based on slides at buildingjavaprograms.com objects and classes, take 2 managing complexity with...

based on slides at buildingjavaprograms.com

Objects and Classes, take 2Managing Complexity with Programmer-Defined Types

Classes are programmer-defined types. All data types other than primitive types (char, int, double, boolean) are written/defined by programmers, including classes from the Java Standard Library

Classes help us manage complexity, since they: tie together related data and operations decompose an application into some number of

objects and their interactionscan be re-used in different applications

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A programming problem Given a file of cities' (x, y) coordinates,

which begins with the number of cities:650 2090 6010 7274 985 136150 91

Write a program to draw the cities on a DrawingPanel, then drop a "bomb" that turns all cities red that are within a given radius:

Blast site x/y? 100 100Blast radius? 75

3

A bad solution

Scanner input = new Scanner(new File("cities.txt"));int cityCount = input.nextInt();int[] xCoords = new int[cityCount];int[] yCoords = new int[cityCount];

for (int i = 0; i < cityCount; i++) { xCoords[i] = input.nextInt(); // read each city yCoords[i] = input.nextInt();}...

parallel arrays: 2+ arrays with related data at same indexes. Poor solution - as the programmer, you must remember the

connection between entries in the int arrays - the program does not tie them together.

4

Observations This problem would be easier to solve if

there were such a thing as a Point object.

A Point would store a city's x,y data.

We could compare distances between Pointsto see whether the bomb hit a given city.

Each Point would know how to draw itself.

The overall program would be shorter and cleaner. A city’s coordinates would be logically connected in

the program.

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Clients of objects client program: A program that uses objects.

Example: Circles is a client of DrawingPanel and Graphics.

Circles.java (client program)public class Circles { main(String[] args) { new DrawingPanel(...) new DrawingPanel(...) ... }}

DrawingPanel.java (class)public class DrawingPanel { ...}

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Classes and objects class: A program entity that represents either:

1. A program / module, or2. A template for a new type of objects.

A blueprint for a collection of similar objects, that have similar attributes and behavior

The DrawingPanel class is a template for creating DrawingPanel objects.

object: An entity that combines state and behavior. object-oriented programming (OOP): Programs

that perform their behavior as interactions between objects.

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Blueprint analogy

iPod blueprintstate: current song volume battery lifebehavior: power on/off change station/song change volume choose random song

iPod #1

state: song = "1,000,000 Miles" volume = 17 battery life = 2.5 hrs

behavior: power on/off change station/song change volume choose random song

iPod #2

state: song = "Letting You" volume = 9 battery life = 3.41 hrs

behavior: power on/off change station/song change volume choose random song

iPod #3

state: song = "Discipline" volume = 24 battery life = 1.8 hrs

behavior: power on/off change station/song change volume choose random song

creates

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Abstraction abstraction: A distancing between ideas and details.

We can use objects without knowing how they work.

abstraction in an iPod: You understand its external behavior (buttons,

screen). You don't understand its inner details, and you don't

need to, in order to use an iPod.

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Class Design In the following slides, we will implement a Point class

as a way of learning about classes.

We will define a type of objects named Point. Each Point object will contain x,y data called fields. Each Point object will contain behavior called

methods. Client programs will use the Point objects.

To create a new class, think about the objects that will be created of this new class type: what the object knows what the object does

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Class Design For a Car class:

What does a Car object know? (What attributes does it have?)

What can a Car object do? (What actions can it carry out?)

Instance variables are an object’s data, i.e., the things the object knows about itself.

Methods are the things an object can do.

Note: Often our classes contain methods that read new values to store in instance variables, write the data stored in the instance variables.

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Point objectsPoint p1 = new Point(5, -2);Point p2 = new Point(); // origin, (0, 0)

Data in each Point object:

Methods in each Point object:

Method name Description

setLocation(x, y) sets the point's x and y to the given values

translate(dx, dy) adjusts the point's x and y by the given amounts

distance(p) how far away the point is from point p

draw(g) displays the point on a drawing panel

Field name

Description

x the point's x-coordinate

y the point's y-coordinate

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Point class as blueprint

The class (blueprint) describes how to create objects. Each object contains its own data and methods.

Point classstate:int x, y

behavior:setLocation(int x, int y)translate(int dx, int dy)distance(Point p)draw(Graphics g)

Point object #1

state:x = 5, y = -2

behavior:setLocation(int x, int y)translate(int dx, int dy)distance(Point p)draw(Graphics g)

Point object #2

state:x = -245, y = 1897

behavior:setLocation(int x, int y)translate(int dx, int dy)distance(Point p)draw(Graphics g)

Point object #3

state:x = 18, y = 42

behavior:setLocation(int x, int y)translate(int dx, int dy)distance(Point p)draw(Graphics g)

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Point class, version 1: Statepublic class Point { private int x; private int y;}

Save this code in file Point.java.

The above code creates a new type named Point. Each Point object contains two pieces of data:

an int named x, and an int named y.

Point objects do not contain any behavior (yet).

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Fields field: A variable inside an object that is part of its state.

Called an instance variable or instance field. Each object has its own copy of each field.

