Lesson3 - De Anza College CIS

Welcome to Lesson 3!

Learning Objectives

By the end of today's lesson, you should know...

What kinds of data the computer can store in its memory
The four most common data types in Java and what kind of data they represent:

int - whole number or integer data
boolean - true or false values
double - numbers with a decimal point
char - character data

How computers store data in memory.
What is a variable.
How to name variables.
How to declare variables.
How to assign values to variables.

1. Lesson 2 Practice Exam Questions

Write a complete program to print out the message Greetings! to the console window.

Assume you are writing it in a file named Greeting.java
Also, make sure that you have a correct comment up top with your name and section information
How much of the code can you remember

Add an inline (single line) comment to your program that states System.out is Java's name for the screen

Place this comment above or next to your command to print the greeting

2. Introducing Data Types

What is a data type?

What is data?

Information produced or stored by the computer.

What is a type?

The kind of data.
For example, numbers vs letters.

Data types are important because the compiler treats different types of data differently:

What kind of operations can be performed. Ex: We can't add and subtract letters.
How much storage to allot. Numbers and letters are stored differently in memory.

There are four basic categories into which each data type falls:

Category	Explanation	Example
Integer	A whole number. No decimal point.	106
Floating Point Number	A number with a decimal point.	3.14
Character	Individual letters, digits and symbols	'A', '%', '9'
Boolean	True/False, 0/1, On/Off	true, false

Java Has a Variety of Data Types:

Type	Bytes	Use
`boolean`	1	A `true` or `false` value.
`char`	2	All Unicode characters, which may require more than 1 byte of storage
`short`	2	Short integers from -32,768 to 32,767.
`int`	4	Integers from -2,147,483,647 to 2,147,483,647.
`long`	8	Integers from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807 (quintillion).

`float`	4	Single-precision, floating-point numbers from about E-45 to E38 with 6 or 7 significant digits.
`double`	8	Double-precision, floating-point numbers from E-308 to E308 with from 14 to 15 significant digits.

Note that the number of bytes used for storage depends on the system and compiler

Group Activity: Name That Data Type!

Which of the 4 most common data types are the values below. Select int, double, boolean or char.

'A' is a(n) _____________________
-213.555 is a(n) ________________
14 is a(n) _____________________
14.0 is a(n) ____________________
false is a ______________________
'$' is a ________________________
-2147483647 is a ______________
true is a _______________________

Most Common Data Types:

Type Memory Used Example Size Range

int 4 bytes 10 -2,147,483,648 to 2,147,483,647

double 8 bytes 10.5 10e-38 to 10e38

char 2 bytes 'c' not applicable

boolean 1 byte true not applicable

Storing Data In the Computer's Memory

Let us pretend that we have a friend, named Grace, and we want to remember her phone number: 555-2368
We can store our friend's phone number in our memory
We even give our friend's phone number a label, like "Grace's phone number"
We do not really know where in our brain we store Grace's phone number
However, whenever we need her phone number, we say to our self, "What is Grace's phone number" and out pops 555-2368
Just like we store our friend's number in our memory, we can store it in a computer's memory
We store data in a computer program using a variable
variable: the name of a place to store data in a computer's memory
Just like we do not know where in our brain we store a phone number, we do not know where in computer memory we store data
We simply give it a name and let the compiler decide where to store the data

Displaying sum of two integers using two variables only in C programming

Recall How Computer's RAM Memory Works:

Each memory location has eight bits, which are known as a byte
The memory address is the number that identifies a memory location
Some data is too large for a single byte
For instance, most numbers are too large for one byte
In these cases, the address refers to the first byte
The next few consecutive bytes store the additional bits for larger data

3. Variables

When we want to instruct a computer to store or retrieve data from its memory, we don't want to have to refer to the memory location in 1s and 0s.
Wouldn't it be nice if we could use a name or label for a location in memory?

Most people can remember a name better than a number.
We would rather remember a name than a memory location in 1s and 0s.

Fortunately, there is a way to give labels to the contents of memory addresses.
Variables are labels used to reference locations in computer memory.

We don't want to have to remember memory addresses like this:

Instead we would rather remember memory addresses like this:

In the above diagram, num1 is a variable that refers to the contents of memory address 1652, num2 is a variable that refers to the contents of memory address 2548, and total is a variable that refers to the contents of memory address 45.
When we use variables, we let the computer remember the address and size of the memory, we just remember the name of the variable.

Variables by Analogy

We can think of variables as boxes made of computer memory
A variable is just like a box that can hold a single thing
We need a way to tell one box from another so we give it a name
As we discussed, we can make up almost any name we want for the variable
For example, we declare a variable "a"
```
int a;
```
When we first declare a variable the box is empty but has a name
Storing values in a variable is like putting items in a box
For example, we assign the variable "a" the value 1
```
a = 1;
```
We can think of the above statement as putting the value 1 in a box with the variable name "a", like the one shown below (images source: David Goodger)
We put names on a box
We put values inside of boxes
When we want to see the value inside a box we look at it with code like:
```
System.out.println("The value stored in box a: " + a);
```
If we change the value of a variable, we are putting a new value in the same box, like:
```
a = 2;
```
Making a new variable and assigning one variable to another makes a copy of the value and puts it into the new box:
```
int b = a;
```
We now have two different boxes that have independent values

Variable Declarations

To create a new variable, you need to tell the compiler what kind of data to expect.

Different data types require different numbers of bytes to store

e.g. integers are stored in 4 bytes while doubles are stored in 8 bytes.

Different operations can be performed on different types of data

e.g. You can't add characters together, but you can add integers together.

