Lecture Content

Question 1

Skeleton (hierarchy) of a Class

Answer 1

/* Package declaration */

/* Import declarations */

class House {

/* Field declarations */

/* Constructors */

/* Methods */

Question 2

Why should an object should hide its implementation details?

Answer 2

• To prevent misuse
• To simplify code (hide stuff you dont need to see)
• To insolute users from implementation changes

Question 3

Classes should present their users with an interface that

Answer 3

• gives them access to well-specified behaviours
• hides internal data representation
• hides algorithmic details
• preserves data integrity

Question 4

Encapsulation: Classes and Users

Answer 4

Classes should allow you to change the internal representation without affecting users

Question 5

A good problem solving strategy: Divide and Conquer

Answer 5

Understand problem, find tools, break into small problems.

1. Try to understand or define the problem
2. Choose the right tools for the job (and read the documentation)
3. Break complex problems into smaller sub-problems
4. Sub-problems can be broken again into yet smaller problems
5. Simple enough problems can be solved directly
6. Small solutions combine to solve bigger problems

Question 6

Java Strategy: Imaginary Methods

Answer 6

Typically, a solution to a problem of reading, manipulating or writing information is a method.

When writing one method, if you need to do a sub-task that is too complex to solve straight-away.. .

1. Imagine there is a method that does that sub-task for you
2. Give it a name, inputs, return type and meaning
3. Then just use it as if it exists

Compile after each method

• The compiler will tell you what new methods need defining.

Question 7

What to keep in mind when designing methods (even for imaginary):

Answer 7

• Meaning of inputs (if any):
  
  • How to interpret the information passed in
• Preconditions (if any):
  
  • Things which must be true as method is called
• Postconditions (if object’s state changes):
  
  • Things guaranteed by end of method execution
• Meaning of return value (if any):
  
  • How to interpret information returned

Question 8

!! variables constrained within some local scope.

Answer 8

• method input arguments…. . . within method definition

• locally defined variables…. . . until the end of method

• this object . . .
. . . within an instance method

• variables initialised in a for-loop. . .. . . within the for-body

Question 9

due to local scope, within a method block can only directly access local parts of the stack, but….

Answer 9

. . . but ‘this’ allows you to access the owning object

. . .‘return’ variables allow information to escape the local scope

Question 10

instance methods

Answer 10

a method that is invoked on a specific object, requesting the object to do something, and maybe changing the object’s state on the way

x = myCircle.area();

Question 11

instance attributes/fields

Answer 11

myCircle.diameter;

Question 12

static methods

Answer 12

aka class methods

Question 13

static fields

Answer 13

aka class variables

Only one static object is created per class– not one per object

Question 14

Static Fields as Global Variables: Global object references

Answer 14

System.out is a global object reference

Question 15

static fields usually declared final or not?

Answer 15

yes, they don’t change

Question 16

how to make methods global

Answer 16

static can be used to make methods global

(static methods, like static fields, are underlined in UML)

Question 17

5 features of Arrays

Answer 17

• Arrays are objects
• Arrays can contain objects
• Arrays can contain other arrays
• All arrays have a length
• Arrays of arrays needn’t be ’rectangular’
  
  • Not all rows need be the same size

Question 18

Consequences of pass-by-value:

Answer 18

• If the method makes changes to the variable’s value, these changes are not visible to the calling code.
• However, the variable could point to some shared memory (heap), and changes there will be visible

Method arguments are passed-by-value.

Question 19

Can a method alter the elements of an array without having to return anything (void)

Answer 19

yes.

Question 20

what is a recursive method?

Answer 20

A recursive method is one that includes a call to itself

Question 21

!! All recursive methods must have the following:

Answer 21

• Base Case: knowing when to stop
• Work toward Base Case: make the problem simpler (standard case), otherwise infiniute recursion
• Recursive Call: method calls itself on a simpler problem

Question 22

good use case for recursive methods

Answer 22

If a problem can be reduced to smaller instances of thesame problem, then a recursive solution is likely to be easy to find and implement.

