Object State with Parallel Arrays

A class can hold parallel arrays as instance variables to represent a collection of related records. Index i in each array refers to the same logical entity — like a row in a database table spread across multiple columns.

This is the AP CSA stepping stone toward ArrayList<CustomObject>. It's especially common in FRQs that pre-date object-of-object designs or where the data is naturally tabular.

The discipline is the same as standalone parallel arrays: every operation must preserve alignment, and the class is responsible for enforcing that invariant.

Class skeleton

public class Gradebook {
    private String[] names;
    private int[] scores;
    private int size;   // number of valid entries

    public Gradebook(int capacity) {
        names = new String[capacity];
        scores = new int[capacity];
        size = 0;
    }
}

The size field tracks how many slots are actually used — distinct from the array's length (capacity).

Add a record (preserves alignment)

public void addStudent(String name, int score) {
    if (size < names.length) {
        names[size] = name;
        scores[size] = score;
        size++;
    }
}

Both arrays get the new value at the same index. Both increase by one logical entry.

Look up a record

public int getScore(String targetName) {
    for (int i = 0; i < size; i++) {
        if (names[i].equals(targetName)) {
            return scores[i];
        }
    }
    return -1;   // not found
}

Search the key array; return the corresponding value at the matched index.

The alignment invariant

The class promises that names[i] and scores[i] always describe the same student. Every method must preserve this. Adding to one array without the other, sorting one independently, or removing from only one breaks the invariant — and the class is broken from that point on.