Booleans: Comparisons, Short-Circuits, equals

Comparisons produce boolean values, and booleans combine with && (AND), || (OR), ! (NOT):

int age = 20;
boolean adult   = age >= 18;          // true
boolean teen    = age >= 13 && age < 18;  // AND: both sides true
boolean weekend = isSat || isSun;     // OR: either side true
boolean invalid = !adult;             // NOT: flips it

&& and || short-circuit: the right side is skipped when the left side already decides the answer. That is not an optimization footnote — it is how you write safe checks:

// && guards the right side: if s is null, the length is never asked
if (s != null && s.length() > 0) { ... }

// Dangerous reversal: asks length() of null → NullPointerException
if (s.length() > 0 && s != null) { ... }

Equality has a trap of its own:

• Primitives compare by value: x == 5 is exactly right.
• Objects (including String) compare by identity with == — "is this the same object", not "same contents". Two different String objects holding "yes" can fail ==. Always compare contents with .equals:

String a = new String("yes");
String b = "yes";
a == b          // false — different objects!
a.equals(b)     // true  — same characters

Write .equals as a reflex for Strings and every other object; make == mean "primitives only" in your head.

Next: practice — the calculator, the converter, the grade book.