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### Understanding Classes and Objects in Java Programming ###

This lecture focuses on the fundamental concepts of classes and objects in Java. Classes serve as blueprints for creating objects, which are instances of the class. Each object can have unique field values, even though all instances share the same field names. The lecture covers field declarations, types, and examples of actual code implementation. It emphasizes the importance of understanding instance variables and their significance in defining object state and behavior. The material prepares students for upcoming discussions on methods and constructors in Java programming. ###

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### Understanding Classes and Objects in Java Programming ###

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  1. Lecture 4:Objects & Fields

  2. Classes vs. Objects • Classes are blueprints describing data type • On their own, classes (usually) cannot do anything • Objects are instances of a class • New objects created (instantiated) using new • Fields describe state of an object • Object’s behavior represented by methods

  3. Instance Variables • All of class's instances have same fields… • … but values can differ between each instance • In a class, each field must have unique name • Different classes can duplicate names of fields • Field declaration must include data type • Will act like variables of that type • Can be primitive, enum, or reference type

  4. Class Example public class Car {/** What kind of car & who made it */private String makeAndModel;/** Color of the car. */private String color;/** Percent full the gas tank is */private float tankLevel;/** Miles recorded on the odometer */private intodometerReading;/* Definition continues from here */

  5. Using Fields (1) Car profHertzCar= new Car();profHertzCar.makeAndModel= “BMW Z4”;profHertzCar.color= “Deep Green Metallic”;profHertzCar.tankLevel= 1.0;profHertzCar.odometerReading= 10000;CaractualCar = new Car();actualCar.makeAndModel = “Subaru Outback";actualCar.color = “Brown”;actualCar.tankLevel = 0.0001;actualCar.odometerReading = 67634;

  6. Using Fields (2) Car dreamCar = new Car();dreamCar.makeAndModel = “BMW Z4”;dreamCar.color = “Deep Green Metallic”;dreamCar.tankLevel = 1.0;dreamCar.odometerReading = 10000;CarrealCar = dreamCar;realCar.makeAndModel = “Subaru Outback”;realCar.color = “Silver”;realCar.tankLevel = 0.0001;realCar.odometerReading = 67634;

  7. Code Example public class Player {public String name;public static voidmain(String[] args){Player player1, player2, player3; player1 = new Player();player1.name= "Homer";player2 = new Player();player2.name= “JJ”;player3 = player2;player3.name = "Hertz";System.out.print(player2.name+" "+player1.name);} }

  8. Tracing With Objects & Fields public staticvoid main(String[] args) {Player player1, player2, player3; player1 = new Player();player1.name= "Homer”;player2 = new Player();player2.name= “JJ”; player3 = player2;player3.name = "Hertz";System.out.print(player2.name+" "+player1.name);}

  9. Instance Variables Review • Each instance gets own copy of every field • Must specify instance whose field should be used • If you have instance, can set value at any time • No limit on use in code, just need the instance • Primitive or reference okay, like with any variable • Declare in class using:typename;

  10. For Next Lecture • Keep reviewing your Java lessons • Will be discussing methods & constructors Friday • Really need to be back up-to-speed at that point • There is weekly assignment problem on Angel • Due next Tuesday at 5PM but could start earlier • With this finish removing rust from lazy summer

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