chapter 05

Java Programming, 3eConcepts and Techniques

Chapter 5Arrays, Loops, and

Layout Managers Using External Classes

2Chapter 5: Arrays, Loops, and Layout Managers Using External Classes

Chapter Objectives

• Create and implement an external class

• Write code to create a constructor class method

• Construct an instance method to initialize instance variables

• Declare and construct an array using correct notation


Chapter Objectives

• Use layout managers with container components

• Code a counter-controlled loop using the for statement

• Correctly employ assignment and unary operators

• Use methods with Frame, Panel, TextArea and Choice components


Introduction

• This chapter illustrates how to create an object, complete with methods and attributes

• Objects have three key characteristics– Identity

• The object can be called and used as a single unit

– State• The object has various properties whose values might

change

– Behavior• The object can perform actions and have actions performed

upon it


The Reservations Program

• A stand-alone windowed application.


Problem Analysis

• When the user enters information and clicks the Book Room button, some visual element should change in the user interface to indicate the current status

• The program should be designed to easily accommodate modifications in the number of party rooms


Design the Solution


Design the Solution

• TextArea components display larger amounts of information than TextFields

• TextFields allow user entries

• A Choice component displays a drop-down list, which ensures valid entries

• A hidden checkbox component is used to clear the checkbox options

• A button triggers an event


Program Design

• The Reservations class is the driver class• Create variable dictionaries

– A class list of variables with their type and purpose• Create method prototypes, headers, and flowcharts


• Methods in the Reservation class

• Constructor methods– Define an instance of an object – Have no return data type – Have the same name as the object– Create an instance of the object internally


Rooms Class


Creating an External Class

• An external class is a class that is not a driver class• The Rooms class is an external class• External classes are declared public to be accessible by

all objects and to allow for inheritance


Arrays

• An array stores multiple data items of the same type in a single storage location

• Declare an array with the element type, a set of square brackets, and the array identifier name– int[] ages; or int ages[];

• Construct an array with the new keyword and the array length, or number of elements– ages = new int[100];– int[] ages = new int[100];


Arrays

• Retrieve the array’s length using the length property– int size = arrayName.length

• Assign values and access array elements using the index number of the element– An index number is assigned to each element,

beginning at zero and progressing by integers– temperature[3] = 78;

• Declare, construct, and assign values with one statement– boolean[] results = {true, false, true, false, true};


Arrays

• Index numbers can be any expression that evaluates to an integer

• Array elements can be used anywhere a variable is used– overtime = (hours[6] - 40) * rate[6] * 1.5

• Two-dimensional arrays are used to create tables of values– Two index numbers represent the number of rows

and columns.– int[][] myTable = new int[4,3];


Constructing an Instance

• Create a constructor method for the class

• Declare a class variable and call the class constructor in the driver class

• A class may have multiple constructors, each with different arguments– The practice of defining multiple methods with the

same name is called method overloading.


Counter-Controlled Loops

• A counter-controlled loop executes a specific number of times

• Java uses a for loop to implement a counter-controlled loop

• Assignment and unary operators are often used to update the counter– An assignment operator performs an arithmetic and

assignment operation all with one operator, thus providing a shortcut

– A unary operator only needs one operand to perform its function


Unary operators behave differently depending on whether the operator is positioned before or after the variable


Unary Operators in for Loops

• A for loop often uses the increment operator to access each element of an array

• To exit a for loop prematurely, assign the counter a number outside the condition range inside the loop


Instance Methods

• An instance method operates or manipulates variables for an external class

• Instance variables are variables manipulated within an instance method, which are local in scope

• When called by a driver class, instance methods must use the class.method() notation

• Instance methods typically include a user-friendly active verb in the method name


The bookRoom() Method


Creating Windowed Applications

• The AWT classes are abstract and provide only essential components which are obtained from the native environment when instantiated

• The Swing components use lightweight Java implementations of standard GUI controls

• A container is an object that contains other components– A Frame is a container for a collection of graphical

AWT components.– A program using a Frame needs to use the word,

extends, in the class header.


The Reservations Class


Choice Components

• A Choice component displays a restricted list through a drop-down list box with a box arrow


Calling Constructor Methods for the Reservations Class


Layout Managers

• Layout managers assist programmers organize components into predefined locations in a window

• If the user resizes the window, the size and position of the components are automatically adjusted


Flow Layout

• Change alignment with setLayout() and the constructor• Add components with the add() method


BorderLayout

• The add() method takes a parameter specifying the desired region

• The setLayout() method takes two integers parameters to specify the number of pixels between components


GridLayout

• All components must be the same size• The constructor takes the number of rows and columns

as parameters


CardLayout

• Each card may have its own layout manager• Only the top card on the stack is visible


GridBagLayout

• Components can be of varying size and can be added in any order


The Reservations() Constructor


Window Event-Handling Methods• A WindowListener is registered with a Frame with the

addWindowListener() method• A WindowAdapter class is used to provide the event-

handling methods for the Frame– Adapter classes implement abstract classes, providing

prewritten method for all the methods, any of which can be overridden by the programmer


The main() Method

• Execution of a program begins with the main() method

• The main() method can be placed anywhere within the class braces

• In a windowed application, the main() method creates the instance of the Frame using a call to the constructor method

• The main() method also sets attributes for the Frame


The main() Method


The actionPerformed() Method


The clearFields() Method

• Clears input fields after a successful booking.


Testing a Windowed Application

• Compile and correct errors• Use multiple test cases to test maximum values• Test with incorrect data


Chapter Summary

• Create and implement an external class

• Write code to create a constructor class method

• Construct an instance method to initialize instance variables

• Declare and construct an array using correct notation


Chapter Summary

• Use layout managers with container components

• Code a counter-controlled loop using the for statement

• Correctly employ assignment and unary operators

• Use methods with Frame, Panel, TextArea and Choice components

Java Programming, 3eConcepts and Techniques

Chapter 5 Complete

Arrays, Loops, and Layout Managers Using

External Classes

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