© 2002 The McGraw-Hill Companies, Inc. All rights reserved. 1 McGraw-Hill/Irwin Programming with...

© 2002 The McGraw-Hill Companies, Inc. All rights reserved.

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McGraw-Hill/Irwin

Programming with Java

Chapter 4

Performing Calculations and Formatting Numbers


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• Perform calculations using the arithmetic operators.

• Calculate and assign the result using the assignment operators.

• Add and subtract using the increment and decrement operators.

• Perform multiple calculations correctly based on the precedence of operators.

• Retrieve string data from the screen and convert to numeric for calculations.

Objectives


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• Convert between data types both implicitly and explicitly.

• Find the formatting rules for the locale where an applet is run.

• Format output for currency, percent or decimal numbers.

• Catch input and calculation errors by using exception handling.

• Declare variables based on the numeric wrapper classes and perform operations using the methods of the classes.

Objectives Continued


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Calculation Operators

• The common arithmetic operators perform basic calculations.

• Then there is the increment and decrement operators.

• The shortcut notation and also the modulus.


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Arithmetic Operators

Arithmetic

Operator

Purpose

+ Addition

– Subtraction

* Multiplication

/ Division

% Modulus (remainder)


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Arithmetic Operators • In Java, = sign does not mean equality but it means

assignment.

• Java is more strict about data types than most programming languages.

• If you mix datatypes then you must be careful.

• For example: fltResult = fltValue/intCount; this statement has mixed data types but Java will allow it because the result is float data type.

• If you assigned to intResult instead of fltResult than it would be an invalid calculation. Java won’t allow it.


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Modulus

• Modulus is the remainder of a division.

• The modulus is sometimes very important such as calculations for hours and minutes.

• For example, the decimal value for the hours does not help to know the minutes.

• Conversion of minutes into hours and minutes, look at the calculation, if intMinutes = 90:

• intHours = intMinutes/60; and intMinutes = intMinutes%60;


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The Order of Arithmetic Operations

• The order of precedence is very important and it is as follows: parentheses – the highest, then it is exponent, then it is multiplication, division and modulus (equal in precedence), then it is addition and subtraction.

• The operation is read left to right.


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Exponentiation

• There is no operator for exponentiation.

• There are 2 ways to raise a number to the power:

• One by simple multiplication.

• The other by using the pow method which is found in the java.math package.

• The pow method requires double precision floating point arguments.


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The pow Method—General Format

Math.pow(double Number, double Power)


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The pow Method—Example

dblFiveSquared = Math.pow(5.0, 2.0);


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Assignment Operators

• In addition to the equal sign there are other assignment operators that perform a calculation and assign the result at the same time.

• They are handy for accumulating totals and counts.

• For example : fltTotal += fltPay;.

• This is the same as fltTotal = fltTotal + fltPay;.

• The assignment operators have the lowest precedence than the binary operators.


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Increment and Decrement Operators

• There are increment operators ++ and decrement operators --

• If we wish to increment one by you would write the statement:

intCount = intCount + 1 or intCount += 1 or use the increment operator intCount++

• Increment operator placed before the variable is known as prefix operation and placed after is known as a postfix operation.


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• Increment, Decrement operators have precedence over binary operators and they are read right to left.

• Prefix operation the variable is incremented before the statement is executed.

• Postfix operation the variable is incremented after the statement is executed.

• Decrement operator is similar to the increment operator with its prefix and postfix operations.

Increment and Decrement Operators Continued


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Converting between Data Types

• In other programming languages there is an automatic conversion between the data types but not in Java.

• In Java, the compiler check the data type for all operands and arguments and insists on the correct data type.

• When calculating with mixed numeric data types some operands may be implicitly converted or “promoted” so that precision is maintained.


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Implicit Numeric Type Conversion

• Java tries to maintain the greatest precision possible.

• This is needed when you have mixed data types but the result data type has to have the higher precision for the implicit conversion otherwise you will have an error.


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• The implicit conversions are as follows:• If both operands are one of the integer types (byte, short,

int, or long) and one of the operands is long, the Java compiler converts the other operand to long and the result of the calculation is long.

• If both operands are one of the floating point types (float or double) and one of the operands is double, Java converts the other operand to double and the result of the calculation is double.

