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Model, plan, test, review — how good software comes about

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Class Diagram and Code in Sync

Python code of the running log on the left, the UML class diagram in exact notation on the right: point at a code line and its counterpart in the diagram lights up, and vice versa. self.laeufe = [] does not become an attribute but the line with the star; the hollow triangle sits at the base class. Eight stages from class Lauf to staff administration, plus part and whole: on deletion the object stage shows that composition and aggregation are not in the code but in the meaning.

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Tests as a Safety Net

A discount function with tiers (5 % from €100, 10 % from €500) and six pytest tests. Seven changes are applied, with the old line struck through. The tests run row by row with traffic lights, E lines and a pytest-style summary. Each change drops into the net as a ball: it is caught when a test turns red, otherwise it falls through. Two broken discount tiers slip past green tests until the missing boundary test rabatt(500) == 50 is added. Plus a refactoring that rightly stays green.

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Token Game in Activity Diagrams

Tokens move tick by tick through the UML activity diagrams of the slides: at a diamond they take one path (tap a condition), at a fork they split, at a join they wait for all parts (“1 of 2”). Running actions show their duration, swimlanes count handoffs. Six examples from the order process to the complaint case, plus four error models: a diamond instead of a bar leads to deadlock or a premature end.

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Trust, but Verify

Five short functions as an assistant delivers them, each plausible and each with a typical error: edge case, invented library function, eval on form input, swapped date format, wrong domain logic. Tap the suspicious line, then check with five methods: read the code, check the docs, example, edge case, recompute. The log shows real Python 3.13 output; the matrix collects which method reveals which error: reading versus trying out.

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Waterfall or Agile?

The same customer portal runs twice: as a waterfall in phases and as Scrum in sprints, week by week. Event cards (new feature, change, cancellation) drop into both lanes in the same week; misunderstandings stay face down until a review or the acceptance test reveals them. Rework appears hatched, and the curves show cumulative benefit and cost up to week 40. Switches for uncertainty, sprint length, feedback and partial deliveries separate the two effects: less rework and earlier benefit.

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WIP Limit and Lead Time

A Kanban board with analysis, development and testing, day by day: cards with number and start day move across the board, people (1/3/1) work on the oldest cards, and a queue forms in front of the testing bottleneck. The WIP limit (none down to 2) is a pull rule with a gauge, next to a cumulative flow diagram with measuring lines. Without a limit throughput 0.5 and a lead time of 26 days; with limit 5 still 0.5, but 10 days. Little's Law, plus a switch to let developers help with testing.

Software engineeringTry it

Software engineering Learn interactively.

All apps. Free in your browser.

Class Diagram and Code in Sync

Tests as a Safety Net

Token Game in Activity Diagrams

Trust, but Verify

Waterfall or Agile?

WIP Limit and Lead Time

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