College Board A.P. Computer Science A Topics Program Design - Read and understand a problem's description, purpose, and goals. Procedural Constructs - Constant declarations ; Variable declarations Standard Data Stuctures - Simple data types (int, boolean, double)

Vocabulary & Terms Reference Variable Keyword Identifier Data type int char double boolean Object Initializing a variable Assignment operator

A reference variable refers to a location in memory that stores an object. Monster fred = new Monster(); Monster sally = new Monster(); In the above example Monster is a class. fred and sally are references to Monster objects. new is a keyword which is used to create an object. It is followed by the constructor method, which is the class name, and parenthesis. The first part: Monster fred – declares a reference variable to hold a Monster object. The second part: = new Monster(); - creates the Monster object, and assigns fred to the location where this object is in memory.

fred Monster Monster fred = new Monster(); 0xF5 Monster fred creates a reference. new Monster(); creates the Monster at location 0xF5. The assignment operator (=) gives fred the address 0xF5 to store

What is a variable?

A variable is a storage location for some type of value. Type the statements below into the interactions pane. Type the name of the variable to see what is stored in the variable. What is stored in yesOrNo? days 102 yesOrNo int days = 102; double taxRate = 7.75; boolean yesOrNo; taxRate 7.75

int days = 102; days 102 int days – declares days as a variable that will store integer values. = 102 assigns days to store the integer value 102.

How do you name variables when defining them?

When naming a variable follow these rules and conventions: Make the variable name meaningful. That means that L is not a meaningful variable name but length is meaningful. When reading your program, I shouldn’t be guessing what the variable is meant to hold. Start all variable names with a lower-case letter. Variable names may also contain letters, numbers and underscores. If a variable name is more than one word long, capitalize each of the other words without adding any spaces. Below are some examples of valid variable names: number sum32 testAverage area_Of_Trapezoid and below are some examples of invalid variable names: 2ndValue test Average question#2

Which of these would be legal variable identifiers? int 1stYear; double jump Up; double feet2Inches; IDENTIFIER is a fancy word for variable.

Java is case sensitive – it interprets upper and lower case letters as different letters. Brandon does not equal brandon. Brandon != brandon

Keywords are reserved words that the language uses for a specific purpose. You cannot use keywords as identifier names. Some that you’ve seen are: int double return voidmain public static long break continue class You can find the complete list of keywords here:

What is a data type?

byte short int long float double char boolean int num = 9; double total = 3.4; We will use int, double and boolean in this course.

TYPESIZERANGE byte8 bits-128 to 127 short16 bits to int32 bits-2 billion to 2 billion long64 bits-big to +big

Think about a light bulb. It has 2 states ON or OFF. The same principals can be applied to a computer. The electronic circuits are either On or Off. Different combinations of On and Off mean different things for the computer. For the computer On is 1, and Off is 0. A number system made up of 1’s and 0’s is known as the Binary Number System. We use the Decimal Number System: digits 0 – 9.

What is a BIT? Each 1 or 0 is called a BIT. It is short hand… take the b at the start of binary and the it at the end of digit… BIT = Binary digIT. Memory consists of bits and bytes. 1 byte = 8 bits Every character on the keyboard has a binary digit associated with it in accordance with the UNICODES. Look at the following site: What is the Binary Number for the letter A? What about a? 7? The more bits you have the more you can store.

int one = 120; int two = ; System.out.println(one); System.out.println(two); (Type the code above into the interactions pane in Dr. Java and check your output.) OUTPUT

Type the following code into Dr. Java exactly as it is written. Compile the code and run the program. Are the results what you expected? //integer example public class Integers { public static void main(String args[]) { int one = 120; //legal assignment int two = ; int three = ; System.out.println(one); System.out.println(two); three = three * 3; //creates an overflow error at runtime System.out.println(three); }

TYPESIZERANGE float32 bits-big to +big double64 bits-big to +big Float has 7 digits of precision and double has 15 digits of precision. We are talking about decimal numbers now! int can NOT store decimal numbers.

double one = 99.57; double two = 3217; double three = 23.32; System.out.println(one); System.out.println(two); System.out.println(three); (type the code above into the interactions pane in Dr. Java and check your output.) OUTPUT

Type the following program into Dr. Java exactly as it is written. Compile the code and then run the program. Are the results what you would have expected? //real number example public class Reals { public static void main(String args[]) { double one = 99.57; double two = 3217; System.out.println(one); System.out.println(two); }

Characters store letters. char let = ‘A’; char is a 16-bit unsigned integer data type. We use single quotes for character literals. We use double quotes for String literals… more than 1 character.

char is a 16-bit unsigned int data type. For example, here is a 16 bit pattern: char let = 65; ASCII VALUES YOU SHOULD KNOW!!! ‘A’ – 65 ‘a’ – 97 ‘0’ - 48

char alpha = 'A'; char ascii = 65; char sum = 'B' + 1; System.out.println(alpha); System.out.println(ascii); System.out.println('B'+1); //’B’ gets converted to its ASCII value //66 and then 1 is added to it. System.out.println(sum); //since sum holds 67 and it is a char, we //get C. OUTPUT A 67 C

Type the following program in Dr. Java exactly as it is written. Compile the code and run the program. Are the results what you expected? public class Char { public static void main(String args[]) { char alpha = 'A'; char ascii = 65; char sum = 'B' + 1; System.out.println(alpha); System.out.println(ascii); System.out.println('B'+1); //char is an integer type System.out.println(sum); }

The boolean data type can store true or false only. boolean heads = true; boolean tails= false;

Type the following program into Dr. Java exactly as it is written. Compile the code and run the program. Are the results what you expected? public class BooleanTest { public static void main(String args[]) { boolean stop = true; boolean go = false; System.out.println( stop ); stop = go; System.out.println( go ); System.out.println( stop ); }

In JAVA, you have 8 primitive data types that take up very little memory. Everything else in Java in an Object. Strings are objects. String temp = "abc";

receiver = 57; receiver is assigned( = ) the value 57. In an assignment statement, the receiver is always on the left of the assignment symbol ( = ). Assignment statements examples : total = 10; amount = ;

You can initialize more than one variable in one statement if they have the same type. Just separate them using a comma. int number = 75, it=99; float taxrate = 7.75; char letter = ‘A’, newlet = ‘a’; boolean isprime = false; String sone = "abc";

Java is a strong typed language. As a result, you must be very careful to look at data types when assigning values. int one=90; char letter= ‘A’; char let= 65; one=letter; letter=let; one=let;

int one = 90; double dec = 234.5; char letter = 'A'; System.out.println( one ); one = letter; //char to int System.out.println( one ); one = 'A'; //char to int System.out.println( one ); System.out.println( dec ); dec = one; //int to double System.out.println( dec ); System.out.println (one); one = dec; //double to int… Illegal System.out.println (one); OUTPUT Error

Type the following program into Dr. Java. Compile the code and run the program. Are the results what you expected? //strong typed language example public class MixingData { public static void main(String args[]) { int one = 95; double dec = 12.4; char letter = ‘B'; System.out.println( one ); one = letter; //char to int System.out.println( one ); one = 'A'; //char to int System.out.println( one ); System.out.println( dec ); dec = one; //int to double System.out.println( dec ); System.out.println( letter ); //letter = dec; //double to int - not legal //System.out.println( letter ); }