Unit 1: Using Objects & Methods

Variables, Strings, the Math class, and what "calling a method" really means.

15–25% of the AP CSA exam

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Primitives vs. objects — the distinction everything builds on

Java has two kinds of data. Primitives (int, double, boolean) store the value itself. Objects (like String) store a reference — an arrow pointing at the data. Half the tricky MC questions in this unit are really testing whether you know which one you're holding.

int score = 42;            // the box holds 42 itself
String name = "Ada";      // the box holds an arrow to "Ada"

Integer division — the #1 point thief

When both sides are int, Java throws away the decimal. It does not round.

int a = 7 / 2;        // 3, not 3.5 — decimal chopped off
double b = 7 / 2;     // 3.0 (!) — division happens as ints FIRST
double c = 7.0 / 2;   // 3.5 — one double makes it real division
int r = 7 % 2;        // 1 — the remainder (mod)

Strings: the methods you must know cold

String indices start at 0, and substring(a, b) goes from index a up to but not including b.

String s = "crash course";
s.length()             // 12 — count of characters
s.substring(0, 5)      // "crash" — indices 0,1,2,3,4
s.substring(6)         // "course" — from 6 to the end
s.indexOf("course")    // 6 — where it starts (-1 if missing)
s.equals("crash")      // false — ALWAYS use equals, never ==

Try it — exam-style questions

1. What does this print?
int total = 9;
double avg = total / 4;
System.out.println(avg);
2. What is s.substring(2, 5) if String s = "computer";?
3. Which comparison correctly checks if two String variables a and b hold the same text?
4. What does Math.random() return?

⚠ Common mistakes that cost points

Unit 1 shaky? It's the foundation for everything else.

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