D286 Java Fundamentals - Set 2 - Part 1

Correct Answer: Encapsulation, inheritance, polymorphism, abstraction

The four pillars of OOP are encapsulation (hiding internal state), inheritance (reusing behavior), polymorphism (one interface, many forms), and abstraction (exposing essentials only).

Correct Answer: extends

A class uses extends to inherit from a superclass. The implements keyword is used separately for interfaces.

Correct Answer: Exactly one

Java permits single inheritance of classes, so a class may extend only one superclass. Multiple inheritance of type is achieved through interfaces instead.

Correct Answer: Object

Every Java class implicitly extends java.lang.Object, inheriting methods such as toString(), equals(), and hashCode().

Correct Answer: A subclass providing its own implementation of an inherited method

Overriding replaces a superclass method with a subclass implementation having the same signature, enabling runtime polymorphism.

Correct Answer: Telling the compiler to verify the method actually overrides a superclass method

@Override causes a compile error if the method does not correctly override a superclass method, catching typos and signature mismatches early.

Correct Answer: References the immediate parent class's members or constructor

The super keyword accesses superclass members or invokes a superclass constructor, commonly used when a subclass extends rather than replaces parent behavior.

Correct Answer: The ability of one reference type to refer to objects of different subclasses

Polymorphism allows a superclass reference to point to a subclass object, with the actual method invoked determined at runtime by the object's real type.

Correct Answer: Determining at runtime which overridden method implementation to call

Dynamic dispatch resolves overridden method calls at runtime based on the actual object type, which is the mechanism enabling runtime polymorphism.

Correct Answer: A class that cannot be instantiated and may contain abstract methods

An abstract class cannot be instantiated directly and may declare abstract methods without bodies, requiring concrete subclasses to supply implementations.

Correct Answer: Yes, it may contain both abstract and implemented methods

An abstract class can mix abstract methods that subclasses must implement with fully implemented methods providing shared behavior.

Correct Answer: A contract specifying methods that implementing classes must provide

An interface declares a set of method signatures forming a contract. Classes that implement it must provide implementations, enabling polymorphism across unrelated hierarchies.

Correct Answer: Any number

A class may implement any number of interfaces, which is how Java supports multiple inheritance of type without the complications of multiple class inheritance.

Correct Answer: A class can implement many interfaces but extend only one abstract class

A class extends at most one abstract class but may implement many interfaces. Abstract classes can also hold instance state, which interfaces traditionally cannot.

Correct Answer: A method with an implementation provided directly in the interface

Default methods, introduced in Java 8, supply an implementation inside the interface, allowing interfaces to evolve without breaking existing implementing classes.

Correct Answer: It can reference any object whose class implements that interface

Declaring a variable by interface type allows it to hold any implementing object, promoting loose coupling and letting implementations be swapped freely.

Correct Answer: Building a class by containing instances of other classes

Composition builds functionality by holding references to other objects rather than inheriting, generally producing more flexible designs than deep inheritance hierarchies.

Correct Answer: It produces looser coupling and more flexible, changeable designs

Inheritance tightly binds a subclass to its parent's implementation. Composition allows behavior to be changed or swapped at runtime with far less ripple effect.

Correct Answer: Returns the class name and hash code representation

Object.toString() returns the class name followed by the unsigned hexadecimal hash code. Classes typically override it to give a meaningful description.

Correct Answer: Collections rely on both being consistent for correct behavior

Hash-based collections require that equal objects produce equal hash codes. Overriding only one breaks that contract and causes objects to behave incorrectly in HashMap or HashSet.