The big idea: A design pattern is a named, reusable solution to a problem that keeps coming up — not a library, and not code you copy.
Its real value is vocabulary: "use an observer here" replaces a paragraph of explanation.
What a pattern is and is not
- Is a description of a structure and why it works
- Is shared vocabulary between developers
- Is not a library or a framework — you implement it yourself
- Is not obligatory: forcing a pattern where there is no problem adds complexity for nothing
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Singleton
Guarantees a class has exactly one instance, with a global point of access.
Used for a configuration store or a connection pool. Widely criticised: it is global state, which makes testing hard and hides dependencies.
Factory
A method decides which subclass to create, so callers ask for what they want rather than naming a concrete class.
Adding a new kind changes the factory only — not every place that creates one.
Observer
Objects subscribe to a subject and are notified when it changes.
The subject knows only that observers exist, not who they are — so new observers need no change to it. Every event system works this way.
Strategy
Interchangeable algorithms behind one interface, selected at run time.
A sort that can be given different comparison rules, or a route planner switching between fastest and shortest.
Each one removes a specific pain: Factory removes scattered constructor calls. Observer removes the subject having to know its listeners. Strategy removes an if-chain choosing an algorithm. Singleton removes duplicate instances — at the cost of introducing global state.
Name the pain, and the pattern justifies itself.
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| Problem | Pattern | What it removes |
|---|---|---|
| One instance must be shared | Singleton | Duplicate, inconsistent instances |
| Which class to create varies | Factory | Concrete class names scattered everywhere |
| Many parts react to a change | Observer | The subject knowing its listeners |
| The algorithm must vary | Strategy | An if-chain selecting behaviour |
Over-application is a real fault: A pattern applied where its problem does not exist adds indirection, files and vocabulary for no benefit.
A factory producing one kind of object is just a constructor with extra steps. In an exam, saying when not to use a pattern shows judgement.
How this is tested — you must match a pattern to the problem it solves and say what it removes. It comes up two ways:
Paper 2 — working with code
- Explain a named pattern, 3-4 marks
- State the purpose of singleton, factory or observer
- Identify which pattern a description matches
Paper 2 — the algorithmic-thinking question
- Choose a pattern for a scenario and justify it
- Explain a drawback of a pattern
The classic trap: Describing what the pattern is without saying what it removes. "An observer notifies subscribers" is a definition; "so the subject never has to know who is listening, and new listeners need no change to it" is the answer.
The problem Observer answers, and the shape it uses.
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A dashboard shows a temperature reading as a number, a graph and a warning light, all updating when a new reading arrives. Suggest a design pattern, justify it, and state one drawback.
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