Unit 2: Types and Expressions

Unit 1 used four types on trust. This unit opens the whole type system: the eight primitive types, int against double, char and boolean, and what a reference variable actually holds. Arithmetic opens up with the full operator set, integer division, the remainder, compound assignment and final. Conversion is taught in three stages, what Java widens for you, what you must cast yourself, and the fact that a cast cuts rather than rounds.

The performance task is The Calculator That Is Honest About Money. The student computes a money result from values declared in the source, with a Type Table giving one row per variable saying why that type and not another, and two Failure Exhibits. Each exhibit shows the broken line, the wrong output copied from a real run, the fixed line and the right output, one for integer division and one for floating point rounding. A calculator that is correct by accident does not meet the standard.

The unit refuses to hand wave two things students will hit. Comparing two String values with == does something surprising, which is shown and then explicitly handed to Unit 3, and 0.1 plus 0.2 does not print 0.3, which is shown with the real output and answered with the two strategies this course allows, whole cents as int or deliberate rounding with Math.round. A teacher gets a plan and a substitute plan for each of the fifteen lessons, answer keys, tiered versions of the hardest handouts, and every document in editable Word and print ready PDF.

The unit runs 15 instructional days.