COMP2511

23T1 Week 8

WEDNESDAY 1PM - 4PM (W13B)

FRIDAY 11AM - 2PM (F11A)

 

Slides by Alvin Cherk (z5311001)

This week

  • Generic programming
  • Singleton pattern

Please let me know in the lab if you want to do assignment-iii as a pair. If not, you are default solo

Generic Programming

Generic Programming

What are generics?

Generics enable types to be passed when defining classes, interfaces or methods

  • Remove casting and offer stronger type checks at compile time
  • Allow implementations of generic algorithms, that work on a collection of different types
  • Adds stability to code by making more of your bugs detectable at run-time

Generic Programming

/**
 * How many different types of T here?
 *
 * It's all about the scope
 */
public class ConfusingClass<T> {
    private T t1;

    public <T> T t2(T t) {
        return null;
    }

    public static <T> T t3(T t) {
        return null;
    }

    public static void main(String[] args) {
        ConfusingClass<String> cc = new ConfusingClass<String>();
        cc.t2(3);
    }
}

Example from Webster

Generic Programming

class ConfusingClass<T1> {
    private T1 t1;

    // private T2 t2; // nope
    // private T3 t3; // nope

    public <T2> T2 t2(T2 t) {
        T1 t1;
        T2 t2;
        // T3 t3; // nope
        return null;
    }

    // ConfusingClass.t3(...)
    public static <T3> T3 t3(T3 t) {
        // T1 t1; // nope
        // T2 t2; // nope
        T3 t3;
        return null;
    }
}

Example from Webster

Code Demo

Stack.java

Code Demo

Inside src/stack, there are a series of stubs for a Stack class which takes in a generic type. There are a series of tests inside StackTest.java which currently fail.

Implement the methods so that the tests pass, using an ArrayList to store the internal data structure. Answer the following questions:

  1. What is E?
    • A generic type
  2. What is the Iterable interface? Why does it have an E as well? What methods does it force us to implement?
    • Iterable: Something that can be iterated over
    • Forces us to implement the .iterator() method

Code Demo

 

  1. When completing toArrayList, why do we need to make a copy rather then just returning our internal ArrayList?

    • Don't want to break encapsulation

  2.  What does the .iterator() method allow us to do?  Look at StackTest.java
    • Allows us to loop through it like a normal collection in a standardized way 

Code Demo

public static Integer sumStack(Stack<? extends Integer> stack);
  1. What does the <? extends Type> and <? super Type> mean?
    • extends : the parameterized type must be a class or subclass of the given type
    • super : the parameterised type must be a class or super class of the given type
  2. What is the difference between ? and E?
    • ? can't be referred to as a type (Type erasure)
    • E can

Singleton Pattern

Singleton Pattern

What type of pattern?

Creational pattern

The singleton pattern ensures that a class has only one instance. It provides a global access point to this instance.

It helps avoid initialisation overhead when only 1 copy of an instance is needed.

Code Demo

Heist.java

Code Demo

Link

Attendance

Feedback

https://forms.gle/R4sMTTQzPC4vqXSN8​