chapter 71© copyright janson industries 2014 avoiding data entry errors ▮ catching an exception...

Chapter 7 1© copyright Janson Industries 2014

Avoiding Data Entry Errors

▮ Catching an Exception

▮ Formatters

▮ StringBuffer

▮ Choice

▮ Iteration

▮ Method Overloading

Non-graded Assg


Lots of different types of errors:

Entering a qty of 1 and price of 1.50, results in cost =

$1.5975

Considered a Soft error (no exception)


Throwing Exceptions

▮ When a hard error is encountered, the JVM throws an exception

▮ Example: trying to divide by "four”

▮ The operating system does not react well…


Normally a user: Enters valid values Clicks the Divide button Result is displayed in the label


Entering the incorrect value “four” and clicking Divide does

not result in an exception being displayed on the

Frame...

... it's displayed in the console


Run Time Exceptions

Or worse yet, in production

Is this how the user should see an exception/error?


Throwing Exceptions

▮ To handle the error programmatically, program must catch the exception

▮ For example, dividing two user supplied values can result in an exception(s)

▮ Here’s the code for calculator example…

Chapter 7 © copyright Janson Industries 2014 8

Calculator Code

Various imports

Various variables


private Checkbox divCB = null;

private Button exitBtn = null;

private Button execBtn = null;

private Label resultLbl = null;

CheckboxGroup arithFunc = new CheckboxGroup();

private double result, firstNum, secondNum;public void windowActivated(WindowEvent e){}

public void windowClosed(WindowEvent e) {}

public void windowClosing(WindowEvent e) {

this.dispose();}

public void windowDeactivated(WindowEvent e) {}

public void windowDeiconified(WindowEvent e) {}

public void windowIconified(WindowEvent e) {}

public void windowOpened(WindowEvent e) {}

public void actionPerformed(ActionEvent e) {

if (e.getSource() == exitBtn)

{ System.exit(0);}

else

{this.doCalc();}}

public void itemStateChanged(ItemEvent e) {

this.doCalc();

}

WindowListener methods

Action Listener methodItem Listener

method

Make result, firstNum, and secondNum class level

variables


The workhorse

private void doCalc() {

String msgBegin = new String("The result of ");

String msgPrep = new String(" ");

firstNum = Double.valueOf(numeratorTF.getText());

secondNum = Double.valueOf(denominatorTF.getText());

if (addCB.getState()) {

msgBegin = msgBegin + "adding ";

msgPrep = msgPrep + "to ";

result = firstNum + secondNum;

} else {

if (subCB.getState()) {

msgBegin = msgBegin + "subtracting ";

msgPrep = msgPrep + "from ";

result = firstNum - secondNum;

} else {

msgPrep = msgPrep + "by ";

if (multCB.getState()) {

msgBegin = msgBegin + "multiplying ";

result = firstNum * secondNum;

} else {


if (divCB.getState()) {

msgBegin = msgBegin + "dividing ";

result = firstNum / secondNum;

}}}}


resultLbl.setText(msgBegin + secondNum + msgPrep + firstNum + " is " + result);

} else {

resultLbl.setText(msgBegin + firstNum + msgPrep + secondNum + " is " + result);

}

}

public static void main(String[] args) {

new Calculator();

}

public Calculator() {

super();

initialize();

}

main

Constructor

Chapter 7 © copyright Janson Industries 2014 12

Visual component objectsprivate Checkbox getAddBox() {

if (addBox == null) {

addBox = new Checkbox();

addBox.setLabel("Add");

addBox.setCheckboxGroup(arithFunc);

addBox.setBounds(new Rectangle(18, 101, 43, 23));

addBox.addItemListener(this);

}

return addBox;

}

private Checkbox getSubBox() {

if (subBox == null) {

subBox = new Checkbox();

subBox.setBounds(new Rectangle(75, 101, 66, 23));

subBox.setLabel("Subtract");

subBox.setCheckboxGroup(arithFunc);

subBox.addItemListener(this);

}

return subBox;

