Unit 0 · Bridge unit
The bridge from AP CSA into honors work. It re-establishes Java fluency and then adds the habits the rest of the course assumes: designing before coding, testing deliberately, and explaining a decision in writing.
What a strong answer looks like
A strong Unit 0 answer explains a design choice rather than only exhibiting working code.
Know
Deciding responsibilities and interfaces before writing code is what makes a program changeable later. Design is a separate activity from coding, not a preamble to it.
Apply
Sketch the classes, their responsibilities, and how they interact before opening an editor.
Watch out
Starting to type immediately, which produces code that works and cannot be modified.
Study move
Take a working project and write, after the fact, the design you would have drawn first.
Know
Abstraction hides how something is done behind what it does. Encapsulation is the mechanism: private state, public behavior.
Apply
Expose the smallest interface that lets callers do their job, and keep representation private.
Watch out
Leaking the representation through a getter, which freezes the implementation permanently.
Study move
Write a class whose internal storage could be swapped without any caller noticing.
Know
A test plan is written from the specification, not from the implementation, so it can catch what the implementation got wrong.
Apply
Write the empty, single, typical, and boundary cases before the code, then run them after.
Watch out
Testing only the path you had in mind while coding, which confirms your assumptions rather than checking them.
Study move
Write a test plan for a method you have not implemented yet, then implement it.
Know
Explaining an algorithm in prose is separately assessed in this course and is a genuine skill.
Apply
Describe what a method guarantees, what it assumes, and what it costs, in that order.
Watch out
Narrating the code line by line rather than explaining what it accomplishes and why.
Study move
Explain a recursive method in four sentences without referring to any line of code.
Connections and techniques that receive extra attention in this unit.
Related topics some schools attach to this unit and others leave out. Covered on request rather than assumed.
Modeling drill
Take an AP CSA-era project and refactor it so that one class could have its internal storage replaced without changing any caller.
