Using Objects and Methods — the one-page version

Tip: If a question shows code and asks "what happens," check syntax first (missing semicolon, mismatched braces, undeclared variable = won't compile), then look for run-time traps (division by zero, null, bad index), then trace for logic.

Tip: Questions on ranges are usually about overflow: Integer.MAX_VALUE + 1 equals Integer.MIN_VALUE, not an error. And a variable declared but never assigned can't be used — that's a compile error, not a default of 0.

Tip: Look at both operands of every /. Both int? Truncate. Either double? Decimal result. For the + trap, scan left to right and note the moment a String appears — from then on everything is concatenation.

Tip: Questions about Scanner are usually "which method reads a whole line vs. one token" (nextLine vs next) or the nextInt-then-nextLine trap. Also remember: = assigns; a question with if (x = 5) is a compile error because an int isn't a boolean.

Tip: Ask two things for every cast: what exactly is being cast (the next value only, unless parentheses say otherwise), and is it truncation or rounding (a bare (int) always truncates). Toward zero: (int) -2.9 is -2.

Tip: Rewrite every compound operator as its long form before tracing: x *= a + b → x = x * (a + b). The parentheses are the part students drop.

Tip: If a question asks why a program won't compile and it uses Scanner or ArrayList without an import, that's the answer. If it asks what a method returns, look at the header's return type — the exam sometimes gives you a header for a made-up method and expects you to use it correctly from the header alone.

Tip: Questions ask "which statement is a precondition?" — look for what must be true of the input before calling. Postconditions describe the result. And when an FRQ states a precondition, don't waste time writing code to check it.

Tip: To decide which overloaded method a call uses, count the arguments and match their types. When an FRQ gives you a method header to implement, copy it exactly — changing the return type or a parameter name costs points.

Tip: Trace method calls by writing down the line you're returning to, evaluating the method with the argument values, then substituting the return value back. Multiple calls on one line are evaluated left to right.

Tip: For "which expression produces a random integer from a to b," check two things: the multiplier equals the count (b − a + 1), and the added value is a. For "what range does this produce," the minimum is the added value and the maximum is that plus the multiplier minus one.

Tip: Class vs. object questions are definitional: class = template/type, object = one instance with specific values. "How many objects are created" = count the new keywords (plus string literals).

Tip: Draw boxes and arrows. Each new is a box; each variable is an arrow. Assignment between reference variables re-points an arrow — it never copies a box. If an arrow points to nothing (null) and you call a method through it, the program crashes.

Tip: When a question gives you a class's method headers and asks what a code segment does, check each call: is it a mutator (state changed for later lines) or an accessor (value used now)? Track object state in a table just like variables.

Tip: Three checks on every String question: (1) substring's second index is exclusive — count characters from from to to - 1; (2) any content comparison must use .equals, never ==; (3) compareTo gives an int, so if (a.compareTo(b)) won't compile — you need < 0, == 0, or > 0.