McGill COMP303 - Software Design Notes

My COMP303 course notes. Available in Markdown. A sample of my notes for Module 3 is available below.

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Object state

Abstract state

An abstract state is a state that impact how an object would be used. For example appending to a linked list when it is empty vs when it has more than 1 element

Optional type

Use Optional.empty() instead of null to help avoid nullpointer exceptions.

See OptionalType.java

NULL OBJECT design pattern

Goal: avoid null and Optional, but still avoid nullpointer exceptions.

Create an implementation of an interface that represents a null class instead of directly passing null so that the code doesn't break when null isn't checked.

See NullObject.java

FLYWEIGHT design pattern

Goal: Avoid redundancy when storing data

Create an array that contains the values, then newly created object simply need to keep track of the index of the value, saving memory. 4 steps to realize this:

1. private constructor
2. static initializing fields that creates the flyweight objects
3. static flyweight store (ex: array) that stores all the flyweight objects
4. static access method that returns the flyweigth objects according to a key

See Flyweight.java

SINGLETON design pattern

Goal: ensure only 1 instance of class (ex: database access, aggregation of states)

This is similar to flyweight, but in this case we only want 1 instance, so no key, and the store is a variable that points to the instance. 3 steps to realize this:

1. private constructor
2. variable that points to the instance
3. static access method

See Singleton.java

Anonymous classes

Create an anonymous class from an interface (ex: using Comparator<T>) without defining it by using return new Interface() {}

See Anonymous.java

Create a high quality equals method

1. null always return false
2. Use == to check if same instance. If so return true
3. Use instanceof to check if same type. If not return false
4. Cast to correct type
5. For each significant field, check that they are equal to eachother
   • float: use Float.compare(float, float)
   • double: use Double.compare(double, double)
   • Primitive fields: use ==
   • Others: Objects.equals(Object, Object)

Additionally, use @Override

See QualityEqual.java

Always override hashCode when overriding equals

Improper implementation of hashcode will prevent proper functioning in collections like hashmap or hashset. Always test whether 2 equal instances have same hash code.

Requirements:

1. must consistently return the same value when called on the same object if no info is modified
2. if two objects are equal according to equals(), they must have the same hashcode
3. two objects that are not equal don't need to have distinct hashcode, but that might improve performance

How to create good hashcode:

1. select random non-zero constant into int result (ex: 17)
2. for each field:
   1. compute hashcode c
   2. Combine hashcode c into result: result = 31 * result + c
3. return result

See QualityEqual.java