AP Computer Science A — Unit 6 Review: Arrays in Java

1. What Is an Array?

• A variable holds one value; an array holds multiple values of the same type in indexed slots.
• Declared once but each element accessed by zero-based index.
• Use case: Manage related data (e.g., runs per inning) with a single structure instead of many variables.

2. Declaring & Initializing Arrays

▶️ Fixed Size Declaration

int[] scores = new int[9];
// size 9, defaults to 0 for ints

• Syntax: type[] name = new type[size];

• Defaults:

• int → 0, double → 0.0, boolean → false, object → null

▶️ Literal Initialization

int[] data = {1, 2, 3, 4};
// size 4, elements set explicitly

3. Accessing & Modifying Elements

array[index] = newValue;   // assign
int x = array[index];      // retrieve

• Index out of bounds if index < 0 or index >= array.length → runtime exception.

4. Memory Model: Primitives vs. Arrays

• Primitives (int a = 3;) store value on the stack.
• Array variables store a pointer on the stack, pointing to an array object on the heap.
• Assigning one array reference to another copies the pointer (both variables refer to same object).

Stack:  c ──▶ [1,2,3,4]
        d ──▶ [1,2,3,4]

Heap:   [1,2,99,4]

(Modifying via either reference affects the same heap object.)

5. Traversing Arrays

▶️ for Loop

for (int i = 0; i < arr.length; i++) {
    System.out.print(arr[i] + " ");
}

• Control start/end/update in header.
• Can modify i behavior (step size, start index).

▶️ for-each Loop

for (int value : arr) {
    System.out.print(value + " ");
}

• Simpler syntax; no indexing and cannot modify original array elements.

6. Common Array Algorithms

▶️ Reverse an Array In-Place

for (int i = 0; i < arr.length/2; i++) {
    int temp = arr[i];
    arr[i] = arr[arr.length - 1 - i];
    arr[arr.length - 1 - i] = temp;
}

• Swap elements from ends toward center.
• Uses length/2 as number of swaps.

▶️ Find Largest Element

int largest = arr[0];
for (int i = 1; i < arr.length; i++) {
    if (arr[i] > largest) {
        largest = arr[i];
    }
}
System.out.println("Largest = " + largest);

• Initialize with arr[0], compare each, update if larger.
• For smallest, invert comparison (<).

7. Key Points & Best Practices

• Zero-based indexing: first element at index 0.
• Always check bounds: 0 <= index < arr.length.
• Use arr.length, not arr.length(), to get size.
• Choose for vs. for-each based on need to modify or skip indices.
• Avoid null-pointer errors by initializing arrays before use.

Master these patterns to handle AP CSA array questions efficiently!