AP Computer Science A practice questions and exam guide

What is printed? int[] arr = {10, 20, 30}; System.out.println(arr[3]);

1. 30
2. null
3. ArrayIndexOutOfBoundsException
4. 0

Answer: C — ArrayIndexOutOfBoundsException

Valid indices: 0, 1, 2. arr[3] is out of bounds (length 3, indices 0..2). Throws ArrayIndexOutOfBoundsException at runtime.

Trap: which direction does inheritance flow? If Dog extends Animal:

1. Dog is a subclass; Animal is the superclass; Dog INHERITS from Animal (gets fields/methods FROM parent)
2. Animal inherits from Dog
3. They share equal status
4. Neither inherits

Answer: A — Dog is a subclass; Animal is the superclass; Dog INHERITS from Animal (gets fields/methods FROM parent)

Inheritance flows from parent (super) to child (sub). Child gets parent's accessible members. Child is more specific. 'super' refers to parent. Reversing super/sub is a common conceptual error.

What is the value of the Boolean expression: (5 > 3) && (2 < 1)?

1. true
2. Error
3. false
4. null

Answer: C — false

Java logical AND (&&): both must be true. First condition true; second false → overall false. Logical OR (||): either true → true. Logical NOT (!): inverts. Short-circuit: && stops if first false; || stops if first true.

An ArrayList in Java differs from a primitive array because it:

1. Provides a dynamic-size, ordered collection with methods like add, remove, get, size — backed by an internal resizable array
2. Stores only primitives
3. Cannot be iterated
4. Has fixed size like primitive arrays

Answer: A — Provides a dynamic-size, ordered collection with methods like add, remove, get, size — backed by an internal resizable array

A) Defining property of ArrayList. B/C/D) Each contradicts.

Integer division in Java (e.g., 7 / 2) yields:

1. 3.5
2. 3 (truncates toward zero); to get 3.5, cast one operand to double: 7 / 2.0 = 3.5
3. 4
4. An error

Answer: B — 3 (truncates toward zero); to get 3.5, cast one operand to double: 7 / 2.0 = 3.5

If both operands int → result int (truncates, not rounds). Common bug. Solutions: (double) 7 / 2 = 3.5, or 7.0 / 2 = 3.5. Modulo: 7 % 2 = 1 (remainder). For negative: −7 / 2 = −3 (toward zero), −7 % 2 = −1.

Where can a private instance field be accessed?

1. Only from inside its declaring class (including its own methods); not from subclasses or other classes
2. Anywhere
3. Only from subclasses
4. Only from same package

Answer: A — Only from inside its declaring class (including its own methods); not from subclasses or other classes

private: visible ONLY within the same class. Encapsulation. Subclasses cannot directly access private fields of parent (use protected or public getter).

How many total prints occur? for (int i = 0; i < 3; i++) for (int j = 0; j < 5; j++) System.out.print("X");

1. 8
2. 5
3. 3
4. 15

Answer: D — 15

Nested loops: outer × inner = 3 × 5 = 15 prints. Standard pattern for 2D iteration.

What is the difference between list.size() and arr.length?

1. ArrayList uses size() (method with parentheses); arrays use length (field, no parens) — common AP CSA trap question
2. No difference
3. Both are methods
4. Both are fields

Answer: A — ArrayList uses size() (method with parentheses); arrays use length (field, no parens) — common AP CSA trap question

Three distinct: array.length (field), ArrayList.size() (method), String.length() (method). Mixing them up is a common syntax error and AP exam target.

What is printed? int a = 10; int b = 3; double c = a / b; System.out.println(c);

1. 3.3333333333333335
2. 3.0
3. 3
4. 3.33

Answer: B — 3.0

Trap: a / b is int division (= 3) BEFORE the result is assigned to double c. So c = 3.0. To get 3.333..., cast first: (double) a / b.

If a class implements the Comparable interface, it must implement:

1. equals() only
2. compareTo(Object other) — returns negative, zero, or positive int based on natural ordering — used by sorting algorithms
3. toString() only
4. hashCode() only

Answer: B — compareTo(Object other) — returns negative, zero, or positive int based on natural ordering — used by sorting algorithms

Comparable<T> interface: int compareTo(T other). Returns negative if 'this' < other, zero if equal, positive if 'this' > other. Used by Collections.sort() and Arrays.sort() to order elements. Should be consistent with equals() (a.compareTo(b) == 0 should imply a.equals(b)).

What does this print? Integer x = 5; int y = x + 3; System.out.println(y);

1. Error: Integer != int
2. null
3. 53
4. 8

Answer: D — 8

Autoboxing/unboxing: Integer x = 5 boxes int 5 into Integer. x + 3 unboxes x to int, then adds. y = 8.

What is printed? String s = "abc"; s.toUpperCase(); System.out.println(s);

1. "ABC"
2. Error
3. "Abc"
4. "abc"

Answer: D — "abc"

Strings are immutable. toUpperCase() returns a NEW string but the original s is unchanged. The return value is discarded here. To save: s = s.toUpperCase().

Why is mergesort more efficient than bubble sort for large arrays?

Mergesort: O(n log n). Bubble: O(n²). For n = 1000: log n ≈ 10, n = 1000 → mergesort ~10,000 operations vs. bubble 1,000,000.

What's the difference between = and ==?

= : assignment. ==: comparison (returns boolean).

Common error: forgetting return statement.

Non-void method must return on every path. Compile error otherwise.

When is `boolean b = (x > 0);` true?

When x is greater than 0.

What is integer division in Java?

Division of two ints truncates toward zero. 7/2 = 3 (not 3.5).

What's the AP CSA Quick Reference?

Provided on exam: signatures of common Java library methods (String, Math, ArrayList, etc.).