IT 114: Introduction to Java

IT 114: Introduction to Java
Class 6

Topics

Review

Variables
Declaring Variables
Assignment Statements
Literals
= Does Not Mean Equals
Variable Names
Choosing Good Variable Names
Java Is Case Sensitive
Printing Variables

New Material

Operators
Arithmetic Operators
The Mod Operator
Two Groups of Data Types
Strings
Concatenation
Concatenating Strings and Numbers
Four Ways To Store Numbers

Homework 3

Homework 3 is posted here.

It is due this coming Sunday, at 11:59 PM.

Review

Variables

One of the most important concepts in programming is the variable
A variable is a place in RAM that holds one value
And has a name
Variables are much simpler than objects
Objects can store many values, not just one
And they can contain code
Objects do not have names
They have locations in RAM

Declaring Variables

A variable in Java can only values of one data type
If you give it a different type of value you will get an error
In Java you must create the variable before you use it
You do this with a variable declaration
A variable declaration is a statement
It uses the following format
```
DATA_TYPE VARIABLE_NAME ;
```

Here are some examples

int number;
double interest;
char ch;
bool finished;

Assignment Statements

You give a variable a value with an assignment statement
It has the following format
```
VARIABLE_NAME = VALUE
```

Here are some examples

number = 5;
interest = .065;
ch = 'x';
finished = false;

When we give a variable its first value we are said to initialize the variable
Declaring a variable does not give it a value
If you try to use a variable before you initialize it you will get an error
Java lets you combine declaring and initializing a variable in single statement with the following format
```
DATA_TYPE VARIABLE_NAME = VALUE ;
```

Here are some examples

int number = 5;
double interest = .065;
char ch = 'x';
bool finished = false;;

Literals

In the statements above we wrote the values of the variables directly inside the the source code file
These values are examples of literals
Programmers sometimes refer to such values as "hard coded"

= Does Not Mean Equals

= is an example of an operator
Operators are special characters that perform an action using one or more values
= is the assignment operator
It takes the value on the right hand sign and gives it to the variable on its left
In arithmetic means equals
But that is not what is really going on with = in Java
Because the value of a variable can change
When you read a line of code like this
```
number = 5
```
You should not say "number equals 5"
You should say "number gets 5"

Variable Names

Variable names in Java have to follow certain rules
The first character must be
- A letter
- An underscore, _
- A dollar sign, $
After the first character you can also use digits
Variable names cannot have spaces
But you can use underscores, _
Here are some examples of legal Java variable names
```
rate
$name
_init
first_name
value1
value_2
_
```
The following names are not legal
```
1st_name
first name
```
Although the first letter can be $ or _, they should be avoided

Choosing Good Variable Names

Just because a variable name is legal doesn't mean it's a good variable name
Consider the following code
```
d = s * t
```
The name of a variable should always describe the value it holds
So speed is a good variable name
But xena_the_warrior_princess is not
Sometimes you need more than one word to describe the value contained in a variable
When this happens, you should use the underscore character, _ to separate words like this
```
course_id
student_name
first_name
```

Java Is Case Sensitive

Both Unix and Java are case sensitive
This means that UPPERCASE (CAPITALS) and lowercase (like this) letters are completely different
This means that the following variable names are are different variables
```
first_name
First_Name
First_name
FIRST_NAME
```
To keep things simple and avoid errors, use lowercase only for variable names

Printing Variables

Once you have defined a variable and given it a value you can use println to print it
```
int number = 1;
System.out.println(number);
```
Notice that there were no quotes, "" around number

New Material

Operators

When you do arithmetic you write things like
```
5.5 + 7
9 - 3.1
```
The characters + and - are operators
Operators are special characters that perform an action
Usually the operator creates a new value
Operators work on one or more values
These values are call operands
Most operators take 2 operands
But some take only 1
There is even an unusual operator that takes 3 operands
There are many different kinds of operators in Java
But we will start out with the operators used in arithmetic

Arithmetic Operators

Java has five arithmetic operators

Operator	Meaning	Example	Result
+	addition	2 + 2	4
–	subtraction	53 – 18	35
*	multiplication	3 * 8	24
/	division	4 / 2	2.4
%	remainder or modulo	19 % 5	4

