What does the statement Student[] group = new Student[3]; create?

The statement creates an array able to hold three Student references, and no Student object exists yet because every element starts as null.

A program executes Student[] group = new Student[3]; and then System.out.println(group[0]);. What is printed?

The statement prints null, because creating the array reserves three reference slots and sets each of them to refer to no object at all.

A program executes Student[] group = new Student[3]; and then group[0].getName();. What happens?

The program stops with a null pointer exception at run time, because element 0 still refers to no object and there is nothing on which getName could be called.

What does each element of an array of objects actually store?

Each element stores a reference to an object held elsewhere in memory rather than the object itself.

Contrast what is stored in an array of integers with what is stored in an array of objects.

An array of a simple type holds the values themselves inside the array, while an array of objects holds references and the objects sit separately in memory.

CCEA A-Level Software Systems Development CL4

Software and the Development Environment

Topic overview Systems software and application software The operating system and file management Source code files Binary, object and executable files Compilers, interpreters and the build process XML files and structured data Utility software and managing files on disk The integrated development environment Object oriented development platforms Open source software and licensing

Defining Data: Types, Variables and Classes

Topic overview Primitive types and their storage Reference types, objects and null Declaring variables, scope and lifetime Type casting and conversion Arithmetic operators and expressions Relational and logical operators Operator precedence and evaluation order Objects as occurrences of a class Abstract, base and inherited classes Attributes and the properties of a class Static variables in a class Interfaces and class reusability

Program Control Structures

Topic overview Sequence and the order of execution Methods and passing control between them Selection with IF and IF ELSE Nested IF and multi-way selection The switch statement Count-controlled repetition with FOR Conditional repetition with WHILE and DO WHILE Nested loops and tracing iterations Breaking out of loops and avoiding infinite loops Choosing and evaluating control structures

Objects, Classes and Data Structures

Topic overview Constructors and creating objects Methods, parameters and return values Encapsulation and access modifiers Getters, setters and validating an object's state Inheritance and the base class Overriding methods and run-time polymorphism Overloading methods Choosing and organising data structures Strings and string handling Static arrays of simple types Arrays of objects

Exception Handling and Managing Input and Output

Topic overview Why errors have to be trapped Types of error and how an exception is raised The try and catch block Catching more than one kind of exception The finally block and releasing resources Throwing exceptions and writing an exception class Checked and unchecked exceptions Reading and validating input from the command line Controlling screen output Text files Binary files Object files and serialisation

Testing an Object Oriented Application

Topic overview Why an object oriented application must be tested Choosing test data: normal, boundary and erroneous Writing a test plan for a class Testing a class method by method Testing constructors and object state Testing inheritance and overridden methods Testing error handling paths Dry runs and trace tables Test harnesses, drivers and stubs Recording and interpreting test results Evaluating a test plan and re-testing

Graphical User Interface Objects and Events

Topic overview Graphical user interfaces and console programs Forms and their common properties Labels, text boxes and text areas Buttons, check boxes and radio buttons Lists, combo boxes and other selection controls Properties, methods and events of a GUI object Containers, panels and laying out a form The event loop and the event queue Event handlers and registering a listener The event object and reading it inside a handler Button, mouse and keyboard triggers

Multiple Forms and Linking to Files

Topic overview Why an application needs more than one form Creating, showing, hiding and closing a form Modal and modeless forms Passing data between forms Menus, menu items and shortcuts Toolbars and navigation buttons Designing navigation for a multi form application Reading and writing text files Binary files and streams Object serialisation and object files Choosing between text, binary and object files

Designing, Testing and Evaluating an Event Driven Application

Topic overview Storyboards and what they show Building a storyboard from user requirements Prototyping and refining a design Error trapping in an event driven application Using try and catch inside an event handler Validating what a user types into a control Designing a test strategy for an event driven application Test plans and choosing test data Testing the paths a user takes through an interface Documenting and interpreting test results Evaluating an application against its requirements Evaluating features, functionality and usability

Reasons for Systems Development and Systems Methodologies

Topic overview Why organisations initiate systems development Analysis and design principles for business solutions Roles in development and the place of the user The purpose of a methodology and how methodologies evolved The waterfall model Iterative, incremental and spiral development Rapid application development The dynamic systems development method Scrum Extreme programming Prototyping in the development process UML modelling and simple diagrams

