Objects: state, behaviour and an interface
| English | 中文 | Pinyin |
|---|---|---|
| instance/ˈɪnstəns/ | 实例 | shí lì |
| encapsulation/ɪnˌkæpsjʊˈleɪʃn/ | 封装 | fēng zhuāng |
What would explain this observation?
- Two bank-account model objects can respond to the same deposit operation while holding different balances.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- A class defines a type with state and behaviour. An object is an instance 实例. Encapsulation 封装 controls access through an interface so operations can preserve invariants. Inheritance models an appropriate type relationship; composition models an object containing or using another object.
- encapsulation: Controlling access to state through an interface; instance: An individual object of a class.
What should prevent an invalid state change?
A method call acts on a particular instance. State changes should satisfy preconditions and postconditions. Polymorphism lets code use a common interface with different implementations when the contract is respected.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- A method call acts on a particular instance. State changes should satisfy preconditions and postconditions. Polymorphism lets code use a common interface with different implementations when the contract is respected.
- Implement a small synthetic account or inventory model. Test two independent instances, rejected invalid operations and boundary values. Keep the model away from real financial accounts and credentials.
Which two habits make the investigation or model in this case more defensible?
Implement a small synthetic account or inventory model. Test two independent instances, rejected invalid operations and boundary values. Keep the model away from real financial accounts and credentials.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: object A starts at 40 units and receives 15; object B starts at 20 and receives 5. Final balances are 55 and 25. Their total is 80, but neither individual object balance is 80.
An instance starts at 12 units, adds 8 and removes 3. Find its final value. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
An instance starts at 12 units, adds 8 and removes 3. Find its final value.
The result is 17 units. Known: object A starts at 40 units and receives 15; object B starts at 20 and receives 5. Final balances are 55 and 25. Their total is 80, but neither individual object balance is 80.
Check the conclusion and its limits
- A class is not the same thing as one instance. Inheritance is not automatically better than composition. Merely hiding a field does not prove that all operations preserve valid state.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Two instances of a class must always share one balance. This claim is false: A class is not the same thing as one instance. Inheritance is not automatically better than composition. Merely hiding a field does not prove that all operations preserve valid state.
Objects: state, behaviour and an interface: A method call acts on a particular instance. State changes should satisfy preconditions and postconditions. Polymorphism lets code use a common interface with different implementations when the contract is respected.
Two instances of a class must always share one balance.
A class is not the same thing as one instance. Inheritance is not automatically better than composition. Merely hiding a field does not prove that all operations preserve valid state.
Controlling access to state through an interface: write the technical term.
encapsulation means Controlling access to state through an interface.