}

private Checkbox getMultBox() {

if (multBox == null) {

multBox = new Checkbox();

multBox.setBounds(new Rectangle(155,101, 59, 23));

multBox.setLabel("Multiply");

multBox.setCheckboxGroup(arithFunc);

multBox.addItemListener(this);

}

return multBox;

}

private Checkbox getDivBox() {

if (divBox == null) {

divBox = new Checkbox();

divBox.setBounds(new Rectangle(228, 101, 54, 23));

divBox.setLabel("Divide");

divBox.setCheckboxGroup(arithFunc);

divBox.addItemListener(this);

}

return divBox;

}

© copyright Janson Industries 2014

private TextField getNumeratorTF() {

if (numeratorTF == null) {

numeratorTF = new TextField();

numeratorTF.setBounds(new Rectangle(196, 37, 36, 23));

}

return numeratorTF;

}

private TextField getDenominatorTF() {

if (denominatorTF == null) {

denominatorTF = new TextField();

denominatorTF.setBounds(new Rectangle(196, 69, 36, 23));

}

return denominatorTF;

}

private Button getExitBtn() {

if (exitBtn == null) {

exitBtn = new Button();

exitBtn.setLocation(new Point(240, 167));

exitBtn.setLabel("Exit");

exitBtn.setSize(new Dimension(50, 23));

exitBtn.addActionListener(this);

}

return exitBtn;

}

private Button getExecBtn() {

if (execBtn == null) {

execBtn = new Button();

execBtn.setLocation(new Point(125, 167));

execBtn.setLabel("Execute");

execBtn.setSize(new Dimension(50, 23));

execBtn.addActionListener(this);

}

return execBtn;

} Chapter 7 13

Visual component

objects


private void initialize() {

resultLbl = new Label();

resultLbl.setBounds(new Rectangle(12, 129, 278, 23));

resultLbl.setAlignment(Label.CENTER);

resultLbl.setText("");

denominatorLbl = new Label();

denominatorLbl.setBounds(new Rectangle(46, 69, 139, 23));

denominatorLbl.setText("Enter second number: ");

denominatorLbl.setAlignment(Label.RIGHT);

numeratorLbl = new Label();

numeratorLbl.setBounds(new Rectangle(74, 37, 111, 23));

numeratorLbl.setAlignment(Label.RIGHT);

numeratorLbl.setText("Enter first number: ");

this.setLayout(null);

this.setSize(300, 200);

this.setTitle("Frame");

this.addWindowListener(this);

this.add(numeratorLbl, null); this.add(denominatorLbl, null);

this.add(getNumeratorTF(), null); this.add(getDenominatorTF(), null);

this.add(getMultCB(), null); this.add(getAddCB(), null);

this.add(getSubCB(), null); this.add(getDivCB(), null);

this.add(getExitBtn(), null); this.add(getExecBtn(), null);

this.add(resultLbl, null); this.setVisible(true); }

}

initialize


Run Time Exceptions

▮ How do you identify statements that might generate exceptions?

▮ Try to cause errors through destructive testing

▮ Input bad data▮ Null value, wrong data type, incomplete value

▮Make resources unavailable▮ Delete an expected file, no Internet connection


Run Time Exceptions

▮ By typing in bad values like “four” and 0, exceptions are generated

▮ Stack trace messages will identify

▮ Line of code that caused the exception

▮ The particular exception that was generated

▮ Read through stack for message pertaining to your class


Run Time Exceptions

Specific exception generated

Class, method, statement number causing exception


Run Time Exceptions

▮ It's doCalc that

▮ Converts the text field data to numeric

▮ Does the divide

▮ These statements can cause ▮ NumberFormat and Arithmetic exceptions


Catching Exceptions

▮ Programs can catch exceptions

▮ Catching tells JVM that the application will handle the exception

▮ Why catch?▮ Solve the problem programmatically▮ Offer users chance to fix problem▮ Display better messages (than the

