CPSC 210
Design Review
(last lecture 😢👋)
We have a guest today...
The maker of Lab4 has a new lab to present...
More workspace practice for the final exam!
Mazen
And now...
Teaching
Evaluations
Please
Design Review
From high-level design principles to a sample of concrete design patterns
- Well-designed software has high cohesion and low coupling
- Most design patterns improve on cohesion and coupling
Cohesion
Every class should have a
single responsibility
Coupling
Measure how system components depend on each other
Liskov Substitution Principle
- Design principle that states:
-
when a subtype is substituted for its super type:
- the subtype must provide the expected behaviours of the super type
-
when a subtype is substituted for its super type:
Refactoring
-
We refactor our code to improve on:
- the design of our codebase
- coupling by abstracting duplicated code
- cohesion by splitting up classes
Design Patterns
We make mistakes
We identify our mistakes
We learn from our mistakes
We prevent mistakes
We Evolve Design Patterns
Lessons Learned!
Composite
I have a tree structure and I don't know all the types of objects in the tree
Run operations recursively through my tree without knowing the types in my tree
Observer
I want objects to know about a state but I don't know what these objects are
Objects can be added and removed as they desire
Singleton
I want to coordinate a state across my system
There is always exactly one instance of this state in my system
Iterator
I have a collection of elements and there should always be a "next" element
Traverse elements without exposing its implementation
ArrayListIterator<E>LinkedListIterator<E>HashSetIterator<E>Iterable<E>
Iterator<E> iterator()
<<interface>>
Iterator<E>
boolean hasNext()
E next()
void remove()
<<interface>>
Collection
<<interface>>
Design patterns help us with the design of our code base
- But language design also improves over time
- Some examples...
Iterator<Item> itr = collection.iterator();
while (itr.hasNext()) {
Item nextItem = itr.next();
// do something with nextItem
}Before Java 5
Since Java 5 (2004)
for (Item next : collection) {
// do something with next
}button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent evt) {
System.out.println("Do something");
}
});Before Java 8
button.addActionListener(e -> {
System.out.println("Do something");
});Since Java 8 (2014)
List<String> list = new ArrayList<>();
list.add("dog");
list.add("cat");
list.add("hamster");Before Java 9
List<String> list = List.of(
"dog", "cat", "hamster");Since Java 9 (2017)
Map<String, Integer> map = new HashMap<>();
map.put("dog", 1);
map.put("cat", 5);
map.put("hamster", 7);Before Java 9
Map<String, Integer> map = Map.of(
"dog", 1, "Cat", 5, "Hamster", 7);Since Java 9 (2017)
Libraries and Frameworks also help us with design
- Sometimes it's a good idea to look what's out there to help us write good code!
- One example... you may want to update your projects... 😉
Lombok
public class User {
private String firstName;
private String lastName;
private int age;
private String email;
public User(String firstName, String lastName,
int age, String email) {
this.firstName = firstName;
this.lastName = lastName;
this.age = age;
this.email = email;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public void setAge(int age) {
this.age = age;
}
public void setEmail(String email) {
this.email = email;
}
public String getFirstName() {
return firstName;
}
public String getLastName() {
return lastName;
}
public int getAge() {
return age;
}
public String getEmail() {
return email;
}
}import lombok.AllArgsConstructor;
import lombok.Data;
@Data
@AllArgsConstructor
public class User {
private String firstName;
private String lastName;
private int age;
private String email;
}Sometimes new languages build on other languages
Kotlin
System.out.print("Hello");
System.out.println("Hello");Java
print("Hello!")
println("Hello!")Kotlin
final int a;
final int b = 21;
int c;
int d = 25;
d = 23;
c = 21;Java
val a: Int
val b = 21
var c: Int
var d = 25
d = 23
c = 21Kotlin
if(str instanceof String){
String result = ((String) str).substring(1);
}Java
if (str is String) {
val result = str.substring(1)
}Kotlin