APCSA Unit 2

Object

An instance of a class that contains variables (data) and behaviors (methods).

Object-Oriented Programming (OOP)

A programming style where programs are organized around objects that contain data and behaviors.

Variable

A piece of data attached to an object that helps define the object's state or characteristics.

Behavior (Method)

An action associated with an object; methods are used to make an object do something.

Example of variables in an object

A Fraction object could have a numerator and denominator as variables.

Abstraction

Hiding the details of how something works because the user does not need to know those details to use it.

Example of abstraction

A driver does not need to know exactly how a car's engine works in order to drive the car.

Math Class

A Java class containing static methods that perform common mathematical operations.

Static

A keyword indicating that a method belongs to the class itself and can be called without creating an object of that class.

Math.abs()

Returns the absolute value of a number.

Math.abs(int num)

Returns the absolute value of an integer.

Math.abs(double num)

Returns the absolute value of a double.

Math.pow()

Raises a number to a specified power.

Math.pow(2, 3)

Returns $8.0$ because $2^3 = 8$.

Math.random()

Returns a random double greater than or equal to $0.0$ and less than $1.0$.

Math.sqrt()

Returns the square root of a number.

Math.sqrt(25)

Returns $5.0$ because $\sqrt{25} = 5$.

Math.pow(double num, double power)

Returns num raised to the given power.

Math class method syntax

Math.methodName(parameters)

Absolute value

The distance of a number from zero; it is always nonnegative.

pow

Short for power; used to raise a number to an exponent.

random

Used to generate a random decimal number from $0.0$ up to, but not including, $1.0$.

sqrt

Short for square root; returns the square root of a number.

String object

An object that stores a sequence of characters.

String.indexOf(String str)

Returns the first position (index) where the specified string occurs; returns $-1$ if it is not found.

word.indexOf("E") for word = "COMPUTER"

Returns $6$.

word.indexOf("P") for word = "COMPUTER"

Returns $3$.

word.indexOf("U") for word = "COMPUTER"

Returns $4$.

word.indexOf("Y") when "Y" is not present in word

Returns $-1$.

Index of the first character in a String

$$0$$

Indexing of String characters

Starts at $0$ and increases by $1$ for each character.

String.length()

Returns the length (number of characters) in the String.

String.substring(int from, int to)

Returns a substring starting at the from index and ending before the to index.

String.substring(int from)

Returns a substring starting at the from index and continuing to the end of the String.

boolean equals(String str)

Checks whether two Strings are equal.

int compareTo(String str)

Compares two Strings lexicographically.

Syntax for creating a String object

String variableName = new String("text");

Example of creating a String object

String joMama = new String("is so fat");

Syntax for calling a String method

object.methodName(parameters)

Example of calling substring on an object

String big = joMama.substring(6);

Printing output in Java

System.out.println(...);

substring(1, 3) for "COMPUTER"

"OM"

substring(6) for "COMPUTER"

"ER"