1. Sorting a list of students records in different orders

• Strategy pattern
• In Java, each ordering is an implementation of a Comparator that can be passed to Collections.sort                                                

2. Model a collection of toys. Some toys like a Box can store other toys

• Composite pattern
• Composite nodes: any toy that can store other toys
• Leaf nodes: any toy that is not a composite                                                                   

3. Adjusting the brightness of computer screens based on a light censor

• Observer pattern
• The light censor is the Subject, the computer screens are the Observers
• When the light censor changes, the corresponding action is that the brightness of the computer screens may change

4. Model a VendingMachine that could either be in idle mode, product selection mode or payment mode.

• State pattern
• Each mode (i.e. Idle, ProductSelection, PaymentPending) should be encapsulated into its own state   
• It's not the strategy pattern because each state defines how it can transition to another state

5. Model PacMan where its ability (i.e. speed, strength) can be boosted when it runs over a buff

• Decorator pattern
• Each buff is a decorator that can be used to boost PacMan. At any given time, it can have different buffs and or multiples of the same buff

Mark, Bill and Jeff are working on a PetShop application. The PetShop has functionality to feed, clean and exercise different types of animals. Mark notices that each time he adds a new species of animal to his system, he also has to rewrite all the methods in the PetShop so it can take care of the new animal.

How do code smells cause problems when developing code?

Is a code smell always a sign of a design problem?

No. For example, sometimes message chaining a.getB().getC() is unavoidable