Implicit Numeric Type Conversion Continued


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• If one operand is an integer type and one is long, the integer is converted to long and the result of the calculation is long.

• If one operand is an integer type and one is float, the integer is converted to float and the result of the calculation is float.

Implicit Numeric Type Conversion Continued


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Explicit Conversion

• You can convert from one numeric data type to another by casting.

• To create a cast operator, you put the primitive data type in parentheses before the variable.

• This converts the data type for calculation into a new variable but does not convert the original variable’s data type.

• For example : fltValue = (float) intTotal / (float) intUnit;


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Converting String to Numeric Data Types

• Java does not automatically convert strings to numeric data types.

• There are two ways to convert strings to numeric data types.

• For example : 1st way : fltValue = Float.parseFloat(strEntered);

• This statement is for Java 1.2 version and above.

• The 1st way only works for browsers that support Java 1.2 and above.

• For example: 2nd way : fltValue = Float.valueOf(strEntered).floatValue();


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Converting String to Numeric Data Types Continued

• This statement is supported by Java1.1 version and above.

• The 2nd way works for all browsers now that support Java1.1 version.

• You will notice that for both ways you are using the wrapper class Float to convert a string to primitive data type float.

• There are other classes such as Double, Integer, and Long. These are some of the numeric “wrapper” classes.

• You can use them in the same way as the example above replacing with numeric data type you want.


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Accuracy of Decimal Fractions

• When you display decimal fractions, you will find the outcome inaccurate, even if it is for the fraction 1/10.

• You probably will get 0.0098 which close to 0.1 but not accurate.

• The reason for this is that floating point values are stored as binary digits and an exponent (which can store very large numbers or very small) rather than decimal digits.

• Different versions of JVM and different browsers produce slightly different results.

• But if you need to keep accurate decimal fractions, such as for dollars and cents, you will have to use the BigDecimal class in the java.math package.


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Invalid Data Entry

• What happens if the user enter text instead of a value, or leaves the textfield blank and you try and convert it to a particular numeric data type.

• The valueOf method throws an exception.

• It will also throw an exception if the user enters a decimal value for a field that should be integer.


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Converting Numeric fields to String

• There are two ways to convert numeric to text.

• One way is an implicit conversion : by concatenation of numeric field to string (or empty string).

• For example: lblOutput.setText(“” + fltPay); or

• lblOutput.setText(“Pay : ” + fltPay);

• The second way is an explicit conversion using the valueOf method from the String class.

• For example:lblOutput.setText(String.valueOf(fltPay));


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Formatting Numeric Output

• Many countries have different formatting rules for displaying numbers and operating system stores the rules for formatting as a locale.

• You can retrieve the formatting specifications for a locale using subclasses of the NumberFormat class, which comes from the text package.

• The formatting classes allow you to format general numbers, currency, and percent fields.

• You can also specify the number of decimal places to display.


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The Instance Methods

• You must include the package, import java.text.*; , if you wish to use the NumberFormat class.

• You must create an object of the Number format class for which formatting you want.


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Decimal Numbers • You set the number of decimal positions you want the value to

display.• With setMaximumFractionDigits method, the numbers are

automatically rounded to the maximum number of digits.• DO NOT forget to set the minimum because if the numbers are

less than the maximum decimal positions it will not round up to the maximum.

• For example if the maximum decimal position is set to 2 and the minimum decimal position is not set and the value is 1 it will not display 1.00, it will only display 1.

• So to achieve for example 2 decimal places, you will have to:fmtDecimal.setMaximumFractionDigits(2); fmtDecimal.setMiniimumFractionDigits(2);


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Decimal Numbers Continued

• For example : strNumber = fmtDecimal.format(fltValue);

• Then it can be displayed as lblOutput.setText(strNumber);

• Besides setting the number of decimal positions, you can also set the number of Integer positions by setMaximumIntegerDigits and setMinimumIntegerDigits methods.


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Currency Formats

• The getCurrencyInstance method retrieves the local format for currency numbers.

• Fractional values are rounded to fit the format so you don’t have to set the maximum and minimum fraction digits.


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Percent Formats

• The getPercentInstance method to retrieve the local percent format.