You also need to give your variable a name.
Each variable declaration (statement) needs to be followed by a semi colon ;
Let's look at some examples

double pi;

int numberStudentsClass;

boolean is_answer_correct;

char grade;

boolean tooCool4U;

All Java program variables must be declared before using them
A declaration statement both names a variable and specifies the type of data it can store

General syntax:

dataType VariableName1, VariableName2, ...;

Where:
- dataType: one of the Java data types
- VariableNameX: the name of the variable
When we declare a variable, the compiler sets aside memory space
However, the contents of the storage space is undefined until we assign a value
For instance, after we declare the following variable, what is its value?

int my_height;

Rules for Naming a Variable

A variable name is a sequence of letters, numbers, or underscore characters
The first character must be either a letter or an underscore character ( _ )
- Cannot be a number
- An _ is allowed but its use is discouraged.
Also, the variable name cannot be one of the Java reserved words (keywords), such as int or double

For a list of reserved words, see: Java Keywords

Spaces are not allowed in variable names

A space is NOT a letter, number or underscore character

Also, variable names are cAsE sEnSiTiVe

id, ID, iD and Id are all valid but different names

Activity 3.1: Variable Declarations (10 pts)

Open up Eclipse and create a new project called Activity3, with a new class named Vars
Add a block comment at the start of your program to include your name and section information. For example
/**
* @author Jennifer Parrish
* CIS 36A
*/
Inside of Vars.java declare 5 new variables as follows:

A variable to store how many quarters you have been at De Anza
A variable to store your age
A variable to store whether or not programming is fun
A variable to store your middle initial
A variable to the value of pi
Invent your own!

Note that I should only see variable declarations inside your code - and the variables should not be assigned any values (that will take place in the next activity)
When you are finished, run your code and notice what happens?

Why is there no output to the screen?
Hint: Are there any System.out.print() statements in the code

Upload Vars.java to Canvas

4. Assigning Values to Variables

Once we have specified the name and data type for a variable, we want to give it a value.
The value should match the data type.
We give a variable a value by using the = sign.
The variable name goes on the left side of the = sign.
The variable value goes on the right side of the = sign.
A semi colon ; always goes at the end of the statement.

variableName = expression;

where expression is the value we assign to the variable.
Below is an example of how to declare and assign a variable.

int numHorses;

numHorses = 12;

We can also combine the two statements above into one statement.

int numHorses = 12;

Either method is valid though the second option is preferred.

Assigning Literals and Complex Expressions to Variables

The simplest expression is a literal value:
```
length = 25;
width = 17.5;
```
Numbers like 25 and 17.5 are called literal numbers, or just literals, because we literally write them in the usual way
In each statement above, the value on right is assigned to the variable on the left
We can assign results of more complex expressions to a variable as well
```
total = num1 + num2;
slope = (y2 - y1) / (x2 - x1);
```
The expression on the right is evaluated (computed) before assignment to the variable on the left
Values placed into a variable replace (overwrite) previous values
Reading variables from memory does not change them

Initializing Variables

Initial values may or may not be assigned when variables are declared:

// These are not initialized when declared
// and have unknown values
int sum, number1, number2;

// These are initialized when declared
int sum = 0;
int number1 = 5, number2 = 10;

Good programming practice: initialize variables when declared
Variables can also change their values as the program progresses

Why are they called variables? Their content varies.

int myAge = 28;

//later in program...

myAge = 29;

Displaying Variables Using System.out.print()

To display the contents of a variable using System.out.print(), the variable should first be declared and have been assigned a value.

System.out.println(myAge); //NO!

int myAge = 12;

To display the variable, place the name of the variable inside of the parenthesis of the System.out.print() statement.

int myAge = 12;

System.out.println(myAge);

It is also possible to print a message along with the variable contents by using the + operator inside of the parenthesis of the System.out.print() statement

int myAge = 12;

System.out.println("I am " + myAge + " years old.");

Activity 3.2: Assigning Values to Variables (10 pts)

Let's practice by giving values to the variables that we declared in our last activity.
Open Vars.java and practice assigning values to each of your variables
After you have assigned a value to each variable, try displaying the variable to the console by placing it inside of a System.out.println() statement.
For example:

int quarters = 3;

System.out.println("I have been at De Anza for " + quarters + " quarters.");

When you have displayed each variable to the console, upload the updated Vars.java to Canvas under Activity 3.2

Activity 3.3: My Weekly Salary (10 pts)

Open up Eclipse.
Inside of your Activity3 project, add a new class class named Salary.
Add a block comment at the start of your program to include your name and section information. For example

/**

* @author Jennifer Parrish

* CIS 36A

Declare a variable called hours and assign it the value 40.
Declare a variable called wage and assign it a value of your choice.
Declare a variable called salary and assign it the value of hours * wage;
Run the program and verify that there are no errors.
Is this program satisfactory?
Now, alter the program to print out the values of the salary you calculated.
For example, you could print out the salary like this:

My weekly salary is $600.

Note the $ and the . above. How do you get those to display?
Submit your program to Canvas.

5. Wrap Up:

Answer the following question:

What Gets Printed to the Screen?

int oranges = 5;

int apples = 8;

int numFruit = apples + oranges;

System.out.println("Total Fruit: " + numFruit);

oranges = oranges + 1;

apples = oranges;

System.out.println( "Apples: " + apples);

System.out.println("Oranges: " + oranges);

Upcoming Assignments:

Assignment 2 due Tuesday at 11:59pm on Canvas
Activities 3.1-3.3 due Tuesday at 11:59pm on Canvas
Assignment 3 due Friday at 11:59pm on Canvas
Quiz 2 due Friday at 11:59pm on Canvas