Lecture 4

Question 1

What is aliasing?

Answer 1

Having a variable refer to an existing object

Question 2

How does Object.clone() work?

Answer 2

All primitive fields are copied

All Object fields are aliased, i.e. refer to the old ones

Question 3

What interface must objects implement to allow cloning?

Answer 3

Cloneable

Question 4

Why do you have to be careful with cloning an object that contains mutable fields?

whats the solution?

Answer 4

Changes to mutable fields will change in every object clone

Solution: clone the mutable field during cloning of the object

Question 5

How are objects serialised using Object streams?

Answer 5

ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(“bank.dat”));

out.writeObject(b);

…

ObjectInputStream in = new ObjectInputStream(new FileInputStream(“bank.dat”));

BankAccount b = (BankAccount)(in.readObject());

Question 6

What can readObject() throw?

Answer 6

ClassNotFoundException and IOException

Question 7

Why do you have to be careful serialising objects?

Answer 7

All object fields will also be serialised

So many many objets might be serialised from one call - potential massive files

Question 8

How can a class be marked as Serialisable?

Answer 8

Implement interface Serializable

Question 9

how to you prevent field being serialised?

Answer 9

Mark it as transient

Question 10

If the same object is serialised twice in one ObjectOutputStream, what happens?

Answer 10

Assigned a serial number and second serialisation just prints that out.

Question 11

What is the version problem?

How is it generally solved?

Answer 11

If the structure of a class changes between serialisation and deserialisation

Give null or empty values to new fields

Question 12

Why is a BufferedReader better than calling Paths.readAllLines(path)

Answer 12

readAllLines reads entire file into memory

BufferedReader goes line by line

Question 13

what is a potential problem with buffered streams?

Answer 13

May have to explicitly flush them

Question 14

How do you format a number?

Answer 14

System.out.format(“sqrt(%d) = %.8f%n”, i, r);

// sqrt(2) = 1.41421356

.8 means 8 digits after decimal point

Question 15

what is GSON?

Answer 15

Open source library to convert Java objects to and from JSON

Question 16

How do you serialise and deserialise objects using GSON?

Answer 16

Gson gson = new Gson();

String oneJson = gson.toJson(1);

Integer one = gson.fromJson(oneJson, Integer.class);

Question 17

What value types are supported in JSON?

Answer 17

Primitives: strings, numbers, booleans, null

Arrays

Objects

Question 18

What is an object in json?

Answer 18

Unordered collection of name/value pairs, e.g:

{“field1”: value1, “field2”: value2..}

Question 19

What is an array in json?

Answer 19

Ordered collection of values:

[value1, value2…]

Question 20

What is needed for deserialisation of generic types in GSON?

Answer 20

TypeToken:

Type listType = new TypeToken<list>&gt;(){}.getType();</list>

List<book> result = GSON.fromJson(reader, listType);</book>

Question 21

What is JAX-RS?

Answer 21

Java framework for RESTful web services

Question 22

What is Jersey?

Answer 22

JAX-RS reference implementation

Question 23

How are Resource methods annotated in JAX-RS?

Answer 23

request method designator:

@GET, @PUT, @POST, or @DELETE

These are invoked in response to HTTP requests

Question 24

What does an @Path annotation determine in JAX-RS?

Give an example

Answer 24

mapping from URLs to class/methods used for response

@Path(“/book”)

public class BookQuery{

…

@GET

@Path(“/{isbn}”)

public string getBook(@PathParam(“isbn”) String isbn) {

// return stuff

}

}

Question 25

@ annotations can be processed by what?

Answer 25

IDEs, JUnit, javac

Question 26

What are the advantages and disadvantages of assertions?

Answer 26

advantage: highlights issues to developers
disadvantage: workload of writing assertions

Question 27

how does JUnit work?

Answer 27

Annotate test method with @Test

use methods like assertTrue or assertEquals, assertNull, assertNotEqual

Question 28

How can you run code before/after each test in JUnit?

Answer 28

@Before and @After