Objects: Instances of Classes: Revision Notes (Computer Science A 1.12)

Short on time? This page is the recap. For explanations and worked examples, use the full study guide.

Recap

• An object is a specific instance of a class with defined attributes.
• A class is the formal implementation, or blueprint, of the attributes and behaviors of an object.
• A variable of a reference type holds an object reference, which can be thought of as the memory address of that object.
• Common attributes and behaviors of related classes can be put in a superclass. Classes that extend it are subclasses; this is an inheritance relationship.
• All classes in Java are subclasses of the Object class.
• Designing and implementing inheritance relationships is outside the course and exam.

Key relationships

Idea	Example
Class	Counter, String, Scanner
Object	one particular Counter; the text "banana"
Reference variable	Counter c; can refer to a Counter
Attribute	data stored in a variable (for Counter, count)
Behavior	a method (increment(), getCount())
Copy a primitive	int y = x; copies the value; later changes to x do not affect y
Copy a reference	Counter b = a; copies the reference; a and b are the same object
Change via one name	visible through every other name for that object
Declaring only	Counter c; creates a variable, not an object

Variable type	Holds	Can be null
int, double, boolean	a value	no
a class such as String or Counter	an object reference	yes

Assumptions and conventions

• In diagrams an arrow means “holds a reference to”. Two arrows to one box mean two names for one object.
• One class can have many objects, each with its own attribute values and the same behaviors.
• Objects are created with new and a constructor (Topic 1.13).
• Methods are called on objects through reference variables (Topic 1.14).

Mistakes to avoid

1. Confusing the class with the object.
2. Thinking a reference variable contains the object.
3. Expecting b = a to copy the object.
4. Being surprised that a change through one name shows through another.
5. Treating Counter c; as if it made a Counter.
6. Forgetting that all classes are subclasses of Object.
7. Spending time on writing inheritance, which is outside the course.

Quick self-check

1. What is the difference between a class and an object? (A class is the blueprint; an object is one instance created from it.)
2. What does String s; create? (A variable that can refer to a String; no String object yet.)
3. After Counter b = a; how many Counter objects exist (if a referred to one)? (Still one: a and b refer to the same object.)
4. After int y = x; x++; does y change? (No.)
5. Which class is a superclass of every class? (Object)

Next: practice questions.