D286 Java Fundamentals - Set 4 - Part 1

Correct Answer: Reusing and extending behavior defined in a parent class

Inheritance lets a subclass reuse fields and methods from a superclass while adding or specializing behavior, reducing duplication across related types.

Correct Answer: The relationship expressed by inheritance between subclass and superclass

Inheritance models an is-a relationship: a Dog is an Animal. Composition models has-a relationships, such as a Car has an Engine.

Correct Answer: When the relationship is has-a rather than is-a

Composition suits has-a relationships and generally produces more flexible designs. Inheritance should be reserved for genuine is-a specialization.

Correct Answer: No, private members are not inherited into subclass scope

Private members are not accessible in subclasses. Protected or public accessor methods must be used to reach that state.

Correct Answer: The superclass implementation executes

Calling super.method() invokes the superclass implementation, letting a subclass extend rather than entirely replace the inherited behavior.

Correct Answer: No, final prevents overriding

Declaring a method final prevents subclasses from overriding it, used to guarantee that critical behavior remains unchanged.

Correct Answer: A method declared without a body that subclasses must implement

An abstract method declares a signature with no implementation. Any concrete subclass must provide an implementation before it can be instantiated.

Correct Answer: No, the class must be declared abstract and cannot be instantiated

A class with any abstract method must itself be abstract, and abstract classes cannot be instantiated directly.

Correct Answer: It can hold any subclass instance, enabling polymorphic code

A superclass-typed reference can hold any subclass object, so code written against the general type works with all current and future subclasses.

Correct Answer: The one belonging to the object's actual runtime type

Java resolves overridden method calls using the object's actual runtime type, not the declared type of the reference variable.

Correct Answer: equals()

The equals() method defines logical equality. Its default implementation compares references, so classes override it to compare meaningful state.

Correct Answer: Equal objects must produce equal hash codes

The contract requires that objects equal under equals() return the same hashCode(). Unequal objects may share a hash code, which is a collision.

Correct Answer: An interface with no methods used to tag a class with metadata

A marker interface declares no methods and exists solely to signal that a class has a certain property, such as Serializable or Cloneable.

Correct Answer: An interface with exactly one abstract method

A functional interface declares exactly one abstract method, allowing it to be implemented concisely with a lambda expression or method reference.

Correct Answer: Providing a concise implementation of a functional interface

A lambda expression provides an inline implementation of a functional interface's single abstract method, replacing verbose anonymous inner classes.

Correct Answer: A standard way to traverse elements of a collection

An Iterator supplies hasNext() and next() methods for sequentially traversing a collection independently of its underlying implementation.

Correct Answer: Direct modification during iteration throws ConcurrentModificationException

Structurally modifying a collection while iterating with for-each throws ConcurrentModificationException. Iterator.remove() safely removes the current element.

Correct Answer: ArrayList gives fast index access; LinkedList gives fast insertion and removal at ends

ArrayList uses a backing array giving constant-time index access, while LinkedList uses nodes giving efficient insertion and removal but slower random access.

Correct Answer: They must be Comparable or a Comparator must be supplied

Sorting requires a defined ordering, either through elements implementing Comparable or by passing an explicit Comparator to the sort method.

Correct Answer: Comparable defines natural ordering inside the class; Comparator defines external orderings

Comparable is implemented by the class itself for its natural ordering. Comparator is a separate object allowing multiple alternative orderings.