Chapter 6 - More About Problem Domain Classes1 Chapter 6 More About Problem Domain Classes

Chapter 6 - More About Problem Domain Classes 1

Chapter 6

More About Problem Domain Classes

Chapter 6 - More About Problem Domain Classes 2

Chapter 6 Topics

• More about writing problem domain classes

• Writing and invoking custom methods• Formatting numerical data for display• Using static variables and methods• Writing overloaded methods• Working with exceptions

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Writing a Definition for the Slip Class

• Process– Write class header– Write attribute definition statements– Write a parameterized constructor

• Argument data types must be assignment compatible with parameter data types

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Writing a Definition for the Slip Class

• Process (cont.)– Write accessors to populate the

attributes– Write a tellAboutSelf method

• Polymorphic method– Two methods with the same name residing in

different classes that behave differently – See Figure 6-2, pp 172– Write a tester class

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• See Figure 6-4 for the Sequence diagram

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Writing Custom Methods

• Standard Methods– Written to store and retrieve values

• Accessor methods– getXXX(), setXXX()

• Custom Methods– Written to do some processing

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Writing Custom Methods

• Custom Methods– Process

• Write method header– Public accessibility (if required)– Appropriate return type to match data type of

value returned (if any)• Write code to implement required process• Write tester class to demonstrate proper

operation

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Formatting Output

• NumberFormat & DecimalFormat Classes– NumberFormat Class

• Member of java.text package• Provides methods to format numerical data

as currency with commas, dollar signs, and decimal points

• Also provides for formatting currency for various countries

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Formatting Output

• NumberFormat & DecimalFormat Classes– NumberFormat Class

• Two steps to format data:– Invoke getCurrencyInstance method to obtain a

NumberFormat instance» NumberFormat currencyFormat =

NumberFormat.getCurrencyInstance();– Invoke the format method for the instance obtained

» System.out.println(“Currency: “ + currencyFormat.format(fee);

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Formatting Output

• NumberFormat & DecimalFormat Classes– DecimalFormat Class

• Member of java.text package• Provides methods to format numerical data

with commas and a decimal point

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Formatting Output

• NumberFormat & DecimalFormat Classes– DecimalFormat Class

• Two steps to format data:– Create an instance of DecimalFormat using the new

operator and pass the format mask» DecimalFormat decimalFormat = new DecimalFormat(“##,##0.00”);

– Invoke the format method for the instance obtained» System.out.println(“Decimal: “ +

decimalFormat.format(fee);

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Formatting Output

• Using Escape Sequences– Escape sequence

• The backslash character (\) followed by the escape character

• Used to display:– Characters that do not appear on the keyboard– Characters that have special meanings within certain

contexts • Example:

– Tab \t– Double quote in an output String “the \”real\” deal”

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Using Static Variables and Methods

• Instance Variables and Methods– A new instance receives its own copy of all

instance variables and methods• Methods not actually copied - to avoid redundancy

• Class Variables and Methods– A new instance shares a copy of all class

variables and methods• Keyword

– Static used to declare class variables and methods as static

• See Figure 6-11, pp. 185

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Overloading Methods

• Method Signature– Consists of:

• Method name• Its parameter list

– Java identifies a method by its signature

• Overloaded method– Methods within the same class having the

same name but a different signature

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Overloading Methods

• Overridden Methods (polymorphism)– A method with the same signature as an

inherited method• Replaces inherited method

• Polymorphic Method– A method in one class has the same

signature as a method in another class

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Overloading Methods• Overloading a Constructor

– Multiple constructors with the same name and different signatures

– Should have a constructor for each way it makes sense to instantiate objects of the class

• Overloading a Custom Method– Any method can be overloaded– Should have a method for each way it makes

sense to input data to perform the required process

• See Figure 6-14, pp. 191

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overloadconstructor

overloadcustommethod

Figure 6-14

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Working with Exceptions

• Exception– An object instance that notifies you of errors,

problems, and other unusual conditions that may occur when your system is running

– Keywords:• try• catch• finally• throw• throws

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Working with Exceptions• Exception Process

– When a client invokes a method that may create and throw an exception, the invoking code must be placed in a try block

– Server method indicates it may throw an exception by including throws keyword in header

– If exception is detected, server sends exception instance to invoking client using throw keyword

– Client catches exception in a catch block– finally block executes regardless of whether an

exception is caught

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Working with Exceptions

• Data Validation– If a method is to create and throw an

exception, its header must contain the throws keyword followed by the exception class it throws• public void setSlipId(int anId) throws

Exception• public void setWidth(int aWidth) throws

Exception

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Working with Exceptions• Catching Exceptions

– If a method contains throws in its header, the invoking code must be prepared to catch the exception

• Otherwise JVM will terminate processingtry {

method that throws exception}catch (Exception e) {

code that handles it}

– See Figure 6-18, pp. 197 – See Figure 6-19, pp. 201