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Encapsulation and controlled state

Encapsulation groups state with the methods that manage it and restricts uncontrolled access. The purpose is not to hide everything: it is to protect a valid interface and preserve rules about state.

Visibility supports an invariant

A ticket object has private remainingUses. The invariant is remainingUses >= 0. A public use() method checks the value before subtracting one; a public getRemainingUses() method returns the value without exposing direct mutation.

If the attribute were freely writable, another part of the program could set it to −12 or a string. Private visibility plus controlled methods creates one place to validate changes. The object is easier to reason about because its own code protects its valid state.

Worked repair

Unsafe interface: public balance, so any caller can assign any value.

Repair: make balance private; initialise it through the constructor; provide deposit(amount) that accepts only positive amounts; provide getBalance() for read access. If withdrawals are required, a method can reject amounts greater than the balance.

Mechanism → direct writes are restricted. Consequence → the class can preserve its rules and callers use a stable interface.

Multiple choice1 mark

Which explanation best describes encapsulation?

AMaking every method private
BStoring all objects in one global variable
CPreventing a class from having methods
DGrouping state and behaviour while controlling access through an interface

Fill in the blanks3 marks

A gap 1 attribute cannot be changed directly by ordinary external code. A gap 2 method forms part of the class interface. Validation helps preserve a state gap 3.

private
public
invariant
paradigm

Written answer6 marks

A TemperatureLog class exposes public highest and lowest attributes. Any caller can set highest below lowest. Redesign the interface and explain how your design preserves valid state.

Name visibility, at least one controlled update method and its validation rule, then state the consequence.

Students type their answer here.

Apply the model independently

The remaining tasks change the context or reduce the support. Complete them without copying the worked model, then check that each explanation connects a mechanism to its consequence.

Written answer6 marks

A game profile has private experience and public methods award(points) and getLevel(). Explain two benefits of this interface compared with public writable experience. Include what award should reject and how the level can be derived consistently.

Develop each benefit as mechanism → consequence.

Students type their answer here.

Fill in the blanks4 marks

Encapsulation groups checkpoint gap 1 and behaviour inside a class. checkpoint gap 2 visibility restricts direct external access. Controlled public methods can validate changes and protect an checkpoint gap 3. Encapsulation does not mean making everything checkpoint gap 4.

Review your understanding

Before submitting, check that you can explain the main distinction in your own words, apply it in an unfamiliar context and justify each consequence rather than only naming a feature.