AP CSA Unit 1 Built-In Classes: Strings, Math, APIs, and Comments

String (Java)

An object representing a sequence of characters (letters, digits, punctuation, spaces); variables of type String store a reference to a String object.

Immutable (String immutability)

A property meaning a String cannot be changed after it’s created; any “modification” method produces a new String object.

String literal

A String value written directly in code using quotes, e.g., "Ava" or "".

Concatenation

Joining strings together, commonly using the + operator (e.g., "Hello, " + name).

String conversion (with +)

When one operand of + is a String, Java converts the other operand to a String and concatenates.

Evaluation order pitfall (String + numbers)

Once concatenation with a String starts, later + operations concatenate as text unless parentheses force arithmetic ("Sum: " + 2 + 3 → "Sum: 23").

length()

A String instance method that returns the number of characters in the string (counts spaces/punctuation too).

Valid String indices

For a String of length n, the valid character indices are 0 through n − 1.

Off-by-one error

A common indexing mistake where code uses an index one too high/low (often forgetting indices stop at length() − 1 or end index is exclusive).

substring(int from, int to)

Returns a new String slice starting at index from (inclusive) and stopping before index to (exclusive).

substring(int from)

Returns a new String from index from (inclusive) to the end of the original string.

Exclusive end index (substring)

In substring(from, to), the character at index to is NOT included; to is the first index not taken.

StringIndexOutOfBoundsException

A runtime error that occurs when substring/char indexing uses an index outside 0..length() (or violates from/to rules).

indexOf(String str)

A String method that returns the index of the first occurrence of str, or -1 if not found.

indexOf(String str, int fromIndex)

An overloaded indexOf that begins searching at fromIndex; returns first match at/after that position, or -1 if not found.

Sentinel value -1 (indexOf)

The value returned by indexOf when the search string is not found; must be checked before using the result as an index.

equals(String other)

A String method that returns true if two strings have the same characters in the same order (content comparison).

== vs equals (Strings)

== compares references (same object); equals compares contents (same characters).

compareTo(String other)

Compares strings in lexicographic (dictionary) order; returns

Lexicographic order (case sensitivity)

Ordering based on character codes; uppercase letters typically come before lowercase, affecting compareTo results.

Static method

A method called using the class name (not an instance), e.g., Math.sqrt(81); you do not create an object to use it.

Math.abs(x)

A static Math method that returns the absolute value of x (distance from 0).

Math.pow(a, b)

A static Math method that returns a^b as a double (even if inputs look like integers).

Math.random()

A static Math method that returns a pseudorandom double in the range [0.0, 1.0) (0.0 inclusive, 1.0 exclusive).

Random integer range formula

To get an int from min to max inclusive: (int)(Math.random() * (max - min + 1)) + min (cast after scaling).