JVM) so users can fix the problem


Try and Catch

▮ Try identifies statements that may throw an exception

▮ Catch(es):▮ Immediately follow try▮ Identify the exception to be caught▮ Contain code to handle the specific

exception try {

statements that may throw an exception

}

catch (exception_object_type exception_variable) {

statements to perform if specific exception occurs

}


RAD and Exceptions

▮ RAD will put in try and catch(es) for the appropriate exceptions that can be thrown▮ Highlight statement(s)▮ Click Source, Surround With, Try/catch Block

▮ Alternatively, specify the Exception superclass and all subclasses will be caught▮ catch (Exception e) { } catches any

exception

▮ Sloppy programming?


private void doCalc() {

msgBeginning = new String("The result of ");

msgVerb = new String(" ");

try {

firstNum = Double.valueOf(numeratorTF.getText());

secondNum = Double.valueOf(denominatorTF.getText());

} catch (NumberFormatException e) {

resultLbl.setForeground(Color.red);

resultLbl.setText("You must enter the value " +

"in number format (ex: 9, 192.4, etc.)");

}

: : : : :

Try and Catch

Must import java.awt.Color also


Let RAD do it for you


Get rid of the stack trace print and …


… add error handling code into catch


When run - oops

Also, when valid data re-entered and executed...


What's going on?

... the valid message is red.


Catching an Exception

▮ Also, catching an exception does not stop the program execution

▮ So, need to change code such that:▮ If input error, no calculation is done ▮ Reset result label text color to black

▮ Lots of ways to implement this:▮ Need a new Boolean variable to indicate

if the data is valid/invalid▮ Create a separate input validation

method


Moved validation from doCalc to validateInput

private boolean isInputValid;

Add code to create the boolean class level variable

▮ At beginning of validateInput, add code to reset label color and boolean value

▮ When exception caught set boolean to false

▮ actionPerformed and itemStateChanged invoke validateInput first and if isInputValid is true, invoke doCalc


Catching an Exception

isInputValid = true;

resultLbl.setForeground(Color.black);


Doh! – the error msg doesn't fit

Run to test


Throwing an Exception

▮ Usually there are lots of edits & audits on input data and class must throw exceptions

▮ For instance, negative numbers should not be accepted but do not cause an exception▮ Check for negative values (with an if)▮ If negative, throw an exception▮ Have the catch:

▮ Blank out the negative value(s)▮ Display a message


Throwing an Exceptiontry {

: : : :

if ((firstNum<0) || (secondNum<0)) {

throw new NumberFormatException(); } }

catch (NumberFormatException err){

: : : :

if (firstNum < 0) {

numeratorTF.setText(" ");

resultLbl.setText("Quit messin' with negative numbers!");

}

if (secondNum < 0) {

denominatorTF.setText(" ");

resultLbl.setText("Quit messin' with negative numbers!");

} }


Click on

Divide

The negative number is erasedAnd the message is displayed


StringBuffers

▮ More efficient than Strings:

▮ Strings are immutable▮ Assigning a new value to a String variable

creates a new String object▮ Old String object still exists and is

unreferenced

▮ Changing the StringBuffer value does not create a new StringBuffer object


StringBuffers

▮ More flexible than Strings:

▮ Append – adds text to the end of a string buffer (i.e. concatenates)

▮ Insert – places text at a specified location within a string buffer

▮ Replace – substitutes text for a specified substring within the string buffer


StringBuffer Examples

▮ Results in: Chicken wings are very tasty.

▮ Results in: Chicken wings are good for you.

StringBuffer a = new StringBuffer("Chicken wings are tasty.");

a.insert(18, "very ");

System.out.println(a);

StringBuffer a = new StringBuffer("Chicken wings are tasty.");

a.replace(18, 23, "good for you");

System.out.println(a);


StringBuffer

▮ Calculator creates message text unique for each operation

▮ The result of dividing 7 by 2

▮ The result of adding 7 to 2

▮ With Strings:▮ Can only use concatenation▮ Each concatenation creates a new

String Object – very inefficient


StringBuffer

▮ Append correct arithmetic verb to msgBeginning StringBuffer

if (addBox.getState());