• The format method converts the argument to a percent and formats it with a percent sign.

• You can set the minimum and maximum fraction digits.


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Selected Methods of the NumberFormat Class

Method Purpose

format Return a string that holds the formatted number.

getInstance Return the default number format for the current locale.

getCurrencyInstance Return the currency format for the current locale.

getNumberInstance Return the general-purpose number format for the current locale.

getPercentInstance Return the percent number format for the current locale.


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Selected Methods of the NumberFormat Class Continued

getScientificInstance Return the scientific number format for the current locale.

getAvailableLocales Return an array of available locales on the current system.

parse Return a number from a formatted string.

setMaximumFractionDigits Set the maximum number of digits to the right of the decimal point.

setMaximumIntegerDigits Set the maximum number of digits to the left of the decimal point.

setMinimumFractionDigits Set the minimum number of digits to the right of the decimal point.

setMinimumIntegerDigits Set the minimum number of digits to the left of the decimal point.


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Specifying a Locale

• You can force your application to use a specific style of formatting by specifying the locale in the getInstance method using the Locale object.

• For example: You can force a formatting adapted by France by:

NumberFormat fmtDecimal = NumberFormat.getInstance(Locale.FRENCH);

• You can also retrieve a list of available locales on the current system using the getAvailableLocales method.


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Handling Exceptions

• When user inputs data and enters the data wrong such as:

• Leaving the textfield blank and trying to convert to numeric.

• The textfield has a floating point value and then you try to convert it to an integer.

• The user enters zero and you try and divide by zero.

• All the above situations generate an Exception object.

• The correct terminology is “throws an exception” and it is your job to “catch” the exception.


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Try and Catch

• The statement(s) that might cause the error go in the try block ({}).

• There is always a catch block associated with a try block.

• If the try block executes successfully, then it will skip the catch block.

• If the try block does not execute successfully, then it will directly jump to the catch block skipping any statements in the try block.


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The try and catch—General Format

try{

// The statement(s) that might generate an exception.}catch(ErrorObject errIdentifier){

// The statement(s) to handle the exception.}


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The try and catch—Example

try{

//Integer division. Zero for intSecond throws an exception.intResult = intFirst / intSecond;lblIntegerResult.setText("" + intResult);

}catch(ArithmeticException err){

showStatus("Error in calculation");}


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Common Exception Objects

Exception Object Purpose

ArithmeticException Error caused by a calculation, such as division by zero.

NumberFormatException Problem converting a string to a number; occurs when the text field is blank or contains a fraction when an integer is required.

IllegalArgumentException Unable to format the value passed to one of the format methods.

FileNotFoundException File does not exist in path specified.

IOException Failure of an input or output operation such as reading from a file.

OutOfMemoryException Not enough memory to create an object.


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Unhandled Exceptions

• What happens if you have an exception and you don’t handle.

• It is passed up a level and if it is not handled there, it passed up again, until it reaches the top of the program, and it may display an error message.

• But in an applet, often it will just sit there with no message or anything to indicate there is a problem.


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Using the Wrapper Data Classes

• Each of the primitive data types has a wrapper class that you can use.

• Each of the data classes have methods that you can use which conforms closer to object-oriented programming.


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The Float Class—Constructors

Float(double) //Create a Float instance converting the double value

Float(float) //Create a Float instance converting the float value

Float(String) //Create a Float instance converting the string value


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The Float Class—Examples

Float FltRate = new Float(txtRate.getText()); //Convert string to float

Float FltHours = new Float(txtHours.getText());

Float FltTotal = new Float(0.0f);


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Some of the Methods in the Integer Class and Float Class


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Partial List of Float Methods

Method Purpose

intValue Returns the value of the Float as an integer.

longValue Returns the value of the Float as a long integer.

floatValue Returns the value of the Float as a float.

doubleValue Returns the value of the Float as a double.

toString Converts the float value to String.

isInfinite Returns true if the Float value is infinity.

isNaN Returns true if the Float value is not a number.


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Integer Class—Constructors

Integer(int)

Integer(String)


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Integer Class—Examples

Integer IntQuantity = new Integer(txtQuantity.getText());

Integer IntCount = new Integer(0);

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