Constructors, invariants and composition in Java

≈ 45 min

Constructors, invariants and composition in Java

A constructor establishes a valid initial state. If an object represents a bank account, its balance, owner and identifier should satisfy the class’s rules immediately after construction. An invariant is a condition that must remain true for every valid object state, such as quantity never being negative or a Loan having both a book and a member while open.

Composition lets one object use another object as a part of its state. A Course can contain an ArrayList of Enrolment objects; each Enrolment can protect its own status. This is often safer than stretching inheritance because the relationship can change without forcing unrelated classes into a hierarchy.

Worked reasoning. A SeatReservation constructor receives a seat number and passenger name. It rejects a blank name and stores an unconfirmed state. confirm() moves the reservation only if payment is recorded. The state transition lives in the object, so every caller follows the same rule instead of each screen implementing a different version.

Exam lens. State an invariant in a sentence, then identify the constructor or method that enforces it. An invariant is a rule about state, not simply a field name.

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Which statement is the most defensible principle for Constructors, invariants and composition in Java?

Use constructors and methods to establish and preserve an object invariant from creation through every update.

Treat an implementation detail as more important than the stated rule.

Assume one successful run proves the system is correct in every case.

Avoid recording assumptions, limits or failure conditions.

Enter the key term for Constructors, invariants and composition in Java. What condition must remain true for every valid object state?

A class represents a percentage. Which invariant should its constructor preserve?

The value should remain within the agreed range, for example from 0 to 100 inclusive.

Use a larger class or broader permission without stating a rule.

Trust the first output and skip a boundary or failure check.

Hide the decision so affected people cannot inspect it.

Name the concise safeguard or principle that completes this lesson’s scenario: A class represents a percentage. Which invariant should its constructor preserve?