{

msgBegin.append("adding ");

result = firstNum + secondNum;

}

StringBuffer msgBegin =

new StringBuffer("The result of ");

StringBuffer msgPrep = new StringBuffer(" ");


if (addCB.getState()) {

msgBegin.append("adding ");

msgPrep.append("to ");

result = firstNum + secondNum; }

else { if (subCB.getState()) {

msgBegin.append("subtracting ");

msgPrep.append("from ");

result = firstNum - secondNum; }

else { msgPrep.append("by ");

if (multCB.getState()) {

msgBegin.append("multiplying ");

result = firstNum * secondNum; }

else { if (divCB.getState()) {

msgBegin.append("dividing ");

result = firstNum / secondNum;

}

}

} }



resultLbl.setText(msgBegin.append(secondNum) .append(msgPrep).append(firstNum)

.append(" is ").append(result).toString());

} else {

resultLbl.setText(msgBegin

.append(firstNum).append(msgPrep)

.append(secondNum).append(" is ")

.append(result).toString());

}

▮ Fix creating the message▮ No concatenation, append▮ To set label, must convert to String

StringBuffer


Editing Data

▮ The best way to avoid data entry errors is to prevent them▮ Limit the values that can be entered

▮ A Choice component only allows a value from a list of values to be selected and entered

▮ Adv: using one choice instead of many visual components (e.g. checkboxes) means less coding


Choice

▮ Combines a TextField and drop-down list

▮ To define a choice and populate it with values:Choice schoolsCB = new Choice();

schoolsCB.add(“Harvard”);

schoolsCB.add(“Yale”);

schoolsCB.add(“Princeton”);

schoolsCB.setBounds(162,100,200,20);

this.add(schoolsCB);


Choice

▮ When user clicks on choice button, the list will be displayed

▮ To retrieve a selected list item:▮ Create and add an ItemListener to the

Choice▮ Define an itemStateChanged method▮ Use getSelectedItem method

public class ChoiceEx extends Frame implements

ActionListener, ItemListener, WindowListener {...

public void itemStateChanged(ItemEvent choice)

{ selectedSchool = schoolsCB.getSelectedItem();

...}


CalcChoice

▮ Pull it all together:

▮ Use choice instead of checkboxes

//import java.awt.Checkbox;

//import java.awt.CheckboxGroup;

import java.awt.Choice;


CalcChoice

▮ One Choice instead of four check boxes and a check box group means▮ One variable instead of five▮ One get component method instead of five

public class CalcChoice extends Frame implements ActionListener, ItemListener, WindowListener {

: : : : : :

private Choice arithFuncChoice = null;

//CheckboxGroup arithFunc = new CheckboxGroup();

//private Checkbox multCB = null;

//private Checkbox addCB = null;

//private Checkbox subCB = null;

//private Checkbox divCB = null;

StringBuffer msgBegin = new StringBuffer("The result of ");

StringBuffer msgPrep = new StringBuffer(" ");


private Choice getArithFuncChoice() {

if (arithFuncChoice == null) {

arithFuncChoice = new Choice();

arithFuncChoice.setBounds(new Rectangle(120, 100, 89, 21));

arithFuncChoice.add("Add");

arithFuncChoice.add("Subtract");

arithFuncChoice.add("Multiply");

arithFuncChoice.add("Divide");

arithFuncChoice.addItemListener(this);

}

return arithFuncChoice;

}

CalcChoice

▮ Added this…

▮ but got rid of…


//private Checkbox getMultCB() {

// if (multCB == null) {

// multCB = new Checkbox();

// multCB.setBounds(new Rectangle(155, 101, 59, 23));

// multCB.setLabel("Multiply");

// multCB.setCheckboxGroup(arithFunc);

// multCB.addItemListener(this);

// }

// return multCB;

//}

//private Checkbox getAddCB() {

// if (addCB == null) {

// addCB = new Checkbox();

// addCB.setBounds(new Rectangle(18, 101, 43, 23));

// addCB.setLabel("Add");

