AP Computer Science A Last-Minute Review (Night Before)

title: "AP Computer Science A Last-Minute Review (Night Before)" description: "The night-before AP CS A checklist: Java syntax, common traps, quick reference, calculator tips, and morning-of advice. Skim in 45 minutes." date: "2026-01-15" examDate: "May AP Exam" topics:

• Java Quick Reference
• Syntax Checklist
• Common Traps
• Exam Strategy

The exam is tomorrow. This is not the time to learn recursion — it's the time to skim, reset, and sleep. Spend 30-45 minutes on this page, then put your notes away.

Primitive Types & Variables

| Type | Range | Default | Notes | |---|---|---|---| | int | -2^31 to 2^31-1 | 0 | Integer division truncates: 7 / 2 = 3. | | double | ~±1.8 × 10^308 | 0.0 | Use for all division unless you want truncation. | | boolean | true or false | false | Use &&, ||, ! for logic. | | char | Unicode character | N/A | Rarely tested, but know 'a' vs "a" (char vs String). |

Casting: (double) x / 2 forces floating-point division. (int) 3.7 truncates to 3.

Math methods: Math.abs(), Math.max(), Math.min(), Math.pow(base, exp), Math.sqrt(), Math.random().

Essential String Methods

| Method | Returns | Example | Notes | |---|---|---|---| | .length() | int | "hello".length() = 5 | No parentheses on length property? No — always .length(). | | .substring(i, j) | String | "hello".substring(1, 4) = "ell" | End index is exclusive. Bounds: [0, length()). | | .charAt(i) | char | "hello".charAt(0) = 'h' | Zero-indexed. Throws exception if out of bounds. | | .indexOf(s) | int | "hello".indexOf("ll") = 2 | Returns -1 if not found. | | .compareTo(s) | int | "apple".compareTo("banana") < 0 | Negative if this < s (lexicographically). | | .equals(s) | boolean | "hello".equals("hello") = true | Use this, never ==. | | .toUpperCase() / .toLowerCase() | String | "Hello".toUpperCase() = "HELLO" | Returns new String; original unchanged. |

Immutability: Strings are immutable. s.substring() creates a new String; s is unchanged.

Array & ArrayList Quick Reference

Arrays

int[] arr = new int[10];           // Length 10, all zeros
arr[0] = 5;                        // Access element
for (int i = 0; i < arr.length; i++) { }  // Traversal

Common algorithms:

• Max: loop through, keep track of max so far, compare each new element.
• Sum: loop through, add to accumulator.
• Search: loop through, return index if found, else return -1.
• Reverse: swap arr[i] with arr[arr.length - 1 - i] for i < arr.length / 2.

ArrayList

ArrayList<Integer> list = new ArrayList&lt;>();
list.add(5);                       // Add to end
list.remove(0);                    // Remove at index 0
list.get(i);                       // Access element
list.set(i, value);                // Replace element
list.size();                       // Current size
for (int x : list) { }             // Enhanced for-loop

Critical trap: If you remove during a forward loop, your index will skip the next element. Solution: loop backwards, or use a separate list.

Class Structure Template

public class MyClass {
    private int field1;            // Instance variable
    private String field2;
    
    public MyClass(int f1, String f2) {  // Constructor
        this.field1 = f1;
        this.field2 = f2;
    }
    
    public void myMethod() {       // Instance method
        // use this.field1, this.field2
    }
    
    public static void main(String[] args) {
        MyClass obj = new MyClass(10, "test");
        obj.myMethod();
    }
}

Key points:

• Constructor name matches class name, no return type.
• Use this.field to refer to instance variables inside methods.
• public methods can be called from outside; private are internal only.
• Static variables are shared across all instances; instance variables are per-object.

Inheritance & Polymorphism

public class Animal {
    public void makeSound() { System.out.println("Generic sound"); }
}

public class Dog extends Animal {
    @Override
    public void makeSound() { System.out.println("Woof"); }
}

Key points:

• Dog inherits all methods and fields from Animal.
• @Override marks a method that replaces a superclass method.
• A Dog object can be stored in an Animal variable: Animal dog = new Dog();
• When you call dog.makeSound(), it calls Dog's version (polymorphism).
• Call parent constructor with super(); in your constructor.

