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University of Limerick 1

Work with API’s


Learning OO programming

Learning a programming language can be broadly split into two part– Learning the language

» Sequence, selection and iteration

– Learning the API» Commonly used structure and services

» GUI’s, Input\Output, Data storage, Networking


Learning the language

abstract

boolean

break

byte

case

catch

char

class

const*

continue

default

do

int

interface

long

native

new

null

package

private

protected

public

return

short

double

else

extends

final

finally

float

for

goto*

if

implements

import

instanceof

static

super

switch

synchronized

this

throw

throws

transient

try

void

volatile

while

N.B. Keyword maked with * are unused


Learning the API

API stands for Application Programming Interface The interface (calling conventions) by which an application

programmer accesses the standard Java classes Java

– 2,000 Classes and Interfaces

– Spread over more than 75 packages


Java Class Library As we can see the Java provides a large collection of

classes that support and simplify many common programming activities– GUIs, TCP/IP sockets, CORBA, Compression (ZIP), 2D

Graphics, Encryption


Commonly used packages

java.lang – Provides classes that are fundamental to the design of the Java

programming language (including the Math class)

java.math – Provides classes for performing math functionality


Scale

At first the number of classes may seem impossible to learn but in most cases programmers are only required to know a specific subset

The subset they know often depends on what they are developing

What is of importance is that you can find and use new classes as you require or discover them in preexisting code


Comparison The Standard Dictionary

– Over 100,000 entries

Commonly used words– First 25 words make up about one-third of all printed material in

English

– First 100 words make up about one-half of all written material

– First 300 make up about sixty-five percent of all written material in English

www.duboislc.org/EducationWatch/First100Words.html


The Math Class

Description provided in Java API Collection of common math functions (sin, cos, sqrt, etc.). And two constants: PI and E Math.PI

– 3.141592653589793

Math.sqrt(25)– 5.0

Math.pow(2,10)– 1024

Math.cos(0)– 1.0

Math.cos(2 * Math.PI)– 1.0


How the Math Class Works

public class Math{ public static final double PI = 3.141592653589793;

public static double sin(double d){ .. } public static double sqrt(double d) { .. }

private Math(){} ..}

> Math.PI3.141592653589793> Math.sqrt(25)5.0


What's different about Math Class

It’s different from a typical Java class – It is a “stateless” class

– We only need one Math class (not multiple instances)

– No need to instantiate it (hence, no public constructor)

– All of its variables and methods are static

– static means “applies to the class as a whole” vs. “applies to an individual instance”


Math Class Description

When we look at the Maths class using the API documentation notice the phrase java.lang at the top of the main panel above the word Math– This means that the Math class is part of the core Java language

and hence can be used directly

Math Class Interface– Field Summary: Has two constants PI and E

– Constructor Summary: has no public constructor

– Methods Summary: many methods all which are static

– Method Details: e.g. sqrt() takes a double and returns a double


Packages and import Statements If a class is not part of java language i.e. java.lang, you'll see

package name

What is a package?– Basically it's a directory that has a collection of related classes – E.g. Random Class description contains: java.util.Random– Indicating that the Random code is stored in a file called

java/util/Random.class somewhere on your machine. – The java/util directory/folder is known as the "util", or utility package.

Since Random is not part of Java Language we need to tell Java where to find it by saying– import java.util.Random; – Another way is to use the asterisk "wildcard character": import java.util.*;


Random Class A class to create Random numbers

Constructor Summary shows the objects of this type can be created

– E.g. Random ran = new Random();

Method Summary shows that it can generate random values of types:

– integers, doubles etc.

– E.g. r.nextInt(6) – Generate a integer numbers between 0 (inclusive) and 6 (exclusive)

– How do I generate a number between 1 and 6 ?


Pseudorandom numbers The computer is not capable of generating truly random

numbers – The computer can only generate pseudorandom numbers--

numbers that are generated by a formula

– Pseudorandom numbers look random, but are perfectly predictable if you know the formula

» Pseudorandom numbers are good enough for most purposes, but not all--for example, not for serious security applications

– Devices for generating truly random numbers do exist » They are based on environmental noise, or on lava lamps

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