// addCB.setCheckboxGroup(arithFunc);

// addCB.addItemListener(this);

// }

// return addCB;

//}


//private Checkbox getSubCB() {

// if (subCB == null) {

// subCB = new Checkbox();

// subCB.setBounds(new Rectangle(75, 101, 66, 23));

// subCB.setLabel("Subtract");

// subCB.setCheckboxGroup(arithFunc);

// subCB.addItemListener(this);

// }

// return subCB;

//}

//private Checkbox getDivCB() {

// if (divCB == null) {

// divCB = new Checkbox();

// divCB.setBounds(new Rectangle(228, 101, 54, 23));

// divCB.setLabel("Divide");

// divCB.setCheckboxGroup(arithFunc);

// divCB.addItemListener(this);

// }

// return divCB;

//}


//this.add(getMultCB(), null);

//this.add(getAddCB(), null);

//this.add(getSubCB(), null);

//this.add(getDivCB(), null);

CalcChoice

▮ Added this…

▮ but got rid of…

this.add(getArithFuncChoice(), null);


If Add selected

If Subtract

CalcChoice

▮ Check the Choice value


if (arithFuncChoice.getSelectedItem()== "Multiply") {

msgBegin.append("multiplying ");

result = firstNum * secondNum;

} else {

if (arithFuncChoice.getSelectedItem()== "Divide") {

msgBegin.append(" dividing ");

result = firstNum / secondNum; }

} } }

if (arithFuncChoice.getSelectedItem()== "Subtract") {

resultLbl.setText(msgBegin.append(secondNum)

.append(msgPrep).append(firstNum).append(" is ")


} else {resultLbl.setText(msgBegin.append(firstNum)

.append(msgPrep).append(secondNum).append(" is ")


}

If multiply

If Divide

CalcChoice


CalcChoice

▮ Create CalcChoice such that

▮ Arithmetic operations are in a Choice

▮ Uses a StringBuffer to build the message

▮ Email CalcChoice.java to

▮ [email protected]


Non-graded Assg


Formatters

▮ Work a little differently▮ You don’t create an instance/object▮ You get an instance from the format

class

▮ 4 steps to format▮ Import format class▮ Get an instance of the formatter from the

format class▮ Set the formatter properties (optional)▮ Use the formatter’s format method


Number Formatter

▮ Results in: 3.0

3.3333333333333335

34.565

System.out.println(9.0/3.0);

System.out.println(10.0/3.0);

System.out.println(34.565);


Number Formatter

▮ Results in: 3

3.333

34.565

34.565

import java.text.NumberFormat; : : : :

NumberFormat nf = NumberFormat.getInstance(); : : : :

System.out.println(nf.format(9.0/3.0));


System.out.println(nf.format(34.565));


Defaults are:3 decimals

maxno decimal

min


Number Formatter

▮ Results in: 3

3.33

34.56

34.57

import java.text.NumberFormat; : : : :

NumberFormat nf = NumberFormat.getInstance();

nf.setMaximumFractionDigits(2); : : : :






Number Formatter

▮ Results in: 3.00

3.33

34.56

import java.text.NumberFormat;

: : : :

NumberFormat nf = NumberFormat.getInstance();

nf.setMaximumFractionDigits(2);

nf.setMinimumFractionDigits(2);





Currency Formatter

▮ Results in…

import java.text.NumberFormat;

: : : :NumberFormat cf = NumberFormat.getCurrencyInstance();

: : : :msgLbl.setText("The cost of this" +

" transaction is: $" + cf.format(cost));

3 items costing $1.77 each with 6.5% tax

▮ Results in: 5.65515

cost =(3 * 1.77) * 1.065

▮ Currency defaults are usually good enough


Currency Formatter


Date and Time Formatters

▮ A little more complicated

▮ First of all, you can get the current date by

▮ Importing java.util.Date

▮ Creating a date objectimport java.util.Date;

: : : : Date d = new Date();

System.out.println(d);

