COMP2511 Week 5

Agenda

  • Admin Stuff
  • Streams and Lambda
  • Strategy Pattern
  • Observer Pattern

Admin Stuff

  • Assignment 1 is due this coming Friday
    • ​Submissions are done via Gitlab, all you need to do is push to your main branch
    • Pipelines slow down close to deadline so avoid making last minute pushes!
    • Test your linting (easy marks) and do quick dryrun for sanity checks
  • Lab05 is due Week 7, Monday at 10am

Streams and Lambda

Streams and Lambda

A pipeline contains:

  • A source (i.e. collection, array, generator function)
  • Intermediate operations which transforms a stream into another stream

What is a stream?

A sequence of elements which runs through the pipeline that transforms from intermediate operations. 

Streams and Lambda

map(): Applies a given function to elements in the stream

filter(): Selects specific elements as per the Predicate (boolean function)

sorted(): Used to sort the stream

Intermediate Operations

collect(): Collects the result of a stream into a list, set, etc.

forEach(): Iterate through every element of the stream

reduce(): Reduce the elements of a stream into a single value

Terminal Operations

Strategy Pattern

Strategy Pattern

What is it?

Behavioural design pattern that lets you define a family of methods that are interchangeable at runtime

 

What problem does it solve?

Introducing new behaviours without modifying existing code, adhering to the Open-Closed Principle. Avoids conditional statements to select algorithms.

 

When should we use this pattern?

When we need multiple ways of achieving the same task

Different transport behaviours (strategies) that can be interchangeable

Strategy Pattern

Strategy Pattern

Currently, the code uses switch statements to handle each of the different cases.

 

How does the code violate the open/closed principle?

Strategy Pattern

Currently, the code uses switch statements to handle each of the different cases.

 

How does the code violate the open/closed principle?

  • Not closed for modification / open for extension.
  • Each new case requires a new switch statement.

 

How does this make the code brittle?

Strategy Pattern

Currently, the code uses switch statements to handle each of the different cases.

 

How does the code violate the open/closed principle?

  • Not closed for modification / open for extension.
  • Each new case requires a new switch statement.

 

How does this make the code brittle?

  • Makes code more brittle as new requirements cause things to break/difficult to extend functionality

Let's refactor it!

Structure

Now that we have refactored, how about we want to add a new pricing strategy? 

Notice how after refactoring, it is a lot easier to add additional functionality and we are no longer breaking the Open-Closed Principle. 

Strategy Pattern

Find out more at Strategy (refactoring.guru)

Observer Pattern

Observer Pattern

What is it?

Behavioural pattern which defines a subscription mechanism that notifies observers about any event that happens to the subject that they are observing

 

What problem does it solve?

Modelling a n-to-many relationship may be difficult. If
not done well, it typically leads to tight coupling

 

When should we use this pattern?

When changing the state of one object requires changing other objects in turn

Observer Pattern

Subject

Maintains a list of observers and notifies them of state changes. During this notification, they are passing data (push/pull)

public void registerObserver(Observer o);
public void removeObserver(Observer o);
public void notifyObservers(data);	// Calls update() of observers

Observer

Register (and unregister) themselves on a subject and to update their state when they are notified.

// Two possible options for update()
public void update(Subject obj);	// PULL data from subject
public void update(data);	// PUSH data to observers

Observer Pattern

Model the system in Java!

Structure

Observer Pattern

 

Find out more at Observer (refactoring.guru)

Mini Quiz

Which design pattern should we use for each of the following?

Represent tasks with subtasks, all supporting markComplete()

Mini Quiz

Which design pattern should we use for each of the following?

Represent tasks with subtasks, all supporting markComplete()

Composite Pattern

Mini Quiz

Which design pattern should we use for each of the following?

Represent tasks with subtasks, all supporting markComplete()

Composite Pattern

In a data analysis tool, switch between different sorting algorithms  depending on the dataset needs

Mini Quiz

Which design pattern should we use for each of the following?

Represent tasks with subtasks, all supporting markComplete()

Composite Pattern

Strategy Pattern

In a data analysis tool, switch between different sorting algorithms  depending on the dataset needs

Mini Quiz

Which design pattern should we use for each of the following?

News app (e.g., email alerts, mobile notifications) notifies when a new article is published

Represent tasks with subtasks, all supporting markComplete()

Composite Pattern

Strategy Pattern

In a data analysis tool, switch between different sorting algorithms  depending on the dataset needs

Mini Quiz

Which design pattern should we use for each of the following?

In a data analysis tool, switch between different sorting algorithms  depending on the dataset needs

News app (e.g., email alerts, mobile notifications) notifies when a new article is published

Represent tasks with subtasks, all supporting markComplete()

Composite Pattern

Strategy Pattern

Observer Pattern

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