Objects, Classes and Data Structures

What Objects, Classes and Data Structures covers

Objects, Classes and Data Structures is examined in Unit AS 1, Introduction to Object Oriented Development — a written examination. It covers 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 and arrays of objects.

The 11 subtopics

Each one has its own page. CCEA AS Software Systems Development students sit this topic in the first year.

• Constructors and creating objectsDefault constructor and constructor overloading.Open →
• Methods, parameters and return valuesParameter and an argument, return type of void and instance method.Open →
• Encapsulation and access modifiersThree principles underpinning object oriented programming, visibility of a class member and usual pattern of modifiers used to encapsulate a class.Open →
• Getters, setters and validating an object's stateAccessor method, mutator method and java naming convention for these two kinds of method.Open →
• Inheritance and the base classDeriving one class from another, what a subclass inherits and what it does not, calling the base constructor, overriding a method, and the is-a test for deciding whether inheritance is the right relationship.Open →
• Overriding methods and run-time polymorphismMethod overriding and abstract method.Open →
• Overloading methodsMethod overloading, key difference between overloading and overriding and risk of overloading a method with very similar parameter lists.Open →
• Choosing and organising data structuresMatching the structure to the job — an array for a fixed run of the same type, a list where the length changes, a record for related fields of different types — and what each costs to search and to grow.Open →
• Strings and string handlingA string as a sequence of characters counted from zero, length, extracting a substring, searching for one string inside another, joining strings, and why two strings are compared by value rather than by identity.Open →
• Static arrays of simple typesStatic array and parallel arrays.Open →
• Arrays of objectsDeclaring 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.Open →

What you need to know

Answer each one before you reveal it.

Take Objects, Classes and Data Structures further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

• An is-a relationship vs A has-a relationship

  An is-a relationship: every instance of the new class counts as a kind of the old one, so inheritance fits. A has-a relationship: it means one class simply owns another as an attribute.

Questions from this topic

Each one with the model answer underneath.

1. 1

   What is a constructor?

   Model answer

   A constructor is a special method that runs automatically when an object is created, and its job is to put the new object's attributes into a valid starting state.

2. 2

   How is a constructor recognised in Java source code?

   Model answer

   A Java constructor carries exactly the same name as its class and declares no return type at all, not even void.

3. 3

   What does the keyword new do in the statement Student s = new Student("Aoife"); ?

   Model answer

   The keyword new reserves memory for a fresh Student object, runs the Student constructor with the argument Aoife, and hands back a reference to that object, which the variable s then holds.

4. 4

   Explain why two objects created from the same class can hold different values.

   Model answer

   Every object receives its own copy of the instance attributes when it is created, so storing a value in one object leaves the attributes of every other object of that class untouched.

5. 5

   What is a default constructor?

   Model answer

   A default constructor is a no-argument constructor that the compiler supplies automatically when the programmer has written no constructor of their own, and every attribute is left at its type's default value.

6. 6

   A class Book declares only the constructor public Book(String title). Explain why the statement Book b = new Book(); will not compile.

   Model answer

   Writing any constructor stops the compiler supplying the no-argument default, so Book offers only the one-argument form and a call that passes nothing matches no constructor at all.

What is inside Levo

Six ways to work the same material.

Objects, Classes and Data Structures revision questions

Is Objects, Classes and Data Structures examined at AS in CCEA Software Systems Development?

Yes. It sits in AS Unit 1, which you take in the first year, so it is on the AS paper as well as counting towards the full A level.

What does Objects, Classes and Data Structures cover in CCEA AS-Level Software Systems Development?

11 subtopics: 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 and arrays of objects. Each one has its own page with questions and model answers.

How is Objects, Classes and Data Structures organised?

By the CCEA CL4 specification, subtopic by subtopic, so it matches the order you are taught it in.