$ cat hands-on/shapes-area.spec HO-03

Area 02 · Object-oriented programming

Shape area report

Read a list of shapes, then report each area, the total and the largest, without the report loop knowing which kind of shape it holds.

Spec no.: HO-03
Level: Warm-up
Time box: 45 min
Reference: Java · C#
Grading: Self-assessed · rubric is ours, not DICT's

01 Coverage

Area	Sub-skills practised	Role
01 Data manipulation	Validation, ArrayList handling	Also
02 Object-oriented programming	Abstraction, Encapsulation, Inheritance, Polymorphism	Primary

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02 Requirements

Abstract class Shape with abstract area() and describe().
Circle(r), Rectangle(w, h) and Square(side). Square extends Rectangle and overrides only describe().
Fields are private or protected, set in the constructor. Every dimension must be greater than zero; the constructor rejects anything else.
Read lines until end: circle r, rect w h or square s. For an unknown shape, a wrong number of values, a non-number or a non-positive size, print Skipped: <line>.
Keep all shapes in one list of Shape. Print describe() and the area to two decimals for each, then Total area and Largest.

03 Input and output

Input

Lines of circle r | rect w h | square s
end to finish

Output

Skipped lines as they are read
Circle(r=2.00) area=12.57 style lines
Total area: x.xx and Largest: <describe()>

Sample input · stdin

circle 2
rect 3 4
triangle 3
square 5
circle -1
end

Expected output · stdout

Skipped: triangle 3
Skipped: circle -1
Circle(r=2.00) area=12.57
Rectangle(3.00 x 4.00) area=12.00
Square(5.00) area=25.00
Total area: 49.57
Largest: Square(5.00)

Match the sample exactly first, then write at least two test inputs of your own.

04 Edge cases to test

triangle 3 (unknown shape)
circle -1 and circle 0
rect 3 (missing value)
An empty list before end: print the total only

05 Starter outline and hints

Outline

abstract Shape { area(); describe(); static positive(v) }
Circle, Rectangle(protected w, h), Square extends Rectangle
parse(line) → Shape or null; catch bad numbers
for each shape: print; total += area; track largest

Hints

Hint 1

Throwing an exception from the constructor keeps invalid objects from ever existing.

Hint 2

Use Math.PI, not 3.14, or your circle area will be off in the second decimal.

06 Practice rubric

Criterion	Sub-skills	Look for	Pts
Abstraction and inheritance	AbstractionInheritance	Abstract base; Square reuses Rectangle's area().	30
Polymorphism	Polymorphism	The report loop has no type checks or casts.	25
Encapsulation	Encapsulation	No public mutable fields; validation inside constructors.	20
Input handling	Validation	Every bad line is skipped with a message; nothing crashes.	15
Output	ArrayList handling	Two-decimal format and the right largest shape.	10
Total			100

Our own weighting for practice. It is not the DICT scoring scheme.

07 Self-check

Tick only what you have verified by running your own program.

My report loop never asks what kind of shape it has.
Square has no area() of its own.
circle -1 is skipped by the constructor's check, not by an if in main.
My total for the sample is 49.57.

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08 Reference solution

Both versions compiled and reproduced the sample output on Java 21 and .NET 8 when this site was built. Reference solutions are in Java and C#. The 2023 FAQ copy also lists C, C++ and Visual Basic 6.0 / .NET; the same structure carries over, but you will need your language's own file and database APIs.

Try the whole brief first. The reference is one correct approach, not the only one, and reading it early removes most of the practice value.

09 Exam-day habits

Work without internet search to simulate exam conditions.
Type the sample input yourself and compare your output line by line before you look at the reference.

Area

Sub-skills practised

Role

01 Data manipulation

Validation, ArrayList handling

Also

02 Object-oriented programming

Abstraction, Encapsulation, Inheritance, Polymorphism

Primary

02 Requirements

Abstract class Shape with abstract area() and describe().

Circle(r), Rectangle(w, h) and Square(side). Square extends Rectangle and overrides only describe().

Fields are private or protected, set in the constructor. Every dimension must be greater than zero; the constructor rejects anything else.

Read lines until end: circle r, rect w h or square s. For an unknown shape, a wrong number of values, a non-number or a non-positive size, print Skipped: <line>.

Keep all shapes in one list of Shape. Print describe() and the area to two decimals for each, then Total area and Largest.

03 Input and output

Input

Lines of circle r | rect w h | square s
end to finish

Output

Skipped lines as they are read
Circle(r=2.00) area=12.57 style lines
Total area: x.xx and Largest: <describe()>

Sample input · stdin

circle 2
rect 3 4
triangle 3
square 5
circle -1
end

Expected output · stdout

Skipped: triangle 3
Skipped: circle -1
Circle(r=2.00) area=12.57
Rectangle(3.00 x 4.00) area=12.00
Square(5.00) area=25.00
Total area: 49.57
Largest: Square(5.00)

Match the sample exactly first, then write at least two test inputs of your own.

05 Starter outline and hints

Outline

abstract Shape { area(); describe(); static positive(v) }
Circle, Rectangle(protected w, h), Square extends Rectangle
parse(line) → Shape or null; catch bad numbers
for each shape: print; total += area; track largest

Hints

Hint 1

Throwing an exception from the constructor keeps invalid objects from ever existing.

Hint 2

Use Math.PI, not 3.14, or your circle area will be off in the second decimal.

06 Practice rubric

Criterion	Sub-skills	Look for	Pts
Abstraction and inheritance	AbstractionInheritance	Abstract base; Square reuses Rectangle's area().	30
Polymorphism	Polymorphism	The report loop has no type checks or casts.	25
Encapsulation	Encapsulation	No public mutable fields; validation inside constructors.	20
Input handling	Validation	Every bad line is skipped with a message; nothing crashes.	15
Output	ArrayList handling	Two-decimal format and the right largest shape.	10
Total			100

Our own weighting for practice. It is not the DICT scoring scheme.

08 Reference solution

Try the whole brief first. The reference is one correct approach, not the only one, and reading it early removes most of the practice value.