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2009 Marty Hall

A Quick Summary

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2009 Marty Hall

For live Java training, please see training courses athtt ://courses.coreservlets.com/. Servlets JSP StrutsClassic, Struts 2, JSF 1. x , JSF 2.0, Ajax (with jQuery,

Dojo, Prototype, Ext, etc.), GWT, Java 5, Java 6, Spring,, .

Taught by the author of Core Servlets and JSP , More

Customized Java EE Training: http://courses.coreservlets.com/ Servlets, JSP, JSF 1. x & JSF 2.0, Struts Classic & Struts 2, Ajax, GWT, Spring, Hibernate/JPA, Java 5 & 6.

Developed and taught by well-known author and developer. At public venues or onsite at your location.

, .venues, or customized versions can be held on-site at your

organization. Contact hall@coreservlets.com for details.

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Agenda

The for/each loop Scanner and command-line input Generics Autoboxing Varargs Printf Reading command-line input

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For/Each Loops Idea

Instead of looping with a numeric index, set a localvariable to each item of a collection or array in order

Basic syntax for T e item: collectionOrArra OfT es

doSomethingWith(item);}

Applies to , , , , Arrays

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For/Each Loops: Example

public static void listEntries(String[] entries) {

System.out.println(entry);}

}

ResultString[] test = {"This", "is", "a", "test"};listEntries(test);

Thisisatest

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Reading Simple Input:

Basic Syntax ac a canner o ys em. n Call nextLine, nextInt, nextDouble, etc

Exam lesScanner inputScanner =

new Scanner(System.in);

String s = inputScanner.nextLine();int i = inputScanner.nextInt();double d = inputScanner.nextDouble();

Notes Scanner for System.in is useful only for simple testing

Use a GUI in a real application , ,

Process, and more

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Example: Printing Random

import java.util.*;

public class RandomNums {

public static void main(String[] args) {System.out.print("How many random nums? ");canner npu canner = new canner ys em. n ;

int n = inputScanner.nextInt();for(int i=0; i

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Autoboxing

You cannot insert primitives into tables, lists, orany ng e se expec ng an ec

Java provides wrapper types for this purpose (int Integer, etc.) In Java 5, s stem converts automaticall Performance Warning

Every insert converts to wrapper type (Integer above) Ever retrieval converts to rimitive t e int above Use arrays for performance-critical access

Old NewList nums = new ArrayList(); List nums =int i = someCalculation();nums.add(new Integer(i));...

new ArrayList();nums.add(someCalculation());...

Integer val =(Integer)nums.get(someIndex);

int num = val.intVal() + 1;

int num =nums.get(someIndex);

nums.add(num + 1);12

Example: Finding State Abbreviations

public class StateMap extends HashMap { public StateMap() {

stateMap = new HashMap();for(String[] state: stateArray) {

stateMap.put(state[0], state[1]);

} public HashMap getStateMap() {

return(stateMap);

private HashMap stateMap;

public String[][] getStateArray() {

} private String[][] stateArray =

{ { "Alabama", "AL" },as a , ,

{ "Arizona", "AZ" }, .}

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Example: Continued

public class MapTest {

StateMap states = new StateMap();HashMap stateAbbreviationTable =

s a es.ge a e ap ;Scanner inputScanner = new Scanner(System.in);System.out.println("Enter state names. " +

"Hit RETURN to quit.");String stateName;String abbreviation;

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Example: Continuedwhile(true) {

" ". .stateName = inputScanner.nextLine();if (stateName.equals("")) {

ys em.ou .pr n n ome aga n soon. ; break;

}a rev a on =

stateAbbreviationTable.get(stateName);if (abbreviation == null) {

abbreviation = "Unknown";}System.out.println(abbreviation);

}}

}15

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Simple Example: printf vs. println

General idea

argument list. %n means newline.

