AP Computer Science A flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Reference variable

A variable that holds the memory address of an object rather than the object's data. Two references can point to the same object.

null

A special reference value meaning no object is referenced. Calling an instance method on a null reference throws NullPointerException at run time.

Method signature

The method name plus the ordered list of its parameter types. Return type and parameter names are not part of the signature.

Pass by value

Java copies each argument into the parameter. Primitives copy values; object references copy addresses, so the referenced object can be mutated but not reassigned.

String immutability

String objects never change after creation. Methods like substring and toUpperCase return new String objects instead of altering the original.

equals versus == for Strings

equals compares character content, while == compares reference identity. Two equal Strings created separately may make == false and equals true.

Integer division

Dividing two int values discards the fractional part and truncates toward zero, so 7/2 evaluates to 3. Mixing in a double gives a double result.

Wrapper classes

Classes such as Integer and Double that hold primitive values as objects. Autoboxing wraps a primitive; unboxing extracts it back.

Short-circuit evaluation

The operators && and || may skip their right operand. If the left side of && is false or the left of || is true, the right side is not evaluated.

De Morgan's laws

Rules for negating boolean expressions: !(a && b) equals !a || !b, and !(a || b) equals !a && !b.

if / else if chain

Conditions are tested top to bottom, and only the first true branch executes. Remaining conditions are skipped once a branch runs.

for loop header

Three parts separated by semicolons: initialization, boolean test, and update. The test runs before each iteration and the update after each body.

while versus do-while

A while loop tests before the body and may run zero times. A do-while tests after the body and always runs at least once.

Enhanced for loop

A loop of the form for (Type x : collection) that visits each element in order. It cannot change the collection's size or array length.

Off-by-one error

A loop bound mistake, such as using <= with an index that should stop at length - 1. It often causes an out-of-bounds exception.

Nested loop iterations

The inner loop completes all its passes for each single pass of the outer loop, so total iterations equal the product of the two counts.

Constructor

A method-like block with the class name and no return type that initializes a new object. Constructors can be overloaded with different parameter lists.

Default constructor

A no-argument constructor supplied by the compiler only when the class declares no constructors. Declaring any constructor removes it.

this reference

A reference to the current object inside an instance method or constructor. It disambiguates a field from a parameter with the same name.

static members

Variables and methods that belong to the class rather than any object. Static methods cannot access instance variables directly or use this.

Accessor and mutator

An accessor (getter) returns object state without changing it. A mutator (setter) changes state, often after validating the new value.

Encapsulation

Hiding an object's data behind private instance variables and exposing controlled access through public methods, which protects state from invalid outside changes.

Method overriding

A subclass method with the same signature as a superclass method. At run time the version matching the object's actual type is called.

super call

A call to a superclass constructor, written as super(...) and required to be the first statement in the subclass constructor when used explicitly.

toString method

A method returning a String description of an object. It is called automatically when the object is concatenated with a String.

Array length

The fixed number of elements in an array, stored in the final field length and accessed without parentheses, as in arr.length.

Array index bounds

Valid indices run from 0 to length - 1. Using any other index throws ArrayIndexOutOfBoundsException at run time.

Default array values

Elements of a new array are initialized to 0 for numeric types, false for boolean, the null character for char, and null for object references.

ArrayList methods

size() returns the count, get(i) and set(i, v) access elements, add(v) appends, add(i, v) inserts, and remove(i) deletes and returns an element.

ArrayList removal while looping

Removing an element shifts later elements left, so a forward index loop can skip elements. Decrement the index after removal or loop backward.

2D array dimensions

A 2D array is an array of arrays. arr.length gives the number of rows, and arr[0].length gives the number of columns in the first row.

ConcurrentModificationException

Thrown when a collection's size is changed while an enhanced for loop is iterating over it, because the loop's internal state becomes invalid.