference to methods.

• Modifications to a parameter affect the original object because of shared memory address.

Handling null & Exception Prevention

• Example scenarios showing handling of null to avoid exceptions.

Package

• Group of related classes with unrestricted access to each other’s protected methods/attributes.

• Benefits of using packages.

Naming in Packages

• Naming conventions for package:

  • Should be in lowercase.

  • Avoid starting with java or javax.

  • Use periods for separation.

• Example Package Declaration: package game.graphic;

Terms and Definitions

• Access Level: Visibility level for class data/methods.

• Attributes: Data storage in class.

• Class: A blueprint for objects.

• Class Attribute: Shared across all instances.

• Class Method: Access class attributes only.

• Constructors: Special methods for constructing objects.

• Default Constructor: Automatically provided if none created.

• Encapsulation: Combining data into a class.

• equals: Method for equality check.

• Getter: Method for accessing private attributes.

• Information Hiding: Preventing unwanted data changes.

• Instance Method: Can access instance and class variables.

• Modifiers: Set properties for methods/attributes.

• null: Denotes no object reference.

• package: Group of related classes.

• protected: Access level for subclasses/same package.

• public: Fully accessible data.

• Setter: Method for altering private attributes.

• Static: Class attribute/method.

• this: Refers to current instance.

• toString: Method for returning a String version of an object.