Exampleublic static void rintSomeStrin s

String firstName = "John";String lastName = "Doe";int numPets = 7;String petType = "chickens";System.out.printf("%s %s has %s %s.%n",

firstName, lastName, numPets, petType);" ". .

" has " + numPets + " " + petType + ".");

} Result:John Doe has 7 chickens.John Doe has 7 chickens.18

Controlling Formatting Different flags

s or s r ngs, or oa ng po n num ers, ordates, etc.

Various extra entries can be inserted To control width, number of digits, commas, justification,

type of date format, and more

printf uses mini-language; complete coverage would take

an entire lecture , For complete coverage, see

http://java.sun.com/j2se/1.5.0/docs/api/java/util/Formatter.html#syntax

Using + instead of , between arguments (printf uses varargs) Forgetting to add %n at the end if you want a newline (not automatic)

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Printf Formatting Options

Stands For Options Example%s String. Can output any data

type. If arg is Object, toStringis called.

%width sGives min num of chars.Spaces added to left if

needed.

printf("%8s", "Hi")outputs

" Hi"

%d Decimal.Outputs wholenumber in base 10. Also %xand %o for hex and octal.

%width d %,width dGives min width; inserts

commas.

printf("%,9d", 1234)outputs

" 1,234"

%f Floating point.Lets you lineup decimal point and controlprecision.

%width .precision f%,width .precision f

width includes comma and

printf("%6.2f", Math.PI)outputs

" 3.14"decimal point.

%t x Time (or date). %tA for day,%tB for month, %tY for year,

Date now = new Date();printf("%tA, %tB ,%tY", now, now, now)

. outputs"Thursday, November 17, 2005"

%n Outputs a newline and a %, respectively. No automatic newline as with println.20

Printf Example: Controlling Width

public static void printSomeSalaries() {

{ new CEO("Steve Jobs", 3.1234),new CEO("Scott McNealy", 45.5678),new e ezos , . ,new CEO("Larry Ellison", 6789.0),new CEO("Bill Gates", 78901234567890.12)};

" ". .for(CEO ceo: softwareCEOs) {

System.out.printf("%15s: $%,8.2f%n" ,. , .

}}SALARIES:

Steve Jobs: 3.12Scott McNealy: $ 45.57

Jeff Bezos: $ 567.98Larry Ellison: $6,789.00

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Printf Example: Controlling Width

public class CEO {

private double salary; // In billions

public CEO(String name, double salary) {this.name = name;this.salary = salary;

}

public String getName() { return(name); }

}

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Variable-Length Arguments The printf method takes any number of

arguments You can use overloading to define a few versions of

,number of arguments

To do this yourself, use " type ... variable " variable becomes an array of given type Only legal for final argument of method

public void printf(String format, Object ... arguments) public int max(int ... numbers)

Can call max(1, 2, 3, 4, 5, 6) or max(someArrayOfInts)

Use sparingly

You usuall know how man ar uments are ossible23

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Varargs: Example

public class MathUtils {

int minimum = Integer.MAX_VALUE;

for(int number: numbers ) {if (number < minimum) {

minimum = number;}

}return(minimum);

}

System.out.printf("Min of 2 nums: %d.%n",

min(2,1));" " min(2,4,6,8,1,2,3));

}}24

Summary For/Each loops are simple

an an e a s ua ons, u very easy n e cases nwhich they do apply

Scanner simplifies basic parsing Easier than String.split and Integer.parseInt

Generics simplify code . .

Autoboxing is convenient But can hide performance costs

Java finally has printf No more clumsy println's with string concatenation

For formatting, math, and other special-purposes methods

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2009 Marty Hall

Customized Java EE Training: http://courses.coreservlets.com/ Servlets, JSP, JSF 1. x & JSF 2.0, Struts Classic & Struts 2, Ajax, GWT, Spring, Hibernate/JPA, Java 5 & 6.

Developed and taught by well-known author and developer. At public venues or onsite at your location.26