Writing New Java Classes 15-121 Fall 2020 Margaret Reid-Miller - - PowerPoint PPT Presentation

Writing New Java Classes

15-121 Fall 2020 Margaret Reid-Miller

Comments

• How was the Lab?
• Homework2: questions? install in Eclipse?
• Quiz will be available tomorrow and due on
• Sunday. I will send email and post on Piazza.
• Next Monday, Margaret's office hours will be

from 12:00pm-2:00pm

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Today

• OOP terminology: classes and objects
• GiftCard class
• Alternator class exercises

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• How do we call a static method?

. ( ) method-name parameters

• How do we call a non-static method?

__ . ___ ( __ )

method-name parameters

• How do we call a constructor?

( ) parameters

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class name

• bject expression

new class-name

Can we write a method that returns multiple values?

• We can bundle together data of the same type in an

array and return the array.

• But even better, we can bundle together data of

different types in an object and return the object.

• Objects allows us to group related data and define

new data types.

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Object-Oriented Programming

• The idea is to divide an application into small

reusable objects.

• Think of them as the nouns that describe the

application.

• Then build the application from reusable

components.

• e.g., we build a car from various components

(wheels, doors, engine…)

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We define new object types by defining classes.

A class definition is like a blueprint for objects.

• We can create many instances of a class (objects)

from a class.

• The differences among these objects are values of its

attributes.

• For example, a class Student would define

attributes common to students:

• name, andrewId, courses enrolled….

but each object would have its own values for these attributes.

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A Java class definition contains fields, constructors, and methods.

• fields store data. Often we cannot access

them directly outside the class.

• constructor(s) initialize all the fields.
• methods interact with the fields, either to

supply or modify them.

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Fields (instance variables) define an

• bject’s properties.

Example: An object from an GiftCard class might have the following fields:

• the name of the store
• the dollar value of the card.
• Once an object is created, each field has some value.
• These values define the current state of the object

and describe the current condition of the object.

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Fields

public class GiftCard { // Fields: The object state private String store; // Name of the store private double balance; // The current dollar value // Methods: The object behaviors }

Fields are usually private:

• visible to methods in the same class and
• hidden to methods in other classes.

By being private, we don't allow other classes to modify the fields directly or expose how the data is maintained.

initial capital

GiftCard class

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An Shopping application (client) can create Giftcard objects.

Use the new operator to instantiate (create) the object, followed by a call to the class constructor. The new operator returns a reference to the new

• bject.

For example in the main method we might write:

GiftCard macys = new GiftCard("Macy's", 100);

store balance

GiftCard

100

macys

String

Macy's

Shopping class

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Constructors initializes all the fields

• f the object.

public GiftCard(String storeName, double dollars) { store = storeName; balance = dollars; // Initial balance }

NOTE: For each parameter, we often use a name different from the field name to distinguish the two.

GiftCard class

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Constructors are like methods with two differences:

• 1. There is no return type.
• The object reference returned always has the

type of the class.

• 2. The name of the constructor is always the name of

the class.

• Like methods, any Java statement can be in the body
• f the constructor. For example, it might check that

the parameter dollars is positive.

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You can have more than

• ne constructor.
• Constructors can be overloaded:

Each constructor has a different parameter list.

• For example, some may supply default values for

fields that have no corresponding parameter.

public GiftCard(String storeName) { store = storeName; balance = 25.0; // Default balance }

GiftCard class

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The behaviors of an object are defined by its (instance) methods

• The methods that define the interface to an object

are usually public. These are the methods given in the Application Programmer Interface (API).

• The program code that creates and uses these
• bjects, called the client code, is now more

expressive and concise.

• The client simply asks the objects to perform their

behaviors (i.e., the objects provide a service).

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Accessors

• Accessors are methods that access an
• bject’s state without changing the state.

Examples:

public String getStoreName() { return store; } public double getBalance() { return balance; }

Accessors should be defined as public (visible by methods in every class).

Names often begin with “get”

GiftCard class

No static keyword

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Mutators

• Mutators are methods that can change an object’s

state.

public void buyGoods(double cost) { if (cost <= balance) { balance -= cost; } }

Mutators should be defined as public (visible to methods of every class).

