OOP class

What is a class?

A blueprint/type that defines the structure and behavior of objects.

What is an object?

An individual instance created from a class at runtime.

What do fields represent?

An object's state/data.

What do methods represent?

An object's behavior/actions.

What is a constructor used for?

To initialize a newly created object's initial state.

What are the key constructor rules?

Same name as the class; no return type; can accept parameters; runs during object creation.

How do you create a new Student object?

Student s = new Student(...);

What is encapsulation?

Combining related state and behavior while limiting direct access to the internal representation.

Why are fields often private?

To protect object state and control changes through a public interface.

What does a getter do?

Returns/reads a value without exposing how it is stored.

What does a setter do?

Accepts a new value and can validate, reject, or transform it before changing state.

What does this refer to?

The current object inside an instance method or constructor.

In this.name = name, what is this.name?

The object's field.

In this.name = name, what is name?

The parameter/local value.

What can this(...) do?

Invoke another constructor in the same class.

What does static mean?

The member belongs to the class rather than an individual object and is shared at class level.

Instance member vs static member?

Instance: belongs to each object. Static: belongs to/is shared by the class.

What can public members be accessed from?

Class, package, subclasses, and anywhere.

What can protected members be accessed from?

The class, same package, and subclasses.

What does no access modifier mean?

Package-private: accessible in the class and same package only.

What can private members be accessed from?

The declaring class only.

What is association?

A relationship where one object knows about or collaborates with another.

What is a has-a relationship?

Composition/association: a class contains or uses another object.

What is an is-a relationship?

Specialization/inheritance: one class is a specialized form of another.

Car has an Engine: has-a or is-a?

Has-a.

GraduateStudent is a Student: has-a or is-a?

Is-a.

When analyzing a problem, what do nouns often suggest?

Candidate classes/entities or attributes.

When analyzing a problem, what do verbs often suggest?

Behaviors/methods and responsibilities.

What are attributes?

What an object knows, such as name, id, status, or price.

What are behaviors?

What an object does, such as enroll(), calculateTotal(), or activate().

What are responsibilities?

What an object owns or is responsible for, such as validating state or coordinating its behavior.

What are the three compartments of a basic UML class box?

Class name; attributes; methods/operations.

What does + mean in UML?

public

What does - mean in UML?

private

What does # mean in UML?

protected

What does ~ mean in UML?

package access

What does UML multiplicity 1 mean?

Exactly one object participates.

What does UML multiplicity 0..1 mean?

Optional: zero or one object.

What does UML multiplicity 0..* mean?

Zero or many objects.

What is the goal of good object-oriented design?

Clear responsibilities and useful relationships among interacting, encapsulated classes.

What is inheritance?

Class reuse and specialization: a subclass inherits accessible state/behavior from a superclass and can add specialized behavior.

What is a superclass?

The parent/base, more general class.

What is a subclass?

The child/derived, more specific class.

What keyword establishes class inheritance in Java?

extends

What kind of relationship should inheritance represent?

An is-a relationship.

Are constructors inherited?

No.

Are private superclass members directly accessible from a subclass?

No.

What is method overriding?

A subclass supplies a specialized implementation of an inherited method.

What must match when overriding a method?

The method name and parameter types/signature; return type must match or be covariant.

Can an overriding method reduce access?

No; it cannot be more restrictive.

Is a private superclass method overridden?

No.

Are static methods overridden?

No. Static methods are hidden, not overridden.

Can a final method be overridden?

No.

Why use @Override?

To catch signature mistakes early and mark the intent to override.

Overriding vs overloading?

Overriding: subclass specializes inherited method, runtime. Overloading: same method name with different parameter lists, compile time.

What does super(...) do?

Calls a superclass constructor.

Where must super(...) appear in a subclass constructor?

As the first statement.

What does super.method() do?

Calls the superclass version of a method.

What is constructor chaining order?

Superclass initialization first, then subclass initialization.

What is polymorphism?

One reference type can work with objects of different concrete types/forms.

What is compile-time polymorphism in this module?

Method overloading.

What is runtime polymorphism in this module?

Method overriding.

What is dynamic method dispatch?

At runtime, Java selects an overridden instance method based on the actual object, not just the reference type.

In Shape s = new Circle(), what is the reference type?

Shape.

In Shape s = new Circle(), what is the actual object type?

Circle.

What does the reference type control in a polymorphic reference?

Which members are visible/can be called directly at compile time.

What does the actual object control in a polymorphic reference?

Which overridden instance-method implementation runs at runtime.

What is upcasting?

Assigning a subclass object to a superclass reference, e.g. Shape s = new Circle();

What is abstraction?

Exposing essential behavior while hiding implementation details.

What is an abstract class?

An incomplete base type that cannot be instantiated directly.

Can an abstract class contain concrete fields, constructors, and concrete methods?

Yes. It can also contain abstract methods.

What is an abstract method?

A method that declares required behavior without providing an implementation/body.

How does an abstract method declaration end?

With a semicolon, e.g. abstract double area();

What must a concrete subclass do with required abstract behavior?

Provide an implementation of the abstract method.

What is an interface?

A contract that implementing classes agree to fulfill; it focuses on required capabilities.

What keyword does a class use to fulfill an interface?

implements

Can a Java class implement multiple interfaces?

Yes.

Abstract class vs interface: primary role?

Abstract class: shared base type + partial implementation. Interface: shared capability/contract.

Abstract class vs interface: constructors?

Abstract class can have constructors; an interface has no constructors.

Abstract class vs interface: inheritance count?

A class extends one class but can implement multiple interfaces.

When is an abstract class a better fit?

For closely related classes sharing state and/or partial implementation.

When is an interface a better fit?

For cross-cutting capabilities/contracts that different classes can implement.

Why is inheritance for a has-a relationship a design mistake?

Inheritance should model is-a specialization; has-a is better modeled with association/composition.

Why can deep inheritance hierarchies be a problem?

More levels do not automatically improve reuse and can make the design harder to understand/maintain.

What may large instanceof/type-checking chains suggest?

Weak abstraction; polymorphism may be more appropriate.

What can empty overridden methods suggest?

The superclass may be asking subclasses to do too much.