Question 23

A recursive method definition normally contains 2 kinds of cases:

Answer 23

◦ one or more base cases without recursive calls

◦ one or more cases that include at least one recursive call

Question 24

Abstraction

Answer 24

Abstraction is a process of hiding the implementation details and showing only functionality to the user.

Question 25

two ways to achieve abstraction

Answer 25

• Abstract class (0 to 100%)
• Interface (100%)

Question 26

!! abract classes (5 features)

Answer 26

abstract says:

• The class is incomplete
• You cannot directly create instances of this class (wil give compile error)
• An abstract class must be declared with an abstract keyword.
• It can have abstract and non-abstract methods.
• It cannot be instantiated.
  
  • It needs to be extended and its method implemented.
• It can have constructors and static methods also.
• It can have final methods which will force the subclass not to change the body of the method.

abstract class A{}

Question 27

how does an interface look like in code

Answer 27

Question 28

what is an interface

Answer 28

An interface is a group of related method declarations with no bodies

• These methods describe a set of capabilities
• A class can implement an interface. If so, the classpromises to provide the capabilities specified by the interface

Question 29

what is the benefit of having an interface

Answer 29

Helps us manage complexity by treating members of various classes uniformly:

• Methods common to all the classes are specified in the interface
• Each class implements the interface
• Client code can refer purely to the interface without
• knowing (or caring) about details of the implementing classes

Duplication can be solved through inheritance.

Question 30

example of an implementation

Answer 30

clear?

Question 31

define inheritance

Answer 31

Inheritance is the process of deriving a specialised class from an existing class

Question 32

What does “A PhD student is-a-kind-of student.” mean?

Answer 32

• Unlike other students, a PhD student: has a supervisor, must write a thesis, and does not take courses.
• Like other students, a PhD student: must register, is in a department, and has a login.

Question 33

what generalisation and specialisation in inheritance

Answer 33

An electric car is-a car that runs with an electric engine

• Car is the superclass or parent-class of ElectricCar
• Car is a generalisation of ElectricCar
• -
• ElectricCar is a subclass or a derived-class of Car
• ElectricCar is a specialisation of Car

Question 34

apparent and actual type

Answer 34

Similarly to case of interfaces:

• The apparent type of an object is that which the compiler can infer at compile time: Lamp l = …
• The actual type is the type the object really has: new AdjustableLamp(…)

Rules:

• Apparent type must be a generalisation of the actual type
  
  • AdjustableLamp l = new Lamp(...); //Compile error
• Methods and fields available according to apparent type
  
  • Lamp l = new AdjustableLamp(...); l.dim(); // Compile error

Question 35

binding

Answer 35

Binding is how Java knows which method to call on what object.

Question 36

protected methods and inheritance

Answer 36

protected methods and fields are both:

• package visible
• and can be accessed from subclasses
  
  • (even in different packages)

Question 37

super keyword

Answer 37

• super refers to the immediate superclass
• Can be used to call the constructor of a superclass
  
  • can only be called from subclass constructor
  • must be called on the first line!
  • different superclass constructors called with different inputs:
    
    • super(on); or super(); or super(arg1, arg2);
• super() is called implicitly if no other constructor is called
• Also, used to call an overridden method from a
  
  • superclass, e.g.super.pressSwitch();

Question 38

to avoid duplication…

Answer 38

…pull duplication into the superclass

Question 39

abstract classes and interfaces?

Answer 39

BUT the interface may be overkill and can be omitted in some cases, though having one does no harm.

Question 40

interface or abstract claass

Answer 40

The interface caters for the case where the abstract class may be too specific

Question 41

abstract methods

Answer 41

Question 42

Rules for use of abstract

Answer 42

Question 43

summary of abstract classes

Answer 43

• Unlike classes, these cannot be instantiated
• Like classes, they introduce types
  
  • but no object can have an abstract class as its actual type

Why use them?

• When there is a set of common features and implementation for all derived classes but:
  
  • We want to prevent users from handling objects that are too generic
  • We cannot give a full implementation for the class