Decomposition and contracts

Decomposition — the design skill the exam's FRQs reward:

1. Read the spec; identify the inputs, the output, and the steps.
2. If the steps are complex, name each one as a helper method.
3. Write the top-level method as a sequence of helper calls.

Small worked example — "how many years until doubling a population?":

public static int yearsToDouble(double start) {
    double pop = start;
    int years = 0;
    while (pop < start * 2) {
        pop *= 1.05;      // 5% growth
        years++;
    }
    return years;
}

Preconditions and postconditions are the contract style the exam uses:

• Precondition: what must be true when the method is called (n >= 0).
• Postcondition: what the method guarantees when it returns (returns the sum of digits of n).

A method may assume its precondition; it must deliver its postcondition. Reading the precondition first tells you which edge cases you may skip and which you must handle.