The Mod Operator

The last operator, %, may not be familiar to you
Division works differently for integers than for decimal numbers
Dividing decimal numbers results in a single value
```
7.0 / 2.0 = 3.5
```
That value is also a decimal
But division with integers returns two values
- The quotient
- The remainder
Let's look at an example
```
17 / 5
```
In integer division, the quotient is always an integer
In the expression above, the quotient is 3
But there is also a remainder which is also an integer
In this case, the remainder is 2
The division operator, / works for both integers and decimals
As does the remainder operator , %
This operator is something called the modulus operator
So 17 divided by 5 can have two results
```
	
17 / 5 = 3
17 % 5 = 2
```
Whenever one integer divides another evenly % returns 0
```
	
10 / 5 = 2
10 % 5 = 0
```
The remainder operator has many uses in programming

Testing whether a number is odd or even

x % 2 = 0  // x is even
x % 2 = 1  // x is odd

Finding the individual digits of a number
```
	
x % 10  // the last digit of x
```

Finding the last four digits of a social security number

ssNum % 1000 // the last four digits of the social security number

Two Groups of Data Types

When I first mentioned data types I said there were 8 Java data types
But we would only use 4
This was a simplification
In truth there are two groups of data types
- Primitive data types
- Classes
Primitive data types can only hold one value
```
int number = 5;
char ch    = 'X;'
```
Classes define objects which can hold many values

Strings

The char data type can only hold one character
What is the data type of "Hello world!'
It cannot be a char because it has more than one character
It is of type String
Which is not a primitive data type
I can declare a String variable like like this
```
String message;
```
Strings are a sequence of characters, one right after the other
Strings can include spaces and tabs
Whenever a program contains a string written out inside the code string literal
In Java, a string literal must be enclosed with double quotation marks
You must use double quotes, not single quotes
A string literal must be on a single line
It cannot continue onto the next line
To reiterate, char variables contains a single character
Character literals use single quotes
```
char ch = 'Z'
```
String variables contain many characters
String literals use double quotes
```
String message = "Hello world!"
```
We will only be using strings in this course

Concatenation

You can make a new string by joining two existing strings
This is called concatenation
We join two strings together using the + operator

When I run

String s1 = "foo";
String s2   = "bar";
System.out.println(s1 + s2);

the output is

foobar

You can concatenate many strings together on a line of code

When I run

String first = "John";
String last  = "Smith";
System.out.println(first + " " + last);

the output is

John Smith

Concatenating Strings and Numbers

What if I wanted to concatenate a string with a variable holding a number?

I can still use the + operator

int result = 5;
System.out.println("The answer is " + result);

gives

The answer is 5

Why does this work"?

If I wrote

int n1 = 5;
int n2 = 10;
System.out.println(n1 * n2);

the output would be

Java knows the data type of each variable and literal
If the value in front of + is a number
It knows that + means addition
If the value is a string, it knows + means concatenation
But then why did Java concatenate a string with in number when I wrote
```
50
```
Java can't concatenate a string with a number
So it turned the number into a string

Four Ways To Store Numbers

Sometimes a value can be stored in more than one data type
Take the number 5 for example
We can store it in an int variable
```
int number = 5;
```
Or in a double
```
double number = 5;
```
But we can also store it in a String
```
String number = "5";
```
Or even a char
```
char number = '5';
```
What does it matter which type we chose?
If we want to do arithmetic need to use int or double
The only thing we can do with String and char is concatenate
Note that we cannot store a number greater than 9 in a char
Because char only hold a single character

IT 114: Introduction to Java
Class 6

Topics

Review

New Material

Homework 3

Review

Variables

Declaring Variables

Assignment Statements

Literals

= Does Not Mean Equals

Variable Names

Choosing Good Variable Names

Java Is Case Sensitive

Printing Variables

New Material

Operators

Arithmetic Operators

The Mod Operator

Two Groups of Data Types

Strings

Concatenation

Concatenating Strings and Numbers

Four Ways To Store Numbers

Class Quiz

Attendance

Class Exercise