Anatomy of a Class: Revision Notes (Computer Science A 3.3)

These notes recap Topic 3.3. For worked examples, read the study guide.

Recap

• A class has a header, instance variables, constructors and methods.
• In this course classes are always public class Name and constructors are always public.
• Instance variables are private unless the specification says otherwise. Each object has its own copy of every instance variable.
• private restricts access to the declaring class. public allows access from classes outside it.
• Encapsulation hides implementation details: outside code uses public methods and never touches private data directly.
• Public methods can be called inside or outside the class. Private methods can only be called from inside it, so they are used as helpers.

Key relationships

Member	Typical form	Who can use it
Class	public class Parcel	Everyone
Instance variable	private double weightKg;	Code inside Parcel
Constructor	public Parcel(double w, String p)	Everyone (to create objects)
Behaviour other classes need	public boolean isHeavy()	Everyone
Helper	private boolean isValid(int points)	Code inside the class

Assumptions

• A constructor has the class’s name and no return type, not even void.
• If no constructor is written, Java supplies a default no-parameter constructor (Topic 3.4).
• Access errors are compile-time errors, for example weightKg has private access in Parcel.

Worked reminder

For Parcel a = new Parcel(25.5, "Leeds"); Parcel b = new Parcel(3.0, "Bath"); a.markDelivered();, a.describe() is "Leeds (delivered)" and b.describe() is "Bath (on the way)": marking a does not change b. For a Scoreboard, after addHome(3), addAway(5), addAway(-2) and addHome(2) both scores are 5, so leader() returns "tied".

Reading a class quickly in the exam

1. Find the header, then list the instance variables and their types: this is the object’s state.
2. Find the constructor: what initial values does it give each variable?
3. For each method, note whether it only reads the state (an accessor) or changes it.
4. Check every line that uses a member against its public or private access and the class it is written in.
5. When code creates two objects, keep a small state table for each, as in the study guide, and update it line by line.

Mistakes to avoid

• Writing void before a constructor name.
• Declaring instance variables public and then accessing them from another class.
• Calling a private method from another class.
• Forgetting that a class with no instance-variable assignment in the constructor leaves defaults (0, 0.0, false, null).
• Believing that changing one object changes all objects of the class.

Quick self-check

1. Which keyword makes a method callable only from inside its class? Answer: private.
2. Why make instance variables private? Answer: so other classes cannot change an object’s state except through public methods, which keeps its data safe (encapsulation).
3. What is wrong with public void Cup(int ml) in class Cup if it is meant to be a constructor? Answer: void makes it an ordinary method; a constructor has no return type.
4. Two Parcel objects exist. How many copies of weightKg are there? Answer: two, one per object.