Unit 2 · Foundational
Where a program starts making choices. The unit covers building boolean expressions, ordering branches so they do not overlap, and the short-circuit behavior that lets a condition guard itself against its own errors.
What a strong answer looks like
A strong Unit 2 answer evaluates each subcondition separately, then states which branch runs first and why the later branches are skipped.
Know
And requires both operands true; or requires at least one; not inverts. De Morgan rules let a negated compound be rewritten without the outer negation.
Apply
Build a truth table for an unfamiliar condition rather than reasoning about it in prose.
Watch out
Negating a compound condition by negating each part without swapping and with or.
Study move
Rewrite a negated compound condition using De Morgan rules and confirm both forms agree on every case.
Know
An else-if chain runs at most one branch, while separate if statements can all run. The order of the branches therefore changes the result when ranges overlap.
Apply
Trace a classifier at the exact boundary values, not at values comfortably inside each range.
Watch out
Writing overlapping conditions in an order where an earlier branch swallows a later one.
Study move
Trace a grade classifier at 89, 90, 79, and 80 and confirm each lands in the intended branch.
Know
Compound conditions combine comparisons, and each part must be evaluated independently before the whole can be judged.
Apply
Evaluate each comparison to true or false first, then combine, rather than reading the condition as a sentence.
Watch out
Chaining comparisons as in mathematics. Java requires each comparison to be written out and joined explicitly.
Study move
Rewrite a range check as an explicit conjunction of two comparisons and test both endpoints.
Know
And stops as soon as a false operand is found; or stops at the first true one. The remaining operands are never evaluated, which is what makes a guard clause work.
Apply
Place the check that prevents an error before the operation that could cause it, relying on short-circuiting to protect it.
Watch out
Reversing the order of a guard, so the risky operation runs before the check that was meant to prevent it.
Study move
Write a condition that safely tests a property of a possibly-null object, then swap the order and explain what breaks.
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
Write a condition that accepts an age between 13 and 19 inclusive, then state what it returns at 12, 13, 19, and 20.