GiftCard class

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Mutators should maintain class invariants

• Mutators should ensure that the object’s state stays

consistent with what the object is modeling, e.g., the gift card balance never goes negative. In another class: macys.buyGoods(10.0); updates the balance but macys.buyGoods(100000.0); doesn't.

Shopping class

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Sample Client Program

public class Shopping { public static void main(String[] args){ GiftCard macys = new GiftCard("Macy's", 100.0); GiftCard target = new GiftCard("Target", 50.0);

Shopping class

Each object has its own copy of the fields.

100.0 name balance GiftCard String Macy's macys String Target target 50.0 name balance GiftCard

Sample Client Program (cont'd)

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if (macys.getBalance() >= 42) { macys.buyGoods(42); } else { System.out.print("can't purchase, " + "insufficient funds"); } if (target.getBalance() < 10){ System.out.println("target card getting low"); } } // end main } // Shopping class

Shopping class

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Exercise: Write a Class Based on Its Use

Example: Using an alternator object

An Alternator object alternates between two integer values.

public static void main (String[] args) { Alternator alt3 = new Alternator(3, 6); System.out.println(alt3.alternate()); // prints 3 System.out.println(alt3.alternate()); // prints 6 // Method both changes state and accesses state! System.out.println(alt3.alternate()); // prints 3 System.out.println(alt3.alternate()); // prints 6 }

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AlternatorTester class

An Alternator object alternates between two integer values.

To write the Alternator class start by determining the fields and ask:

• What does an object need to remember?
• What are its attributes/properties?

How should we declare a fields? _________ _____ ________; type name

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private

Alternator class

Alternator class

• What is the main purpose of the constructor?
• How do you declare a constructor?

public ______________( . . . ) { //What should usually go here?

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initialize the fields class name field1 = …; field2 = …;

More on writing classes

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Every class should have toString()method.

• A toString() method returns a string that

represents the current state of the object

public String toString() { return "store = " + store + " balance = " + balance; } Required signature

GiftCard class

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Often toString() is used for debugging.

public class Shopping { public static void main(String[] args){ target = new GiftCard("Target"); target.buyGoods(10.50); System.out.println("target: " + target); Java invokes toString() on objects in

• print statements and
• string concatenation expressions.

Shopping class

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Every class should have an equals()method.

An equals() method compares two objects and returns true if they have the same state and false

• therwise.

public boolean equals(GiftCard other) { if (other == null) return false; return store.equals(other.store) && balance == other.balance; }

In another class, call this method with two objects

target.equals(macys)

GiftCard class

Use dot to access an object’s fields.

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The this Reference

• The reserved word this allows an object to refer to

itself within its class. (It is sometimes called the implicit parameter.)

public GiftCard(String name, float dollars) { this.name = name; … }

shadows the field field

public boolean equals(GiftCard other) { return this.name.equals(other.name) && … } Usage: if (target.equals(macys)) …

GiftCard class

Encapsulation hides object data and method implementations.

Who knows how to use a radio? Who knows how to build a radio from low-level electronic parts?

• A client (user) only needs to know what the object

does (its interface).

• A server (implementor) decides how the object does

it. To maintain object data dependencies and maximize reusability the server should

• not allow the client direct manipulation of the data
• nor expose the details of the implementation.

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Encapsulation reduces data dependency and maximize reusability.

• Advantages:
• Reduces complexity for the client.
• If you change the object’s underlying data

structure or algorithms, the client works as before.

• Since there is no direct access to fields, you can

ensure internal data dependencies (class invariants) are maintained.

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Classes (and their parts) have visibility modifiers:

• public: accessible to everyone
• protected: inside package, inside class,

inside subclass

• package-private (default, no modifier

used): inside package, inside class

• private: accessible only within the class

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Data fields are usually private

Data (fields):

• Whatever the object needs to store
• Available to clients via “getter” and “setter”

methods.

• Are private to encapsulate the object data

from clients and to ensure class invariants.

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Public vs Private Methods

public: Only those methods that are part of the interface (the methods the client can call).

• They can’t assume the client will supply correct

arguments.

• These methods must ensure that the pre- and post-

conditions are met and post-conditions maintain the class invariants. private: “helper” methods inside the class

• Since clients cannot call these methods, and the

server calls meet the pre-conditions, these methods need not check the pre-conditions.

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