Recursion Template

public static int factorial(int n) {
    if (n <= 1) return 1;          // Base case
    return n * factorial(n - 1);   // Recursive case
}

Key points:

• Base case stops recursion (e.g., n ≤ 1).
• Recursive case calls itself with a simpler argument.
• Each call uses stack memory; too deep causes StackOverflowError.
• Tracing: write out 5-7 nested calls on paper to verify logic.

Control Structures

If/Else

if (condition) {
    // code
} else if (other) {
    // code
} else {
    // code
}

Loops

for (int i = 0; i < 10; i++) { }           // Index-based
for (int x : arr) { }                      // Enhanced (no index)
while (condition) { }                      // Condition checked first

do { } while (condition);                  // Condition checked after

Top 10 Traps (Prevent These)

1. == on Strings: Always use .equals(). "hello" == "hello" may be false.
2. Integer division: 7 / 2 = 3, not 3.5. Cast to double if you need decimals.
3. Array index out of bounds: Loop must be i < arr.length, not i <= arr.length.
4. ArrayList remove during forward iteration: Skips next element. Loop backwards instead.
5. Forgetting return statement: Method doesn't return a value even though return type is non-void.
6. .substring() end index is exclusive: "hello".substring(0, 2) = "he", not "hel".
7. Null pointer exception: Call method on uninitialized object. Always initialize or check for null.
8. Off-by-one in 2D arrays: grid[row][col], not grid[col][row].
9. Modifying ArrayList while iterating forward: Add/remove changes indices. Use a separate list or iterate backwards.
10. Swapped parentheses or braces: if (x > 5) (condition in parentheses) followed by { code } (body in braces).

Syntax Checklist (Do These Right)

• ✅ All opening braces { have a closing brace }.
• ✅ All semicolons ; end statements (not after class/method definitions).
• ✅ Strings use double quotes: "hello". Single quotes are for chars: 'a'.
• ✅ Array declaration: int[] arr, not int arr[] (both work, but first is style).
• ✅ ArrayList uses angle brackets: ArrayList<Integer>, not ArrayList(Integer).
• ✅ Method calls include parentheses: .length() for Strings, .size() for ArrayList.
• ✅ Comparison uses == for primitives, .equals() for objects.
• ✅ Assignment in conditions: if ((x = 5) > 3) works, but don't confuse = (assign) with == (compare).

Morning-Of Checklist

• ✅ Get 8 hours of sleep tonight. A tired brain will confuse == and .equals().
• ✅ Eat a good breakfast. Low blood sugar kills your second hour.
• ✅ Bring two calculators (or two batteries). One dies, you're not stuck.
• ✅ Bring an extra pencil and eraser. FRQs require scratch work.
• ✅ Arrive 15 min early. Calm your brain, review your notes one last time.
• ✅ On exam day: scan all 4 FRQs in the first 3 min. Identify the easiest, start there.
• ✅ Allocate ~22 min per FRQ. Do not overspend on one problem.
• ✅ If stuck, skip and come back. Partial credit on all four beats perfection on two.

Score Boundaries (Recent Years)

Out of 108 total points (40 MCQ + 4 FRQs):

• 5: ~71-108 points (~66%+)
• 4: ~57-70 points (~53-65%)
• 3: ~43-56 points (~40-52%)
• 2: ~33-42 points (~31-39%)
• 1: Below ~33 points

You only need ~66% correct to earn a 5. You can leave questions blank and still win. Don't panic if the exam feels hard — everyone feels that way.

Final Reminders

• Arrays are 0-indexed. The last element is at length - 1.
• ArrayList is 0-indexed too. The last element is at size() - 1.
• String.substring(0, 3) on "hello" returns "hel" (index 3 is excluded).
• String.indexOf(s) returns -1 if not found; always check before using the result.
• Use .equals() on Strings; == compares object references, not contents.
• Integer division truncates: 7 / 2 = 3. Cast to double if you need 3.5.
• ArrayList can't use primitive types directly: use ArrayList<Integer>, not ArrayList<int>.

Good luck tomorrow. You've got this.

Related guides:

• 3-day cram plan
• 7-day study plan
• 1-month study plan
• FRQ practice guide