Classes and Objects

Adapted from

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Java Software Solutions

for AP* Computer Science A

2nd Edition

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by John Lewis, William Loftus, and Cara Cocking

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Java Software Solutions is published by Addison-Wesley

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Presentation slides are copyright 2006 by John Lewis, William Loftus, and Cara Cocking. All rights reserved.

Instructors using the textbook may use and modify these slides for pedagogical purposes.

*AP is a registered trademark of The College Entrance Examination Board which was not involved in the production of, and does not endorse, this product.

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1

Writing Classes

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• Focuses on:
• class definitions
• encapsulation and Java modifiers
• Java compiler and JVM

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2

Objects

• An object has:
• state - descriptive characteristics
• behaviors - what it can do (or what can be done to it)
• The state of the coin is its current face (heads or tails)
• The behavior of the coin is that it can be flipped
• Note that the behavior of the coin might change its state

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Classes

• A class is a blueprint of an object
• It is the model or pattern from which objects are created
• For example, the String class is used to define String objects
• Each String object contains specific characters (its state)
• Each String object can perform services (behaviors) such as toUpperCase()

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4

Classes

• The String class was provided for us by the Java standard class library
• But we can also write our own classes that define specific objects that we need
• For example, suppose we want to write a program that simulates the flipping of a coin
• We can write a Coin class to represent a coin object

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5

Classes

• The Java language requires that the code for each public class must be stored in its own text file.
• The filename of the text file that contains a public class must have the same name (case-sensitively) as the class, and must have the file extension ".java"
• Java requires that every method definition must appear within a class definition

ie. public class Coin needs to be save to a file called Coin.java

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6

Java Compiler and JVM

• 2 Stages
1. each program file (.java) that contains a class definition is converted by a process called compilation into a sequence of byte codes (.class)
2. the computer on which the program is being executed carries out the actions of the byte codes.
• Java Virtual Machine resides on target computer

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7

Errors && Exceptions

• Compile Time Error
• Syntax Error
• Program Structure Error
• ie. no return statement
• Data Type Error
• ie. assigning an int to a String
• Run -Time Error (Exceptions)
• ArithmeticException
• ArrayIndexOutOfBoundsException
• ClassCastException
• NullPointerException
• Logical Error

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8

Garbage Collection

• Java virtual machine periodically recycles memory
• This recycling process is called garbage collection.
• Virtual machine inspects each value and determines whether the value is still needed by the program.
• Virtual machine reclaims the memory for later reuse.

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Classes

• A class contains data declarations and method declarations

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public class Coins {

Data Scope

• The scope of data is the area in a program in which that data can be used (referenced)
• Data declared at the class level can be used by all methods in that class
• Data declared within a method can be used only in that method
• Data declared within a method is called local data
• Scope is defined within { }

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Instance Data

• The face variable in the Coin class is called instance data because each instance (object) of the Coin class has its own
• A class declares the type of the data, but it does not reserve any memory space for it
• Every time a Coin object is created, a new face variable is created as well
• The objects of a class share the method definitions, but each has its own data space
• That's the only way two objects can have different states

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Constructors Revisited

• Recall that a constructor is a special method that is used to initialize a newly created object
• When writing a constructor, remember that:
• it has the same name as the class
• it does not return a value
• it has no return type, not even void
• it typically sets the initial values of instance variables
• The programmer does not have to define a constructor for a class
• All classes have a default constructor implied

Class Design

public class Person

{

private double myHeight;

private double myWeight;

private String myHair;

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public Person( double height, double weight, String hair )

{

myHeight = height;

myWeight = weight;

myHair = hair;

}

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// ...

}

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Class Header

Instance Variables

Constructor

Initialize Instance Variables

complete Person Class Ex. 94 [slide 5], 96, 97

Local Data

• Local variables can be declared inside a method
• The formal parameters of a method create automatic local variables when the method is invoked
• When the method finishes, all local variables are destroyed (including the formal parameters)
• Keep in mind that instance variables, declared at the class level, exists as long as the object exists
• Any method in the class can refer to instance data

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Method Decomposition

• A method should be relatively small, so that it can be understood as a single entity
• A potentially large method should be decomposed into several smaller methods as needed for clarity
• A service method of an object may call one or more support methods or helper methods to accomplish its goal
• Support methods could call other support methods if appropriate

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Encapsulation

• We can take one of two views of an object:
• internal - the variables the object holds and the methods that make the object useful
• external - the services that an object provides and how the object interacts
• From the external view, an object is an encapsulated entity, providing a set of specific services
• These services define the interface to the object
• Recall that an object is an abstraction, hiding details from the rest of the system

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Encapsulation

• An object should be self-governing
• Any changes to the object's state (its variables) should be made only by that object's methods
• We should make it difficult, if not impossible, to access an object’s variables other than via its methods
• The user, or client, of an object can request its services, but it should not have to be aware of how those services are accomplished

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Encapsulation

• An encapsulated object can be thought of as a black box
• Its inner workings are hidden to the client, which invokes only the interface methods

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Client

Methods

Data

Visibility Modifiers

• In Java, we accomplish encapsulation through the appropriate use of visibility modifiers
• A modifier is a Java reserved word that specifies particular characteristics of a method or data value
• We've used the modifier final to define a constant
• We will study two visibility modifiers: public and private

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Visibility Modifiers

• Members of a class that are declared with public visibility can be accessed from anywhere
• Public variables violate encapsulation
• Members of a class that are declared with private visibility can only be accessed from inside the class
• Members declared without a visibility modifier have default visibility and can be accessed by any class in the same package

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Visibility Modifiers

• Methods that provide the object's services are usually declared with public visibility so that they can be invoked by clients
• Public methods are also called service methods
• A method created simply to assist a service method is called a support method
• Since a support method is not intended to be called by a client, it should not be declared with public visibility

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Preconditions and Postconditions

• A precondition is a condition that should be true when a method is called

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• A postcondition is a condition that should be true when a method finishes executing

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• These conditions are expressed in comments above the method header

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• Both preconditions and postconditions are a kind of assertion, a logical statement that can be true or false which represents a programmer´s assumptions about a program

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Other Resources

"Difference between Abstraction and Encapsulation in Java - OOPS." Java67 Java Programming tutorials and Interview Questions.

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<http://www.java67.com/2012/08/difference-between-abstraction-and-encapsulation-java-oops.html>

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