IT 114: Introduction to Java

IT 114: Introduction to Java
Class 5

Topics

Homework 2

Homework 2 is posted here.

I want you to write a Java program that will draw a rectangle.

Be sure to read the specification carefully and draw a rectangle of exactly the right size.

If the rectangle is not the right size you will lose points.

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

Announcements

The first graded quiz will be given next week.

The questions will be taken from Class Quizzes 2 - 6.

Tips and Examples

Patience

The first few weeks of this course can be frustrating
There are a lot of details to learn
And some things have to be done on the Unix command line
It may seem, at first, that nothing works
Don't let this get to you
In the beginning you will make mistakes
Everyone does
But if you keep at it you will make fewer mistakes
One of the things you have to learn in technical courses is frustration tolerance
You should expect that some errors will occur
And deal with them one by one
But you might have trouble doing this if you wait to the last minute
Sometimes the best way to deal with a problem is to walk away from it for a while
And try it again tomorrow
I do this all the time
I come across a bug and I just can't fix it
If I do something else
And then come back to it later, it's sometimes obvious what I missed
When you are working hard on a problem you can get tunnel vision
Coming back to it the next day you approach the problem with a fresh perspective

Review

Why You Do Java Program Have Their Unusual Structure?

All Java program have the following structure


public class CLASS_NAME {
    public static void main(String[] args) {
        ...
    }
}

Why?
To understand this we have to talk about two important ideas
- Objects
- Classes

Objects

Java is an object oriented language
But what are objects
An object is a section of a computer's RAM
An object can hold many values
Along with code that can work with these values
There is no limit to the number of values an object can contain
Objects only exist when the program that created them is running

Classes

Objects have to be defines before you can create them
The definition are a type of software called a class
Classes are software written in Java
Objects are the contents of RAM created from a class when a Java program is run
There is only one class for each type of object
A single class can create many objects

Statements

A Java program consists of a number of statements
A statement is a line or lines of code that represent a complete action
There are two kinds of statement
- Simple statements
- Compound statements
A simple statement can be written on a single line and contains no other statements
A compound statement contains other statements within it
All statements must end with a semicolon, ;
If you don't you will get an error when you compile it

Printing a Line in Java

The first Java statement you need to learn prints a line of text
It has the following form
```
System.out.println(STRING);
```
This may seem like a lot of characters to perform a simple action
The statement looks like this due to Java's object oriented design
The javac compiler objects because it does not know println
If the parentheses are empty it simply prints a blank line

Data Types

The basic job of any program is to create and manipulate data
And data comes in many forms
Each of these forms must be represented in RAM in the most efficient way possible
To do this, each computer language has different data types
Each of these data types specifies how a value is to be represented in RAM
Some languages, like Python and Perl, are very easy going with respect to data types
But Java is not one of them
Java is a type-safe language
Another term for this type of language is strongly typed
In a type-safe language, you have to assign a data type to each value
The data type of a value determines what you can do with it

The Java Data Types

There are 8 data types in Java
But we will only use 4 of them

int integer

double decimal

bool boolean

char character

New Material

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
Variables are used to store values that you will use later

Declaring Variables

I have said that Java is fussier than scripting languages like Python and Perl
One place where you can see the difference is how they treat variables
In Python and Perl a variable can hold a value of any type
Integer, decimal, boolean, it does not matter
Not only that, but a variable can hold a decimal one minute
And a boolean the next
That is not true of Java
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 Python and Perl you create a variable when you use it
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

Variables are only useful when they have a value
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"
In general you should avoid literals except for values like 0 or 1
Literals make a program less flexible
If you were writing a program to calculate interest you would not want the interest rate to be a literal
Because then you would have to rewrite the source code every time the rate changed

= 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
= is the assignment operator in all the languages I have studied
Is unfortunate
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
```
Can you guess what this is calculating?
How about this
```
distance = speed * time
```
Both expressions use legal variable names but the second is more informative than the first
It is more readable
The code you write makes sense to you right now
But will it still make sense a year from now?
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
```
Another way to combine multiple words in a variable name is to capitalize the first letter of each new word
Like this
```
courseId
studentName
firstName
```
This technique is called camel case
I prefer to use underscores since I think they are more readable
But you are free to use either technique
You must do something to make the start of each word in name stand out
I don't want to see
```
courseid
studentname
firstname
```
They are not easy to read
This is much better
```
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