1

A Sorting Example

(a start for Lab 4 problem number 3)

2

Alphabetizing Words

How do we sort a list of words into alphabetic order? compare first letter, if different then done otherwise, compare 2nd letter repeat

In general, this way of ordering is called lexicographic ordering

3

To Start: Alphabetizing Characters This is easy… if everything is lower case

for characters char1 and char2: char1 < char2 just in case char1 precedes char2

It is just as easy if everything is upper caseString s = “MixED uP cASe”;

s = s.toLowerCase();

// resulting string is “mixed up case” After converting, then alphabetize

4

Idea

Given input strings s1 and s2 of equal length.

boolean isBefore(String s1, String s2)if(s1.charAt(0)<s2.charAt(0))

return true;else if(s1.charAt(0)>s2.charAt(0))

return false;else // this means they are equal

check charAt(1); ….

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What About Different Lengths? To complete the solution

What happens if s1 and s2 have different lengths? e.g. “dump” comes before “dumpster”

After completing isBefore(s1,s2) How can you use it to sort an array of strings?

6

Classes and Objects

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Problem Solving

Solving a problem involves many activities: understand the problem design a solution consider alternatives and refine the solution implement the solution test the solution

These activities are not purely linear – they overlap and interact

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Problem Solving

Key point: break problem into manageable pieces e.g. submethods, recursive calls

When writing software, we design separate pieces that are responsible for certain parts of the solution

An object-oriented approach lends itself to this kind of decomposition breaks problems into objects and classes

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Objects

Informally – bits of code that represent real objects in the world

Example: an object might represent an employee at a company

Each employee object handles the processing and data management for that employee

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Objects

An object has: state – descriptive characteristics behaviours –

what it can do what can be done to it

The state includes fixed and variable characteristics

Behaviours can change the state

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Objects

Consider a bank account object state includes:

account balance (varies over time) account number (fixed)

behaviours include: deposit/withdrawal (change balance) opening/closing

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Classes

Java uses the word “class” in two different ways collections of methods (like class libraries) descriptions of an object type

So far, we have only really worked with the first kind of class

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Classes

An object is defined by a class Think of a class as a blueprint for creating objects

of a certain type Multiple objects can be created from the

same class “Instances” of the class

The class that contains the main method represents the whole program

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Objects and Classes

Bank Account

A class(the concept)

John’s Bank AccountBalance: $5,257

An object(the realization)

Bill’s Bank AccountBalance: $1,245,069

Mary’s Bank AccountBalance: $16,833

Multiple objectsfrom the same class

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Inheritance One class can be used to derive another

via inheritance

Classes can be organized into hierarchies

Bank Account

Account

Charge Account

Savings Account

Checking Account

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Containment

Sometimes an object “contains” objects of a different type This is not the same as inheritance

For example, a class Bicycle may involve: 2 Wheel objects 1 Chain object 1 Frame object A Bicycle itself might inherit from Vehicle A Bicycle might belong to a Rider

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Bicycle

Containment A graphical “contains-a” relationaship

WheelWheel

Chain Frame

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Creating Objects

Generally, we use the new operator to create an object

title = new String ("Java Software Solutions");

This calls the String constructor, which isa special method that sets up the object

Creating an object is called instantiation

An object is an instance of a particular class

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Creating Classes

Instantiating the class with new creates an object in memory

Each class has one or more special methods called “constructors” called when the object is created used to set initial state parameters sometimes arguments are passed …define our own next class…

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Example: String Objects

We have already seen the String class//constructor

String s = new String(“example”);

//String methods:

char c = s.charAt(3);

// c = ‘m’;

String s1 = s.concat(“123”);

//s==“example” & s1==“example123”

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String Class Constructors

String s1; //create reference… no constructor

s1 = new String(); //initialize to empty string

String s2 = new String(“blah blah”);// initialize from a string “blah blah”

char[] chars = {‘a’,’b’,’c’};String s3 = new String(chars);

//initialize from an array of characters

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A Reminder about References For primitive types:

num1 38

num2 96Before:

num2 = num1;

num1 38

num2 38After:

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A Reminder about References For object references:

name2 = name1;

name1

name2Before:

"Steve Jobs"

"Steve Wozniak"

name1

name2After:

"Steve Jobs"

"Steve Wozniak"

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Garbage Collection

When there are no longer any references to an object, it can no longer influence the running of the program e.g. the string “Steve Wozniak”

In this case, Java automatically returns this memory to the system This is called garbage collection

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Class Variables

The state of an object is given by class variables or data members the BankAccount class may have a class

variable called AccountBalance Typically these are declared private (later…) Methods can use and change these variables

getBalance() deposit(500)

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Getters and Setters

The state of an object is usually manipulated with methods Instead of accessing data members Assures the state remains consistent e.g. getName(); setName()

Methods that read and write data members are called accessors and mutators … or getters and setters

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Using “static”

Two types of methods in a class definition code library vs. object description How do we tell which is which?

Using static to differentiate static = code library non-static = object method

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Static Methods

Won’t be copied with each new object created

Can be called as functions, no names needed x = methodName(); x = ClassName.methodName();

Need all values passed as parameters

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Non-static Methods

Copied into each object instance Must be called with object reference

MyClass o = new MyClass();

x = o.MethodName(); Implicitly use the object that they are part of

as a parameter Used in classes that describe an object type

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Enumerated Types

Simple types that allow you to enumerate all possible values

The values are identifiers of your own choosing

Any number of values can be listed Example:

enum Season = {winter, spring, summer, fall};

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Enumerated Types

Once an enumerated type is defined, variables of that type can be declaredSeason time;

time = Season.fall; Only admissible identifiers are allowed

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Why?

Just a simple way to specify a simple type This could easily be done with full class

declarations Could also be done with integer arrays (internally, types are stored as integers)

Can not assign numeric identifiers We really won’t use these much in class

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Wrapper Classes

The java.lang package contains wrapper classes that correspond to each primitive type:

Primitive Type Wrapper Class

byte Byte

short Short

int Integer

long Long

float Float

double Double

char Character

boolean Boolean

void Void

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Wrapper Classes

The following declaration creates an Integer object which represents the integer 40 as an object

Integer age = new Integer(40);

An object of a wrapper class can be used in any situation where a primitive value will not suffice

For example, some objects serve as containers of other objects

Primitive values could not be stored in such containers, but wrapper objects could be

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Wrapper Classes

Wrapper classes also contain static methods that help manage the associated type

For example, the Integer class contains a method to convert an integer stored in a String to an int value:

num = Integer.parseInt(str);

The wrapper classes often contain useful constants as well

For example, the Integer class contains MIN_VALUE and MAX_VALUE which hold the smallest and largest int values

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Autoboxing

Autoboxing is the automatic conversion of a primitive value to a corresponding wrapper object:

Integer obj;int num = 42;obj = int;

The assignment creates the appropriate Integer object

The reverse conversion (called unboxing) also occurs automatically as needed

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How To Make a Wrapper

class IntValue{

private int iValue;

public int getValue(){

return iValue;}

public void setValue(int i){

iValue = I;}

}

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How To Make a Wrapper

You don’t need to do this… Just use Integer It is better in that it has many more methods

But this code shows why these are called wrapper classes It’s like wrapping an integer inside a package

Essentially the only time we use wrapper classes is for holding primitive types