Advanced Development in Java

Question 1

What is a nested class?

Answer 1

A nested class is one where we put the class definition of a class inside the definition of another class.

Question 2

Can a class nested inside another access its private variables?

Answer 2

Yes, any class nested within another may access any of its functionalities. The nested class is simply there for the sake of abstraction.

Question 3

Why may you need to use a nested class?

Answer 3

You typically want classes to be specialised to a certain task. When you have two classes that rely heavily on each other, it can get annoying having to declare getters/setters or other access functions every time you want to add a new thing.

Nesting the class allows them to share functionality amongst themselves while also ensuring sound encapsulation.

Question 4

What is the difference between a static nested class and an inner class?

Answer 4

Static nested classes do not have access to instance members, while inner classes do.

Question 5

What access modifiers may a nested class have?

Answer 5

A nested class may have any of the standard modifiers, as it is treated like a variable of the outer class it is within.

Question 6

Can inner classes be accessed from the outside?

Answer 6

Yes, as long as they have the appropriate visibility modifier.

One may be instantiated, for example, in the following form: OuterClass.new InnerClass();

Question 7

How does the ‘this’ keyword work with inner classes?

Answer 7

‘this’, when cast in the inner class, refers to the inner type instance.

OuterClass.this, then, refers to the instance of the surrounding class.

Question 8

What is a local class?

Answer 8

A local class is defined, and exists solely within, a block (typically a method).

Question 9

Can local classes have access modifiers?

Answer 9

They may not, as they may not be accessed from anywhere other than the private scope.

Question 10

Can local classes access method-scoped variables?

Answer 10

No, unless they are declared as final. The class is its own encapsulated “zone”.

Question 11

What is an anonymous class?

Answer 11

An anonymous class is a class declared without any class name at all.

Question 12

How may an anonymous class be instantiated?

Answer 12

Anonymous classes are instantiated by creating an instance of an interface, and filling out the functionality required within curly brackets.

Question 13

Where may an anonymous class be instantiated?

Answer 13

An anonymous class may be instantiated anywhere with a scope.

Question 14

What is an enumerated class?

Answer 14

An enumerated class is a data type consisting of a set of named variables with a specific range.

They are not instantiated, and therefore are effectively static and final.

Question 15

Can enumerated classes be declared locally?

Answer 15

No. They must be declared as a class or a class member, because they are static and final.

Question 16

What may an event/listener system be used for in Java?

Answer 16

Events and listeners are used to solve a problem where your program will have to wait for some thing to happen.

Question 17

What is event-driven programming?

Answer 17

Event-driven programming entails having an action wait for an event to happen. Instead of checking at intervals and wasting memory, we release our resources, and let the program tell it when an event happens.

Question 18

Where may you use an event/listener?

Answer 18

A GUI, where you press a button and something happens. In this case, the event of a button being pressed would call an action or method.

Question 19

What is the delegation model in Java?

Answer 19

Upon a change in the internal state of an event source, an event is generated, which is an object that typically contains information about said state change. A listener, that has been registered to the event, is then notified of that event happening.

Question 20

How do you register a listener in Java?

Answer 20

An event source, such as a button, will have an addCategoryListener() method, where Category may be an Action, a Key, etc.

An object that extends CategoryListener may then register itself (or have itself registered) using that method. When that event is called, it will then activate the categoryPerformed(ActionEvent e) method, where CategoryEvent is our distributed Event object.

Question 21

How may we read the contents of an Event object?

Answer 21

When the Event source generates an Event object, it will get passed in as an argument to the categoryPerformed method. We can then read the contents.

Question 22

How could we create our own Event system?

Answer 22

We need…

..a Listener class that extends EventListener
..a corresponding Event class that extends Event
..and a Listener instance that extends your created Listener class.

Question 23

What are generics?

Answer 23

Generics allow algorithms to be written for types that may be specified at a later point. They let you reuse the same code with different input types.

Question 24

How may we declare a generic type in a class?

Answer 24

If the class has an identity of Class<T>, then we may choose for one of our variable types to simply be T variable.</T>

Question 25

How may we declare a generic method?

Answer 25

A generic method is declared slightly differently than a generic type, where we place the type before the function name.

e.g. object.<String>functionName("Hello!")</String>

Question 26

What are bounded types?

Answer 26

Bounded types are used to allow a class/method to operate not just with generic types, but with types that exhibit certain properties.

We may do this by declaring that <S>, meaning that S must be equal to or a subclass of Class. We may also do this for interfaces.</S>

Question 27

How does the compiler use type inference to help with your generic types?

Answer 27

If your method has some input, the compiler will infer the intended generic type based on what the method block contains.

When instantiating a class with a generic type, you may leave the generic bounds (<>) empty.
e.g. Class<int> class = new Class<>()</int>

Question 28

Is Container<Double> a subtype of Container<Number>?</Number></Double>

Answer 28

No. This is because the compiler cannot use the given metadata to be sure that the Double is of type Number.

Question 29

Is Container<? extends Number> a subtype of Container<Number>?</Number>

Answer 29

Yes. ? is a wildcard, meaning that it is an unknown type to be declared at runtime.

Question 30

What is meant by type erasure?

Answer 30

Type erasure is a phenomenon in the Java compiler where when the program is compiled and built, all type information from parameters and arguments within generic classes/methods is erased, allowing the compiler to ensure backwards compatibility with earlier versions.

Question 31

What is known as the raw type?

Answer 31

The raw type is the legacy type notation for Java. All generic classes and methods are broken down into raw types at compile time.

Question 32

What is heap pollution?

Answer 32

Heap pollution is when a variable of a declared parameterised type refers to an instance that is not of that parameterised type. This is caused when type erasure causes the type information to be unavailable at runtime.

Question 33

What is a collection?

Answer 33

A collection is an object that groups multiple elements into a single unit. For example, a collection of cards, or a collection of emails.

Question 34

What is the difference between a collection and a map?

Answer 34

A collection is a group of elements, while a map is a grouping of elements that links values known as keys to each element.

Question 35

What is the Set interface?

Answer 35

A Set contains no duplicate elements - a disjoint set. Two sets are equal if they contain the same elements, even if the containers are different.

Question 36

What is the List interface?

Answer 36

A List is an ordered collection of elements that may contain duplicates. Two sets are equal if they contain the same elements, in the same order.

Question 37

What is the Queue interface?

Answer 37

A Queue is a collection of elements, iterated over using first-in-first-out ordering.

Question 38

What is the difference between add/offer, remove/poll and element/peek in a Queue?

Answer 38

The former of the two does its task, but returns an Exception upon failure. The latter returns null instead.

Question 39

What is the Deque interface?

Answer 39

A Deque is a double-ended queue, where removal can happen at both the head and the tail.