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AP Computer Science A — Classes, Arrays and Algorithms flashcards

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Units 3–4 of the revised AP Computer Science A course: writing classes, arrays, ArrayList, 2D arrays, searching, sorting and recursion. Inheritance and polymorphism are left out because the revised course dropped that unit.

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Learn deck · part of AP Computer Science A · written against: College Board AP Computer Science A Course and Exam Description (Java subset; 2025 revision). Updated 2026-10-10.

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  1. Why are instance variables normally declared private?
    For encapsulation: outside code can reach the data only through the class's methods, so the class controls how its state is read and changed and can keep it valid.
    Class Creation › Anatomy of a Class
  2. What is the difference between an accessor and a mutator method?
    An accessor (getter) returns information about an object without changing it; a mutator (setter) changes the object's state and is often void.

    Example: getBalance() versus deposit(50).
    Class Creation › Anatomy of a Class
  3. What is the difference between private and public access?
    A private member can be used only by code inside its own class; a public member can be used from any class. In AP style, classes, constructors and most methods are public, while instance variables are private.
    Class Creation › Anatomy of a Class
  4. Inside a BankAccount method, can code read the private balance of a different BankAccount object passed as a parameter?
    Yes. Access is checked per class, not per object, so any code written inside BankAccount may use other.balance directly.
    Class Creation › Anatomy of a Class
  5. What are a method's precondition and postcondition?
    A precondition is what must be true before the method is called, which the method is not required to check; a postcondition is what the method guarantees will be true when it finishes.
    Class Creation › Anatomy of a Class
  6. What does new int[5] contain right after it is created, compared with new String[5]?
    Five 0 values for the int array; five null references for the String array. Arrays are filled with the type's default value.
    Data Collections › Arrays
  7. Given int[] a = {3, 1, 4, 1, 5}; which expression gives the last element?
    a[a.length - 1], which is 5. Indices run from 0 to length - 1.

    Watch out: a[a.length] throws an ArrayIndexOutOfBoundsException.
    Data Collections › Arrays
  8. How do you get the size of an array, a String and an ArrayList?
    Array: the field arr.length (no parentheses). String: the method s.length(). ArrayList: the method list.size().
    Data Collections › Arrays
  9. Why does for (int v : arr) v *= 2; leave the array unchanged?
    In an enhanced for loop, v is a copy of each element, so changing v changes only the copy. Use an indexed loop with arr[i] *= 2 to modify the array.
    Data Collections › Arrays
  10. An enhanced for loop runs over an array of Student objects and calls s.setGrade(12) on each. Are the objects changed?
    Yes. The loop variable holds a copy of each reference, so calling a mutator through it changes the actual objects. Only assigning the loop variable a new object would have no effect.
    Data Collections › Arrays

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About these cards

Review status: Second-reader reviewed, with corrections applied.

Checked against:

  • Topic outline follows the College Board AP Computer Science A Course and Exam Description (revised course, first exam May 2026), Units 3–4 (the AP name is used nominatively; no course or exam text reproduced).
  • Every code trace was checked by compiling and running the equivalent Java (OpenJDK); behavior checked against the Java SE API documentation (reference only, no text copied).
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