Managing Projects

Topic overview The purpose of project management Project management concepts and the project life cycle Roles and responsibilities in project management Resources needed to develop a project Estimating effort, duration and cost Gantt charts and the schedule Milestones and checkpoints Network diagrams and PERT Finding the critical path Float and how it is used Monitoring and controlling progress Managing risk and change

Testing Strategies

Topic overview Why testing matters throughout the development cycle Testing at the design stage Program and unit testing Integration testing System testing White box testing Black box testing Normal, boundary and erroneous test data User acceptance, alpha and beta testing The personnel involved in testing What a test plan contains Building a test plan for a scenario

Database Concepts and Entity Relationship Modelling

Topic overview What a database is and why one is used Database terminology: fields, records and keys The relational model The hierarchical model The network model Entities and attributes in an ER model Relationships and their degree Building an ER model for a problem From ER model to a working design Comparing the three database models

Normalisation and Structured Query Language

Topic overview Why data is normalised Un-normalised data and repeating groups First normal form Second normal form Third normal form Working a full normalisation through The advantages and limits of normalisation The principles of SQL SQL commands for defining and changing data SQL statements for retrieving data Joining tables and grouping results

Project Management and the Systems Approach in Practice

Topic overview Breaking a development into tasks and deliverables Estimating durations, effort and dependencies Documenting the plan as a Gantt chart Working out the critical path and float Milestones, reviews and user sign-off points Planning people, resources and skills Building a plan capable of reflecting change Recording progress against the plan Updating the plan to reflect slippage Recording change requests and versions of the plan Evaluating different approaches to a business problem Selecting and justifying a specific approach

Defining Requirements and Documenting the Design

Topic overview Eliciting requirements from the user Writing functional requirements Writing non-functional requirements Specifying the software and hardware the solution needs Producing and agreeing the user requirements specification Designing the data model and the tables Designing the inputs, the screens and the navigation Designing the outputs and the reports Documenting the processes and the logic of the solution Testing the design against the desired user outcomes Presenting the design and the solution using appropriate media

Building a Desktop Solution with a Database

Topic overview Applying relational techniques to a real solution Choosing the database and the development tool Connecting the application to the database Data adaptors Data sets Data commands and parameterised queries Binding the database to the interface Multiple forms and navigation Saving changes and protecting the data Testing and documenting the built solution

Testing and Evaluating the Solution

Topic overview Planning the testing of the finished solution Choosing test data: normal, boundary and erroneous Testing validation and error handling Testing the database and its queries Testing the user interface and the navigation Testing the whole solution and testing with the user Documenting test outcomes as evidence Diagnosing failures and retesting Evaluating the test procedures Evaluating the solution against the user requirements Evaluating delivery against the project plan Evaluating your own performance on the development

Objects, Classes and Data Structures

Arrays of objects

3 things to know
AS Unit 1

Overview

Declaring an array whose elements are objects, why every element starts empty, creating each object before it is used, and the error raised by reaching through an element that holds nothing.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Describe how the object with the highest value of an attribute is found in an array of objects.

Key term

A library program declares an array of 200 Book references but currently holds only 40 books. What is the cost of that decision?

Key term

What has to be done to an element of an array of objects before it can be used?

Questions from this subtopic

Each one with the model answer underneath.

1
What does the statement Student[] group = new Student[3]; create?
Model answer
The statement creates an array able to hold three Student references, and no Student object exists yet because every element starts as null.
2
A program executes Student[] group = new Student[3]; and then System.out.println(group[0]);. What is printed?
Model answer
The statement prints null, because creating the array reserves three reference slots and sets each of them to refer to no object at all.
3
What has to be done to an element of an array of objects before it can be used?
Model answer
Each element must be given an object of its own, as in group[0] = new Student("Aoife");, before any method can be called through that element.
4
A program executes Student[] group = new Student[3]; and then group[0].getName();. What happens?
Model answer
The program stops with a null pointer exception at run time, because element 0 still refers to no object and there is nothing on which getName could be called.
5
What does each element of an array of objects actually store?
Model answer
Each element stores a reference to an object held elsewhere in memory rather than the object itself.
6
Contrast what is stored in an array of integers with what is stored in an array of objects.
Model answer
An array of a simple type holds the values themselves inside the array, while an array of objects holds references and the objects sit separately in memory.

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Software and the Development Environment

Defining Data: Types, Variables and Classes

Program Control Structures

Objects, Classes and Data Structures

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A null is the of one.

absencepresencetype

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AA null means no value has been recorded.

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