Declaration syntax:<access> <type> <name>;

Example:

public class Student { private String name;// each Student object private double gpa;// name and gpa field

}

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Accessing fields Other classes can access/modify an object's fields, if

permitted by the access specifier for the field.

access: variable.field modify: variable.field = value;

Example:Point p1 = new Point();Point p2 = new Point();System.out.println("the x-coord is " + p1.x);// accessp2.y = 13;// modify - usually disallowed (encapsulation)

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A class and its client Point.java is not, by itself, a runnable program.

A class can be used by client programs.

PointMain.java (client program)public class PointMain { ... main(args) { Point p1 = new Point(); p1.x = 7; p1.y = 2;

Point p2 = new Point(); p2.x = 4; p2.y = 3; ... }}

Point.java (class of objects)public class Point { int x; int y;}

x 7 y 2

x 4 y 3

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PointMain client examplepublic class PointMain { public static void main(String[] args) { // create two Point objects Point p1 = new Point(); p1.y = 2; Point p2 = new Point(); p2.x = 4;

System.out.println(p1.x + "," + p1.y); // 0,2

// move p2 and then print it p2.x += 2; p2.y++; System.out.println(p2.x + "," + p2.y); // 6,1 }}

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Methods: An Object’s Behavior Methods of objects (methods that are non-static) define

the behavior of the object.

public class Point {private int x;private int y;

public void setLocation(int newX, int newY) {x = newX;y = newY;

}}

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More on Methods + this keyword

The keyword this allows an object to refer to itself:public void setLocation(int x, int y) { this.x = x;// this.x is instance var for this object this.y = y;

}

this is used to distinguish between the instance variable and parameter of the same name.

an instance method is executed from the context or perspective of a particular object

this lets you refer to the object on which the method is running.

this is called the “implicit parameter”

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Constructors Method that executes when a new object is created Same name as the class No return type - implicitly returns new object Used to initialize object’s data fields

public class Point {private int x;private int y;

public Point(int x, int y) {this.x = x;this.y = y;

}}

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Accessors or Getters Many classes include methods that get, or return, the

value of an instance variable. This is needed if clients need access to variable values, since we protect our instance variables (by declaring them private)

This methods are called accessors or getters, and usually have the name get<VariableName>.

Getter for x coordinate, in Point class:public class Point {

private int x;// x cannot be modified from outside classprivate int y;

public int getX() {return x;

}

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Mutators or Setters Many classes contain methods that allow a user to

modify the value of an instance variable, often with restrictions on the type of modifications allowed.

These methods are called mutator or setter methods.

public class Point {private int x;

public void setX(int newX) {x = newX;

}}

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Point class - Exercise Write a complete Point class that contains:

a constructor that takes as arguments the coordinates of the point

a constructor that takes no arguments, and initializes the point to represent the origin (0, 0)

mutator methods for both the x and y coordinates accessor methods for both the x and y coordinates a distanceToOrigin method that computes the

distance to (0, 0) from the current point a distance method that takes a Point p as its

argument, and returns the distance from this point to p

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Point Objects Instance method: method that exists inside each objectPoint p = new Point(2, 3);p.setLocation(4, 1);Point q = new Point(5, 5);q.setLocation(0, 0);

p x y

public void setLocation(int x, int y) { ... }

q x y public void setLocation(int x, int y)

{ ... }

2 3

5 5

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More on Constructors Initializes the state of a new object Runs when the client creates a new object using the new keyword + the class/constructor name

If a class contains no constructor, Java provides the class with a default constructor that takes no arguments

Syntax:public <ClassName> (<type1> <parm1>, ... ) {

<statements that initialize instance variables>}

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Common Constructor Errors Including a return type:public void Point(int newX, int newY) {

...}

Remember that a constructor automatically returns the new Point object.

Assigning values to local variables (the constructor’s parameter) rather than instance variables:

public void Point(int newX, int newY) {newX = x; // assign parameter the value of // instance variableint y = newY; // create local variable y

}

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Arrays of objects null : A reference that does not refer to any object.

The elements of an array of objects are initialized to null.

String[] words = new String[5];DrawingPanel[] windows = new DrawingPanel[3];

index 0 1 2 3 4

value null null null null null

index 0 1 2

value null null null

words

windows

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Things you can do w/ null store null in a variable or an array element

String s = null;words[2] = null;

print a null referenceSystem.out.println(s); // output: null

ask whether a variable or array element is nullif (words[i] == null) { ...

pass null as a parameter to a method

return null from a method (often to indicate failure)

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Null pointer exception dereference: To access data or methods of an object with the dot

notation, such as s.length(). It is illegal to dereference null (causes an exception). null is not any object, so it has no methods or data.

String[] words = new String[5];System.out.println("word is: " + words[0]);words[0] = words[0].toUpperCase();

Output:word is: nullException in thread "main" java.lang.NullPointerException at Example.main(Example.java:8)

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Looking before you leap You can check for null before calling an object's

methods.

String[] words = new String[5];words[0] = "hello";words[2] = "goodbye";// words[1], [3], [4] are null

for (int i = 0; i < words.length; i++) { if (words[i] != null) { words[i] = words[i].toUpperCase(); }} index 0 1 2 3 4

value "hello" null "goodbye" null nullwords

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Two-phase initialization1) initialize the array itself (each element is initially null)

2) initialize each element of the array to be a new object

String[] words = new String[4]; // phase 1for (int i = 0; i < words.length; i++) { coords[i] = "word " + i; // phase 2}

index

0 1 2 3

value "word 0"

"word 1"

"word 2"

"word 3"

words