▮ Results in: Tue May 31 13:11:59 EDT 2014


DateFormat

▮ DateFormat class has predefined formats

▮ To use a DateFormat:▮ Import DateFormat class

▮ Get a DateFormat instance (getDateInstance) and specify the format to use

▮ Create a date object

▮ Use date format object’s format method and pass the date object as a parameter


DateFormat import java.text.DateFormat; : : : :

Date d = new Date();

DateFormat dfShort= DateFormat.getDateInstance(DateFormat.SHORT);

DateFormat dfMed = DateFormat.getDateInstance(DateFormat.MEDIUM);

DateFormat dfLong = DateFormat.getDateInstance(DateFormat.LONG);

DateFormat dfFull = DateFormat.getDateInstance(DateFormat.FULL);

System.out.println(d);

System.out.println(dfShort.format(d));

System.out.println(dfMed.format(d));

System.out.println(dfLong.format(d));

System.out.println(dfFull.format(d));

Results in: Tue Mar 13 14:38:59 EDT 2012

3/13/12

Mar 13, 2012

March 13, 2012

Tuesday, March 13, 2012


DateFormat for Time

System.out.println(d.getTime());

DateFormat tfShort = DateFormat.getTimeInstance(DateFormat.SHORT);

DateFormat tfMedium = DateFormat.getTimeInstance(DateFormat.MEDIUM);

DateFormat tfLong = DateFormat.getTimeInstance(DateFormat.LONG);

DateFormat tfFull = DateFormat.getTimeInstance(DateFormat.FULL);

System.out.println(tfShort.format(d));

System.out.println(tfMedium.format(d));

System.out.println(tfLong.format(d));

System.out.println(tfFull.format(d));

Results in: 13068621902251:16 PM1:16:30 PM1:16:30 PM EDT1:16:30 PM EDT

Time stored as a number

In USA, LONG & FULL are the same


SimpleDateFormat

▮ Allows you to define a format

▮ To use a SimpleDateFormat:▮ Import SimpleDateFormat class▮ Create a SimpleDateFormat object and

specify the format using date format symbols▮y – year, s – seconds, a – (AM or PM)▮M – month, m – minute▮H – hour (0-23), h – hour (1-12)▮d – day of month (1-31), D – day of year▮z – time zone


SimpleDateFormat

Results: 05/31/201401:19 PM

1 O'clock PM, Eastern Daylight Time

151

import java.text.SimpleDateFormat;

: : : :

SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy");

SimpleDateFormat stf = new SimpleDateFormat("hh:mm a");

SimpleDateFormat st2f = new SimpleDateFormat("h 'O''clock' a, zzzz");

SimpleDateFormat sd2f = new SimpleDateFormat("D");

System.out.println(sdf.format(d));

System.out.println(stf.format(d));

System.out.println(st2f.format(d));

System.out.println(sd2f.format(d));


Iteration

▮ WHILE and DO WHILE allow looping based on a condition

▮ FOR allows looping for a set number of iterations based on a ▮ Starting value

▮ Increment value

▮ Ending value


Iteration

▮ WHILE allows looping based on a condition

▮ FOR allows a certain number of iterations

while (eof == 0) {

statements to be repeated;

}

for (int j = 0; j < max; j++) {


}


Iteration

▮ DO WHILE checks the condition after the statements are executed

▮ Condition is checked at end of loop ▮ Therefore, the loop will be executed at

least once

do {


} while (eof == 0);


Iteration

▮ BREAK and CONTINUE provide exits from a loop

▮ Results in:

for (int j = 0; j < 5; j++) {

if (j == 2) {

break;

}

System.out.println("j = " + j);

}

j = 0

j = 1


Iteration

▮ Results in:

for (int j = 0; j < 5; j++) {

if (j == 2) {

continue;}

System.out.println(“j = “ + j);

}

j = 0

j = 1

j = 3

j = 4


Method Overloading

▮ Having methods with the same name but different signatures

▮ Signature = method name + parameters

▮ Why? Overloading makes your class more flexible/easier to use

chapter 71© copyright janson industries 2014 avoiding data entry errors ▮ catching an exception...

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